THE NUTRITION OF FARM ANIMALS THE MACMILLAN COMPANY NEW YORK • BOSTON • CHICAGO • DALLAS ATLANTA • SAN FRANCISCO MACMILLAN & CO., LIMITED LONDON • BOMBAY • CALCUTTA MELBOURNE THE MACMILLAN CO. OF CANADA, LTD. TORONTO THE NUTRITION OF FARM ANIMALS BY HENRY PRENTISS ARMSBY, PH.D., LL.D. • i DIRECTOR OF THE INSTITUTE OF ANIMAL NUTRITION OF THE PENNSYLVANIA STATE COLLEGE; EXPERT IN ANIMAL NUTRITION, UNITED STATES DEPARTMENT OF AGRICULTURE; FOREIGN MEMBER, ROYAL ACADEMY OF AGRICUL- TURE OF SWEDEN gork THE MACMILLAN COMPANY 1917 All rights reserved COPYRIGHT, 1917, BY THE MACMILLAN COMPANY. Set up and electrotyped. Published June, 1917. ' \ ) Nortoaoft J. S. Gushing Co. — Berwick & Smith Co. Norwood, Mass., U.S.A. PREFACE THE manner in which the subject of the nutrition of farm animals is presented to the student will naturally differ ac- cording to the ultimate end in view. If the prime purpose is to impart practical skill in the feeding of live stock, the study of the principles of nutrition is likely to be regarded as pre- liminary and to partake of the nature of an information course, and chief stress will be laid upon familiarity with the results of experience, particularly as related to the business aspects of the subject, and to the acquisition of practical skill. But while by no means disposed to minimize the significance of this aspect of the subject, the writer is nevertheless convinced that for the students of our agricultural colleges a somewhat different procedure is desirable. He believes that greater emphasis than they sometimes receive may wisely be laid upon the chemical and physiological laws which underlie the practice of feeding, both on account of their intrinsic importance and because the subject may thus be made a real collegiate disci- pline which shall contribute to the training as well as to the information of the student. Accordingly, the present volume attempts to deal primarily with the natural laws governing the nutrition of farm animals, as distinguished from the broader field of animal husbandry, and only secondarily with the specific details of practice. It seeks to avoid so far as may be mere dogmatic statements, and, although not attempting complete citation of literature even upon important points, to present the experimental evidence with sufficient fullness to indicate something of the limitations of present knowledge and of the opportunities for further in- vestigation. Its aim is to discuss the fundamental principles upon which successful stock feeding is consciously or uncon- sciously based in the firm persuasion of the truth so pithily expressed almost half a century ago by the father of agricul- tural science in the United States, Professor Samuel William v 380112 VI PREFACE Johnson, that, "Other qualifications being equal, the more advanced and complete the theory of which the farmer is the master, the more successful must be his farming. The more he knows, the more he can do. The more deeply, comprehen- sively, and clearly he can think, the more economically and advantageously can he work," and that "A true theory is the surest guide to a successful practice." In short, the book is intended for the student rather than directly for the farmer and assumes a certain degree of prelim- inary training on the part of the reader, including an elementary knowledge of chemistry and physics. The author is under obligations to The Honorable Secretary of Agriculture, for permission to reproduce, in Chapter XVIII, a part of Bulletin No. 459 of the United States Department of Agriculture; to the Macmillan Company for the similar use in Chapter XV of material from Bailey's "Cyclopedia of American Agriculture"; and to Messrs. Henry and Morrison for permis- sion to base the tables of the net energy values of feeding stuffs contained in the Appendix upon their extensive compilations in the fifteenth edition of "Feeds and Feeding." He is like- wise indebted to the following publishers for the use of the cuts named : The Carnegie Institution of Washington, Figure 24. The F. A. Davis Company, Figure 17. Ginn & Company, Figures 2, 3, 6, 7, 8, 16, 19, 20, and 22. The Macmillan Company, Figures 4, 15, 29, 31, 32, 33, 34, 37, 43, 44, and 45. The W. B. Saunders Company, Figures 1,5, and 14. John Wiley & Sons, Inc., Figures 18 and 40. STATE COLLEGE, PA., May, 1917. CONTENTS PAGE INTRODUCTION vii PART I THE MATERIALS OF NUTRITION CHAPTER I THE COMPONENTS OF PLANTS AND ANIMALS 3 § i. Dry matter; organic matter ; ash 3 § 2. The carbohydrates 7 § 3. Fats and related bodies. — The Lipoids . . . .16 § 4. The proteins ........ .24 § 5. The non-proteins 36 § 6. Sundry ingredients . . . * . . . . -39 CHAPTER II THE COMPOSITION OF ANIMALS AND OF FEEDING STUFFS . . 42 § i. The cell 42 § 2. Animal tissues and organs 45 § 3. The composition of £he animal as a whole . . . .61 § 4. The composition of feeding stuffs 66 PART II THE PROCESSES OF NUTRITION CHAPTER III DIGESTION AND RESORPTION . . . . . . . -77 § i. The organs of digestion 77 § 2. The chemistry of digestion ....... 89 § 3. Resorption — The feces 105 § 4. The determination of digestibility in vii Vlll CONTENTS CHAPTER IV PAGE CIRCULATION, RESPIRATION, AND EXCRETION 123 § i. Circulation .......... 123 § 2. Respiration 132 § 3. Excretion 139 CHAPTER V METABOLISM 144 § i. General conception . . . . . . . .144 § 2. Enzyms as agents in metabolism . . . . . .148 § 3. The metabolism of the carbohydrates 152 § 4. The metabolism of the simple proteins . . . . .160 § 5. The metabolism of the nucleoproteins . . . . .168 §6. The metabolism of the fats .171 § 7. Metabolism of ash ingredients 178 § 8. Functions of the nutrients 182 CHAPTER VI THE BALANCE OF NUTRITION 192 § i. General conception . . 192 § 2. Methods of investigation 194 § 3. The balance of matter . 202 § 4. The balance of energy . 216 § 5. Significance of results 241 PART III THE FEED REQUIREMENTS CHAPTER VII THE FASTING KATABOLISM 249 § i. The protein katabolism in fasting 251 § 2. The energy katabolism in fasting . . . . . .256 § 3. Conditions affecting the fasting katabolism . . . .258 CONTENTS ix CHAPTER VIII • PAGE MAINTENANCE — THE ENERGY REQUIREMENTS . . . .267 § i. Net energy values for maintenance 271 § 2. Maintenance requirements of farm animals . . . .280 § 3. Factors affecting the maintenance requirement . . . 304 § 4. The relation of the maintenance requirement to external temperature 3°8 CHAPTER IX MAINTENANCE (Continued) — THE REQUIREMENTS OF MATTER . 313 § i. The protein requirements for maintenance . . . . 313 § 2. The ash requirements for maintenance 332 § 3. Accessory substances 348 CHAPTER X THE FATTENING OF MATURE ANIMALS 35° § i. Composition of the increase in fattening . . . -35° § 2. Feed requirements for fattening ...... 359 CHAPTER XI GROWTH 37i § i. General nature of growth . . . . . . . 371 § 2. The utilization of feed in growth . . . . . .381 § 3. The feed requirements for growth 396 CHAPTER XII MEAT PRODUCTION 424 § i. Nature of meat production 424 § 2. The animal as a factor in meat production . . . .428 § 3. Feeding for meat production 444 § 4. Influence of external conditions 453 CHAPTER XIII MILK PRODUCTION .......... 459 § i. The physiology of milk production 459 § 2. The animal as a factor in milk production .... 47° § 3. The influence of environment on milk production . . 478 § 4. The utilization of feed in milk production .... 488 § 5. Feeding for milk production . . . • . . . - 5°° X CONTENTS CHAPTER XIV , PAGE WORK PRODUCTION . .531 § i. The physiology of work production . . . . .531 § 2. The efficiency of the body as a motor ..... 544 § 3. Feed requirements for work 560 PART IV THE FEED SUPPLY CHAPTER XV THE FEEDING STUFFS 571 § i. Roughages, or coarse fodders . . . . . .572 § 2. Roots, tubers and fruits . . . . . . .578 § 3. The concentrates 579 CHAPTER XVI RELATIVE VALUES OF FEEDING STUFFS 591 § i. Direct comparisons of feeding stuffs . . . . . 591 § 2. Relative values based on composition and digestibility . 597 § 3. Conditions affecting digestibility . . . . . .601 CHAPTER XVII THE PRODUCTION VALUES OF FEEDING STUFFS .... 630 § i. General considerations 630 § 2. Production values as regards energy — Net energy values . 634 § 3. The computation of net energy values ..... 667 § 4. Production values as regards protein 678 CHAPTER XVIII THE COMPUTATION OF RATIONS ....... 689 § i. Feeding standards 689 § 2. Feed requirements . . . . . . . .691 § 3. Method of computation . . . . . . .697 CONTENTS XI APPENDIX ESTIMATED PROTEIN AND ENERGY REQUIREMENTS OF FARM ANIMALS Table I. Maintenance requirements of cattle and horses, per day and head Table II. Maintenance requirements of sheep and swine, per day and head ....... Table III. Requirements for fattening with no considerable growth — all species — in addition to the maintenance requirement .... Table IV. Requirements for growth with no considerable fat- tening ........ Table V. Requirements for milk production .... Table VI. Requirement%for work production by the horse PAGE 711 711 711 712 712 AVERAGE DRY MATTER, DIGESTIBLE PROTEIN AND NET ENERGY VALUES OF FEEDING STUFFS PER 100 POUNDS . . .714 Table VII. Values per 100 pounds for ruminants . . . 715 Table VIII. Values per 100 pounds for the horse . . .721 Table IX. Values per 100 pounds for swine . . . .722 Table X. Mineral elements of feeding stuffs — per 100 pounds of dry substance 723 REFERENCES The full-face numbers in parenthesis in the body of the text refer to the numbered paragraphs and not to pages. ILLUSTRATIONS PAGE FIG. 1. Different types of cells composing the body • • • • 43 2. One end of a muscle fiber ........ 50 3. Part of a muscle fiber . . . . . . . . -51 4. Fat cells in muscles -52 5. Scheme of a fat cell 5g 6-8. Successive stages in the formation of adipose tissue . . 59 9. Sheep's stomach . go 10. Stomach and duodenum of horse .81 11. Stomach of hog gx 12. Intestines of cattle g4 13. Ccecum of horse g^ 14. Section of villi IO5 15. Steer in digestion stall H3 16. Blood corpuscles I24 17. Diagram of mammalian heart . . . . . -125 18. Scheme of circulation of blood . . . . . . .127 19. Relation of cells to blood vessels and lymphatics . . . 130 20. Main lymphatic trunks 13! 21. Alveoli of lung !33 22. Section of two alveoli 133 23. Diagrammatic scheme of metabolism 182 24. Scheme of closed-circuit respiration apparatus .... 209 25. Original Regnault-Reiset apparatus 210 26. Regnault-Reiset apparatus as used by Zuntz . . . . '211 27. Scheme of Pettenkofer respiration apparatus . . . -213 28. Pettenkofer respiration apparatus, explanatory sketch . -213 29. The Mockern respiration apparatus . . . . . .214 30. Horse equipped for experiments with Zuntz apparatus . -215 31. Lavoisier's ice calorimeter 222 32. Section of bomb calorimeter . 224 33. The Zuntz tread power dynamometer ...... 226 34. Dulong's water calorimeter 237 35. The respiration calorimeter at The Pennsylvania State College . 238 36. Rubner's calorimeter ......... 239 37. The marbling of meat 356 38. Rate of gain of protein per 1000 pounds live weight . . -379 xiv ILLUSTRATIONS FIG. PAGE 39. Rate of gain of energy per 1000 pounds live weight . . . 398 40. Lobule of milk gland .462 41. Alveoli of milk gland 463 42. Structure of milk gland . . . . . . . . 463 43. Partial section of wheat grain 582 44. Partial section of oat grain . 584 45. Partial section of maize kernel . . . . . . 588 INTRODUCTION THE problems of nutrition concern the farmer both directly and indirectly - — indirectly because his function in society is to furnish the materials for the nutrition of man ; directly, because an essential part of that function consists in the economical conversion of vegetable into animal products by means of farm animals. Particularly is this true regarding the inedible prod- ucts of the farm. It is a well-recognized fact that only the smaller portion of the solar energy or of the proteins which are stored up in the farmer's crops is directly available for man's use. Even in distinctively food crops, such as wheat, for ex- ample, more than two-thirds of the energy which they contain may be unavailable for human nutrition, while the grasses and legumes, so important in all systems of agriculture, are of no direct value as food for man. The essential function of the animal in a permanent system of agriculture is the conversion of as large a proportion as possible of these inedible products into forms whose matter and energy can be utilized by the human body. It is true that animal products contribute largely to our supply of clothing and also that, as a motor, the work animal plays an important part in agriculture and industry. In both respects, however, substitution is possible to a greater or less extent. Vegetable fibers may to a degree replace animal fibers in our textiles, while inanimate motors seem destined to fill an increasing role in power production in all its aspects. But for the conversion of the by-products of the farm and fac- tory into human food, there is as yet no suggestion of an agency which can take the place of the animal body. With the growth of the non-agricultural population it is in- creasingly important that this function of conserving the food supply through the utilization of inedible soil products shall be performed with a maximum of efficiency. This requires, on the one hand, as intimate a knowledge as possible of the funda- mental laws governing the nutrition of farm animals, so that XVI INTRODUCTION the transformation may be effected with the least possible waste, and, on the other hand, the ability so to apply these laws as to secure the greatest economic return, since it must never be forgotten that the criterion of success in agriculture is not a maximum production but a maximum profit. It is with the former portion of this complex problem that the present work attempts primarily to deal. Without entering into the controversy between the vitalist and "the mechanist, the nutrition of the animal, whatever its guiding principle, may be regarded as a physico-chemical pro- cess, including the entire complex of reactions by which the crude materials of the feed are converted into substances suited to maintain the activities of the body cells or capable of being built up into living structures. In other words, the study of nutrition is a study of the chemistry and physics of the changes through which the crude products of the soil yield animal tissues or secretions on the one hand and excretory products on the other. The earlier investigators dealt with the food as a supply of matter, dividing it into inorganic and organic constituents and distinguishing among the latter between the nitrogenous and non-nitrogenous substances. In other words, they studied the problems of nutrition substantially as problems of biological chemistry. Rubner's fundamental investigations went far to shift the emphasis to the physical side of the problem. It has come to be clearly recognized that the animal body is essen- tially a transformer of energy — a mechanism for the conversion of the chemical energy of its feed into mption energy while more or less incidentally a reserve of energy-containing material may be stored up which can be utilized for human food. It is this capacity of the animal body to store up in itself or in its secretions a part of the matter and energy of the feed it con- sumes which gives the animal its economic significance as a conserver of the food supply. Its value in this respect depends upon the proportion of its feed which it is able thus to set aside — i.e. upon the balance between the income and outgo of matter and of energy — and it is from this point of view that the present volume undertakes to present the nutrition of farm animals. From this standpoint, the subject naturally falls into four principal divisions. INTRODUCTION xvij First, since nutrition involves chemical changes by which feed substances are converted into body substances, there is required some knowledge of the chemical compounds concerned and of their occurrence and proportions in plants and animals. Second, the conversion of feed substances into body sub- stances is a function of the living organism and it becomes necessary, therefore, to learn something of the processes by which the body effects these changes or, in other words, to study the physiology of nutrition. Third, in order to apply the principles of the chemistry and physiology of nutrition to the practical problems arising in the feeding of farm animals it is requisite to determine quantita- tively the amounts of matter and of energy which are required by different species of animals for their support and for the production of meat, milk or work. Fourth, to supply the feed requirements as thus ascertained in the most economical manner demands a knowledge of the available feed resources, both as to the nature and quantity of nutriment which they contain and as to the proportion of this nutriment which can be utilized by the body. Accordingly, the general subject of the nutrition of farm ani- mals is treated of under four general heads, viz. : — Part I, The Materials of Nutrition. Part II, The Processes of Nutrition. Part III, The Feed Requirements. Part IV, The Feed Supply. PART I THE MATERIALS OF NUTRITION NUTRITION OF FARM ANIMALS CHAPTER I THE COMPONENTS OF PLANTS AND ANIMALS § i. DRY MATTER; ORGANIC MATTER; ASH 1. Dry matter. — The material composing the plant or animal may be regarded as consisting of water and dry matter. The two are ordinarily separated by maintaining the material at or above the boiling point of water until it ceases to lose weight. The loss in weight is regarded as consisting solely of water, while the residue is, of course, the dry matter. , 2. Water. — Water is by no means to be regarded as an accidental or incidental component of plants or animals. The necessity for an adequate water supply to living beings is too well known to require mention, while very little reflection is needed to show that the water is as essential a part of the organ- ism as any other ingredient. In the supporting tissues of the plant or animal it has a mechanical function, lending elasticity combined with strength. It acts as a solvent and carrier of food materials and waste products and the osmotic pressures of the solutes are an important factor in physiological processes. Finally, its action in dissociating electrolytes appears to be very intimately related to the chemistry of living matter. Water is usually abundantly supplied to live stock. The study of animal nutrition, therefore, deals chiefly with the dry matter, its supply and transformations, not because this is fundamentally any more essential than the water but because ordinarily it is economically more important. 3 4 KUtRITIQN OF FARM ANIMALS X Organic matter. — By the action of oxygen at a high tem- perature, the dry matter of plants or animals may be separated into two portions, one being converted into the gaseous state, while the other remains behind in the solid form. Following the older nomenclature, it is customary to distinguish these two portions as " organic " and " inorganic," or " ash," in- gredients. The terms, however, are to some extent misnomers, since no such sharp distinction exists as was once supposed between organic and inorganic compounds. Organic matter in the sense in which the term is commonly used may be said to be broadly equivalent to the carbon compounds of the organ- ism, but even this definition is inexact and the same element may be volatilized during oxidation or may appear in the ash according to circumstances. For example, the element sulphur is an essential ingredient of the proteins. When these are burned in air part of the sulphur escapes in the gaseous form, but a part also combines with any bases present and appears in the ash as sulphates. Even the element carbon, distinctive of so-called organic matter, may appear in part in the ash of the plant or animal in the form of carbonates when the bases of the ash are in excess of the acid radicles. These examples serve to show that an element may be an integral part of the molecules which make up the organic matter and yet appear after incineration in the ash. Thus it has recently been shown that the phosphorus of wheat bran and other feeding stuffs is present chiefly in the form of a complex carbon compound, yet when these materials are burned the phosphorus appears in the ash in the form of phosphates. Organic matter is usually regarded as consisting of the ele- ments carbon, hydrogen, oxygen, nitrogen and sulphur, phos- phorus being sometimes added to the list, but doubtless other elements like potassium, sodium, chlorin, etc., also enter into the structure of the " organic " molecules. 4. Subdivision of organic matter. — The number of individ- ual organic compounds found in the animal body or in the plant is very great. For the present purpose, however, it is not necessary to consider separately each individual substance but only the general properties of the important groups into which they may be classified. The organic constituents of the body may be subdivided into THE COMPONENTS OF PLANTS AND ANIMALS 5 non-nitrogenous and nitrogenous substances. Under the former are included the carbohydrates, the fats, the organic acids and various other minor groups. The nitrogenous substances in- clude the proteins and a variety of simpler nitrogenous sub- stances sometimes classed together as the non-proteins. In the following sections these various groups will be considered as far as is requisite for an intelligent study of their behavior in the animal body, it being assumed that the reader has already some knowledge of their general properties, both chemical and physical. 5. Mineral matter, or ash. — To what extent the elements found in the ash and commonly reckoned as the mineral ele- ments, namely, potassium, sodium, calcium, magnesium, iron, phosphorus, sulphur, chlorin, silicon, etc., are actually present in the living plant or animal as electrolytes and to what extent as ingredients of complex organic molecules, it is at present im- possible to state with any defmiteness. In ordinary usage the term ash is equivalent to the residue remaining after incineration at as low a temperature as possible, usually not exceeding a dull red heat. The proportion of ash in ordinary feeding stuffs varies con- siderably according to the kind of plant, the portion of the plant used (seeds, stems, leaves, roots, etc.), the maturity of the plant and various other conditions. Wolff gives the following as general averages for the proportion of ash in the dry matter : — GRAIN STRAW Cereal crops ... 2% 5.25% 3% 5.00% Oil plants 4% 4.50% The proportion varies most in the straw and least in the grain. In the animal, the presence of ash is most evident in the bones. About two-thirds of the dry matter of the clean bone (free from fat) consists of ash. Ash is by no means absent from the soft tissues of the body, however, of which it forms an essential in- gredient. The proportion varies in different organs, but as a rough general average the body, inclusive of the skeleton, con tarns about 3.5 per cent of ash in the fresh substance, 6 NUTRITION OF FARM ANIMALS equivalent to about 7.1 per cent of the dry matter. The pro- portion of ash to dry matter is greater in the young than in the mature animal and greater in the lean than in the fat condition. The more important elements found in the ash are as fol- lows:— Potassium. — This metal is indispensable to plant growth and is found in all parts of the plant, but especially in the active, growing parts. In the animal body it is found abundantly in the tissues, such as the muscles, glands, nerves, etc., while the fluids (blood, plasma, lymph, etc.) contain relatively small amounts of it. Sodium. — Unlike potassium, sodium is not indispensable to plant growth, although it apparently is useful to the plant under some con- ditions. It is found especially in the stems and leaves of plants, although not so abundantly as potassium. Seeds contain but little of it. In the animal body it is especially abundant in the fluids, which, as just noted, contain relatively little potassium. Calcium. — Like potassium, calcium is necessary for the growth of plants. It is found especially in the leaves and stems of plants and to a much less extent in the seeds. It appears to be equally essential to the animal and is found in all parts and organs of the body. Its most striking use, however, is in the formation of the skeleton, the mineral portion of which (81) consists chiefly of calcium phosphate and carbonate. Both these compounds being scarcely at all soluble in water, they are well adapted to form the framework of the body. In the skeletons of the higher animals calcium phosphate is the chief mineral ingredient, while in the lower animals like shellfish and Crustacea, the shell, which corresponds to the bones of domestic ani- mals, contains chiefly calcium carbonate. Magnesium. — Magnesium is also one of the elements essential for plant growth. It is found throughout the plant in smaller amounts than calcium, but is more abundant than the latter in the seeds and seems to aid in seed formation. In the animal body, magnesium usually accompanies calcium, but in much smaller amounts. Iron. — A small amount of iron is required by the higher plants for the formation of the green coloring matter (chlorophyl) by means of which they assimilate the carbon dioxid of the air. In the ani- mal, iron in small quantity is necessary for the formation of the red coloring matter (haemoglobin) of the blood which is the agent for conveying the oxygen of the air to the tissues. While, therefore, but a very small amount of iron is required by either plants or animals,1 it is nevertheless essential to the most fundamental processes of life. 1 It is estimated that the blood of an adult man contains about 3 grams of iron. THE COMPONENTS OF PLANTS AND ANIMALS 7 Phosphorus. — Phosphorus is another of the elements essential to plant growth, its chief function seeming to be to aid in the produc- tion and transportation of the proteins. It is found in all parts of the plant but accumulates especially in the seeds. Plants may contain more or less phosphorus in the form of phos- phates, especially in their vegetative organs. Even in the latter, however, a considerable share of it is in "organic" combination, while in the seeds but very small amounts of " inorganic" phosphorus are found. The " organic." phosphorus of plants is contained chiefly in three classes of compounds, viz., the phosphatids (37, 38), or so- called phosphorized fats, the nucleo- and phospho-proteins (52, 55), and phytin, the latter being the chief phosphorus compound of seeds. Phytin is a compound of phosphoric acid and inosit and may be split up into these constituents by hydrolysis and also by an enzym found in seeds. In the animal, the great store of phosphorus is found in the skele- ton, where it exists, as already stated, chiefly in the form of calcium phosphate. It is also found somewhat abundantly in the soft tissues of the body, of which it is an essential ingredient. Here it seems to exist largely in "organic" combination in the phosphatids and the nucleo- and phospho-proteins. Sulphur. — Sulphur is taken up by the roots of the plant in the form of sulphates, and when plant or animal substances are burned, more or less of the sulphur which they contain is found as sulphates in the ash. For these reasons, sulphur has been commonly regarded as one of the ash ingredients of plants and animals. As a matter of fact, however, as already pointed out, it is usually as truly an "or- ganic" ingredient as nitrogen or carbon. In particular, it is one of the elements of which the proteins are composed, and seems to exist in the plant and animal chiefly in this form. Chlorin. — Chlorin is found in plants associated with sodium. It does not seem to be necessary to plant life. In the animal it is an essential element in the gastric juice. Small amounts of fluorine and traces of iodin and of manganese and other catalysts also occur, but their specific functions are obscure except that fluorin is an ingredient of the enamel of the teeth. § 2. THE CARBOHYDRATES 6. Occurrence. — Although substances belonging to this group of compounds are found in the bodies of animals, they are especially characteristic of plants. Starch, one of the most familiar of them, is the first visible product of the assimilation 8 NUTRITION OF FARM ANIMALS of carbon dioxid by chlorophyl-bearing plants, and the great mass of vegetable tissue is composed either of carbohydrates or of their nearly related derivatives. The more common carbohydrates have been known for a long time. Starch is familiar to us in the mealy portion of grains and in certain tubers, and cellulose in cotton and linen and, in im- pure forms, in the woody fiber of plants. Of the sugars, cane sugar has been known since almost prehistoric times, while the presence of this and other sugars in plant juices, in sweet fruits, honey, etc., is a familiar fact. The more common sugars were separated and identified quite early in the history of chemistry. 7. Classification. — The carbohydrates contain hydrogen and oxygen in exactly the proportions to form water, and their name is derived from this fact, although compounds exist which contain two atoms of hydrogen to one of oxygen and yet are not carbohydrates, such, for example, as acetic acid, C2H4O2. The simplest of the carbohydrates are the simple sugars, more exactly designated as the monosaccharids. By polymerization, with elimination of water, the monosaccharids yield more complex carbohydrates which are conveniently classi- fied as di-, tri-, and polysaccharids. Monosaccharids, or simple sugars 8. Composition. — The monosaccharids may be represented by the general formula Cn H2n On. Substances having this gen- eral formula are known whose molecules contain from one to nine carbon atoms and which, from a chemical point of view, may be called carbohydrates. The simplest of these is formal- dehyde, CH2O, which is believed by many to be the first step in the synthesis of carbohydrates by the green plant. Only the CG and C& compounds, however, known respectively as the hexose and pentose carbohydrates, are of importance in their relations to nutrition. 9. Hexoses. — The most important hexose monosaccharids are dextrose, levulose, galactose and mannose. Dextrose, ^-glucose, or grape sugar, is generally regarded as an aldose of the hexatomic alcohol sorbite. ' Sorbite : CH2OH- (CH - OH)4- CH2OH Dextrose : CH2OH- (CH • OH)4- CHO THE COMPONENTS OF PLANTS AND ANIMALS 9 It occurs almost universally in the juices of plants along with levulose and cane sugar, and is found also in small amounts in the blood of mammals. Sixteen isomers of this compound are possible, twelve of which are known. Galactose and mannose are isomers of dextrose, occurring in nature only in combination as di- or polysaccharids. Levulose, or fruit sugar, is a ketose of sorbite, having the formula CH2OH-(CH • OH)3-CO-CH2OH, eight isomers being theoretically possible. It occurs mixed with dextrose in plant juices and in honey. The hexose monosaccharids are all soluble in water and readily diffusible and have a more or less sweet taste. All those found in nature are optically active, rotating the plane of polarized light. Thus dextrose, as its name implies, has a right-handed rotation and levulose a left-handed rotation. They reduce an alkaline solution of metallic salts, especially of copper, and this fact is utilized both as a qualitative test for them and as a means of quantitative determination. They are fermented by yeast, yielding as the chief products ethyl alcohol and carbon dioxid. 10. Pentoses. — The pentoses are simple sugars, correspond- ing to the hexoses but having the general formula C5Hi0O5. Those occurring in nature are aldoses. Like the hexoses, they reduce metallic oxids, but unlike them they are not ferment- able by yeast. Arabinose. — By the hydrolysis of gum-arabic or cherry gum, there is produced dextro-rotatory arabinose (/-arabinose). Levo- rotatory arabinose (^-arabinose) has been prepared artificially. The inactive or racemic form (£-arabinose) has been found in human urine in small amounts. Xylose. — By the hydrolysis of wood gum there is produced a dextro-rotatory pentose known as /-xylose. The levo-rotatory form of the same sugar (d-xylose) is obtained in the hydrolysis of certain nucleo-proteins, the pentose group seeming to be a constituent of the molecule of those compounds. Rhamnose is a derivative of the pentose sugars in which an atom of hydrogen has been replaced by methyl. It occurs somewhat widely in the vegetable kingdom. 10 NUTRITION OF FARM ANIMALS Glucosids 11. The monosaccharids not only occur in the free state but also in combination with a great variety of substances in the so-called glucosids. The glucosids readily undergo hydrolytic cleavage into their two (or more) constituents, either by the action of chemical reagents or of enzyms. For example, the amygdalin of the bitter almond yields two molecules of dextrose, one of benzaldehyd and one of hydrocyanic acid, and cerebron, a constituent of the brain, splits up into cerebronic acid, sphin- gosin and galactose. Among other more or less familiar glucosids may be mentioned salicin, saponin, phloridzin and digitalin. Disaccharids 12. The hexose group. — The disaccharids may be regarded as polymers or anhydrids of the monosaccharids, formed by the union of two molecules of the latter with the elimination of one molecule of water. The only disaccharids at present known be- long to the hexose group and their formation may be repre- sented by the equation C6Hi2O6 + C6Hi2O6 = Ci2H22On + H2O. From another point of view they are termed by some writers glucosids of the monosaccharids. Sucrose. — Sucrose, or cane sugar, has probably been longest known of the more familiar carbohydrates. It is found in the juices of the sugar cane and sorghum, in the sugar beet and in the sap of the maple, all of which are utilized as commercial sources of sugar. In smaller amounts it is present in a large number of plants. By the action of heat, aided by a dilute acid or alkali, or by the action of certain enzyms, notably the invertase of yeast, the reverse of the general reaction for the formation of the disaccharids may be brought about, one molecule of sucrose combining with one molecule of water to yield one molecule each of dextrose and levulose. Ci2H22Oii + H2O = C6Hi206 + C6Hi2O6 Sucrose rotates the plane of polarized light to the right, while, owing to the fact that the rotatory power of levulose is greater than that of dextrose, the mixture of equal parts of the two which is formed in the foregoing reaction rotates to the THE COMPONENTS OF PLANTS AND ANIMALS II left. On account of this fact, this breaking up of cane sugar has been called inversion and the use of this term has been extended to designate in general the hydrolytic cleavage of di- saccharids into their constituent monosaccharids. Lactose. — Lactose, or milk sugar, is a characteristic ingredi- ent of the milk of mammals. Like sucrose, it may be broken up, with the addition of one molecule of water, into two mole- cules of monosaccharids, in this case dextrose and galactose. It is less soluble than sucrose and therefore less sweet to the taste, having a gritty feel in the mouth. It is not found in plants. Maltose. — By the action of certain ferments upon starch during the germination of seeds and also in the digestive tract of animals, a disaccharid known as maltose is produced. It is therefore present abundantly in malt, whence its name. This sugar when hydrolyzed yields two molecules of dextrose. 13. General properties. — The disaccharids are crystalline, soluble in water and optically active. Sucrose does not reduce an alkaline copper solution, but lactose and maltose do. The disaccharids are not fermentable. Any cases in which they are apparently fermented are found to be preceded by some action which inverts or breaks up the disaccharids into their con- stituent monosaccharids. Trisaccharids 14. By the union of three molecules of CeH^Oe with the elimination of two molecules of water, there may be formed the compound CigH-BOie, called a trisaccharid. One such, known as raffinose, is present in the sugar beet, the cotton seed, in barley and in wheat. Upon hydrolysis it yields one mole- cule each of dextrose, levulose and galactose. Polysaccharids 15. Chemical structure. — The polysaccharids, like the di- saccharids, are anhydrids, but are formed by the combination of many molecules of the monosaccharids and have a correspond- ingly high molecular weight. The general formula of the hexose polysaccharids is (C6H10O5)n, the value of n doubtless varying 12 NUTRITION OF FARM ANIMALS through a wide range, but the molecular weights of the in- dividual polysaccharids have not been finally determined. The polysaccharids are tasteless and usually amorphous sub- stances which, with the exception of cellulose, are more or less soluble in water. They are optically active but in general are not diffusible through membranes. They are hydrolyzed easily, especially by the action of heat and acids and by enzyms, yield- ing ultimately monosaccharids. In addition to their common names, they are designated by terms derived from the monosaccharids out of which they are built up. Thus starch, which is an anhydrid of dextrose and yields only this sugar upon hydrolysis, is a dextran. Similarly, there are levulans, galactans, mannans, arabans, xylans, etc., yielding the corresponding sugars when hydrolyzed. In the same manner, it is customary to distinguish between the hexosans, derived from the hexoses, and the pentosans, the anhydrids of the pentoses. 16. The hexosans. — This group of carbohydrates includes those which are most abundant in the vegetable kingdom and of the greatest significance as sources of nutriment for man and animals, viz., starch, the dextrins and gums, and cellulose and its various derivatives. It will be convenient to consider the more important hexosans somewhat in the order of their re- sistance to solvents. 17. Cellulose. — Cellulose constitutes the basis of the cell walls of plants and is also found in certain lower animals (tuni- cates). Clean cotton consists of nearly pure cellulose, each fiber being a single cell from which the contents (protoplasm) have nearly disappeared. Linen and the best qualities of paper are other examples of nearly pure cellulose. A crystalline form has also been described. Cellulose is insoluble in water and comparatively resistant to reagents in general. Plants, however, contain enzyms (cytases) which are able to bring it into solution in the processes of plant growth, and apparently these enzyms play some part in its digestion by animals. It is also attacked and dissolved by some species of bacteria. Concentrated sulphuric acid dis- solves it, and the solution, on dilution and boiling, undergoes hydrolysis, yielding dextrose. Cellulose is therefore a dextran. Its molecular weight is unknown. THE COMPONENTS OF PLANTS AND ANIMALS 13 18. Hemicelluloses. — These polysaccharids differ from true cellulose in being hydrolyzed by comparatively short boiling with dilute acids and further in the fact that the hydrolysis, instead of yielding only dextrose, as in the case of cellulose, produces a variety of both hexose and pentose sugars, the former including galactose, mannose and levulose, as well as dextrose, and the latter arabinose and xylose. The hemicellu- loses must be regarded, therefore, as containing both hexosans and pentosans, but whether in mixture or chemical union is uncertain. While true cellulose constitutes the framework of the plant, the hemicelluloses serve to a greater or less extent as reserve material. In the conventional method of feeding stuffs analysis, the hemicelluloses are found both in the " crude fiber " (109) and in the " nitrogen-free extract " (110). 19. Lignin. — In the young plant, the cell walls consist of nearly pure cellulose. With advancing maturity they become thickened, not only by the formation of additional cellulose and of hemicelluloses but by the deposition of numerous " incrusting substances," the most important group of which has received the collective name of lignin. These substances contain a con- siderably higher percentage of carbon than cellulose (54 to 60 per cent) and may be separated from the latter by oxidizing agents. The substances of the lignin group contain methoxyl (— O- CH3) and ethoxyl (— O • C2H5) groups in considerable amount, and by some are regarded as substituted celluloses. 20. Crude fiber. — The so-called " crude fiber " (109) of plants contains most of the cellulose and lignin of the cell walls and in addition a third group — the cutin group1 — whose per- centage of carbon is still higher (60-75 Per cent). Cutin appears to be indigestible. 21. Starch. — Starch is one of the most common and impor- tant of the vegetable carbohydrates. In the growth of plants, starch is formed in the green leaves by the aid of light, and is the first visible product of assimilation. In the mature plant, it is stored up in large quantities in the seed or in the tuber to supply the needs of the new plant. Hence the common grains, corn, wheat, oats, barley, etc., as well as potatoes, are rich in starch and form commercial sources of it. The seeds of most legumes contain it in less amounts but still abundantly. In 1 Compare Konig: Landw. Vers. Stat., 65 (1907), 55. 14 NUTRITION OF FARM ANIMALS the oil seeds it is replaced by oil. It is not found in the animal body. Starch occurs in plants in the form of microscopic granules, which have a peculiar form for each species, so that we may speak of the starches rather than of starch. These grains consist of a surrounding envelope consisting of a variety of cellulose in- closing a more soluble substance or substances known as granu- lose. When treated with much hot water the starch grain swells and bursts the envelope and the enclosed granulose dissolves, probably after undergoing more or less hydration. Starch may be hydrolyzed readily by dilute acids or alkalies or by heat. The final product of its hydrolysis is dextrose, which in an impure form constitutes commercial glucose or starch sugar. Starch is therefore a dextran. As already noted, certain enzyms, notably those formed in germinating seeds and others secreted in the digestive tract of animals, act upon starch readily with the production of maltose. Starch is also acted upon by some species of bacteria with the formation of lactic, butyric and other acids, methan and in some cases hy- drogen. 22. Galactans. — Galactans occur more particularly in le- guminous plants, other feeding stuffs being comparatively free from them. 23. Inulin. — The roots of the artichoke, dahlia, dandelion, chicory and other composites contain instead of starch a quite similar carbohydrate, inulin, which on hydrolysis yields levulose instead of dextrose, i.e., it is a levulan, 24. The dextrins. — In the hydrolysis of starch a series of ill-defined, intermediate compounds is produced, collectively called dextrins. Commercial dextrin is made by heating moist starch to about 235° Fahrenheit. It is likewise produced in the cooking of starchy materials, the brown crust of bread, for example, consisting largely of dextrin. Various dextrins have been separated and described, but it seems questionable whether the investigators have worked with definite chemical individuals. For the present, it seems wiser to speak collec- tively of the dextrins as intermediate products between starch and the simpler di- and mono-saccharids. 25. Glycogen. — In the liver and muscles of animals, and to a less degree in other parts of the body, there is found in rather THE COMPONENTS OF PLANTS AND ANIMALS 15 small amounts a carbohydrate called glycogen. Glycogen has the same percentage composition as starch and has sometimes been called animal starch, although improperly, since its proper- ties are quite different from those of starch. It has important functions in the animal, as will appear later. It is not found in the plant. It is readily soluble in water, yielding an opalescent solution. The empirical formula of glycogen is the same as that of starch. When hydrolyzed it yields only dextrose, and is therefore a dextran. 26. The gums. — Familiar examples of this class of sub- stances are gum arabic and the gums of the cherry, peach and plum. The mucilage of flax seed closely resembles the gums, and other seeds also contain gum-like "bodies. Upon hydrolysis, the gums yield hexoses, especially galactose, showing that they contain galactans. In addition to hexoses, however, they yield sugars belonging to the pentose group. 27. The pentosans. — The pentosans may be regarded as polymers or anhydrids of the pentoses, corresponding in this respect to the polysaccharids of the hexose group. Their general formula is (CsHgC^),,, but their molecular structure is unknown. Araban. — This is a constituent of gum arabic and other gums, as shown by the fact that these gums, as already noted (10), yield /-arabinose when hydrolyzed. Xylan. — This compound is also known as wood gum. It can be extracted from various woods, from the cob of maize and from various other vegetable materials by the action of dilute alkalies, and yields /-xylose when hydrolyzed. In the plant, araban and xylan appear to be in a more or less close chemical combination with hexosans, especially in the cell walls of the more mature plant, constituting the so-called hemi- celluloses (18). Pectins. — Most ripe fruits, as well as the flesh of beets, turnips and similar roots, contain a group of substances called the pectin group. As they exist in the roots or fruits they are insoluble in water, but by cooking they are converted into sub- stances which form the basis of fruit jellies. On hydrolysis they yield pentoses, chiefly arabinose. 1 6 NUTRITION OF FARM ANIMALS § 3. FATS AND RELATED BODIES — THE LIPOIDS 28. Classification. — Under the rather vague term " lipoids," or fat-like substances, there are included, besides true fats, a large number of chemical individuals of varied and complex molecular structure. Chemically, these substances (with the exception of the cholesterins) are characterized by containing radicles of the so-called fatty acids, principally the higher ones of the series. Physically, the lipoids have been defined, prin- cipally from the standpoint of the physiological chemist, as substances which are soluble in organic solvents, such as ether, alcohol, chloroform or benzol. This latter definition, however, includes substances, such as the cholesterins, which would be excluded by the chemical definition just given. For the present purpose, the principal lipoids may be conveniently grouped under five heads : (i) fats, (2) waxes, (3) cholesterins, (4) phosphatids or phospholipins, (5) cerebrosids or galactolipins. The Fats 29. Occurrence. — It is a familiar fact that the bodies of animals contain a not inconsiderable amount of fat, the per- centage seldom falling below six in the very lean animal while it may rise as high as forty in the very fat animal. The fat is the reserve material of the body and is contained in what is called adipose tissue (94) , consisting of cells of connective tissue more or less filled with fat. Larger or smaller amounts of adi- pose tissue are found in all parts of the body but especially in the subcutaneous tissues, the tissues surrounding the intestines, and, particularly in fat animals, in the muscles. In plants, fats are usually less abundant. They occur in all parts of the plant but are especially stored up in the seeds, where they serve as reserve material which is metabolized during germination. Some seeds, like those of cotton, flax and rape, contain fat so abundantly that they are commercial sources of oil. In the plant, the fat is not deposited in special tissues but is usually distributed through the protoplasm of the cell. Both animal and vegetable fats are mixtures of various simple fats, often containing also small amounts of free fatty acids. THE COMPONENTS OF PLANTS AND ANIMALS 17 30. Molecular structure. — The simple neutral fats are tri- glycerids, that is, they are esters of the triatomic alcohol glycerol with monobasic fatty acids, the hydrogen atoms of the three hydroxyls being replaced by the acid radicles. Their general formula is as follows, RI, R2 and R3 representing the acid radicles, which may or may not be the same : — Glycerol CH2 - OH — CH . OH — CH2 . OH Neutral fat CH2 . ORi — CH • OR2 — CH2 . OR3 The fatty acids may be divided into the saturated and the unsaturated. The saturated fatty acids have the general for- mula CraH2nO2 and are the normal acids of the aliphatic series, the two lower members of which are familiar as formic and acetic acids. The general formula of these acids is as follows, each carbon atom being united to the adjacent ones by a single bond. CH3-(CH2)n -COOH The two principal saturated acids contained in the animal fats are stearic acid, Ci8H36O2, and palmitic acid, CieH32O2. Besides these two, however, others are also found in small amounts. In butter fat, especially, several of the lower acids of the series are present, the principal ones being butyric, C4H8O2, caproic, C6Hi2O2, caprylic, C8Hi6O2, capric, Ci0H2oO2, lauric, Ci2H24O2 and myristic, Ci4H28O2. In the body fats there have been found also higher acids of the same series, particularly arachnic acid, C2oH4oO2. The unsaturated fatty acids differ from the saturated acids in containing two or more carbon atoms united by two bonds instead of one and consequently in containing less hydrogen than the saturated acids. Of the unsaturated acids, the most abundant in animal fats is oleic acid, having the formula CH3-(CH2)7-CH = CH-(CH2)7-COOH The eruic acid of rape oil also belongs to this series, and the linoleic acid, Ci8H32O2, of linseed oil and other drying oils belongs to a related series of unsaturated acids of the general formula CnH2n_4O2 with two double unions of carbon atoms. It is a noteworthy fact that nearly all the fatty acids occurring in the animal body contain an even number of carbon atoms. 1 8 NUTRITION OF FARM ANIMALS 31. Chemical reactions. — Of the chemical reactions of the fats, the one of most importance physiologically is that known as saponification, or more strictly as hydrolysis. It consists of a cleavage and hydration of the molecule, yielding glycerol and fatty acids. The most familiar instance of this reaction is in the process of soap making. For example, if tri-stearin is acted upon by potassium hydrate the final result is as represented by the following equation : — C3H5(C18H3502)3 + (KOH)3 = (KC18H3502)3 + C3H8O3 Tristearin Potassium hydrate Potassium tristearate Glycerol In this reaction, the alkali salt of the fatty acid, that is, a soap, is obtained. By the action of water at temperatures con- siderably above 100° C., essentially the same result is reached except that the free acid is obtained instead of the salt. The same decomposition may also be effected by means of acids, which probably act as catalyzers. Of most importance physiologically is the hydrolysis of fat by means of enzyms. Such enzyms are produced by certain plants and are also found in various digestive juices, notably in the secretion of the pancreas. These enzyms have received the general name of Upases. The hydrolysis of fats by enzyms appears to be a reversible reaction, at least with the glycerids of low molecular weight. In other words, the same enzym may effect the cleavage of a glycerid or the combination of glycerol and the fatty acid, the reaction in either case reaching an equilibrium at a certain stage.. 32. Physical properties. — Certain general properties are common to all the fats. Their specific gravity is in all cases less than one, so that they float on water. They have a fatty feel and leave a permanent grease spot on paper or fabric. They are almost insoluble in water, although water is soluble to a not inconsiderable extent in fats. They are readily soluble in ether, benzol, carbon disulphid and most of them in petroleum ether, but only sparingly in alcohol. The melting point of the fatty acids increases with the molecular weight. The exact melting point of a fat is diffi- cult to determine, but for the three common glycerids and the corresponding acids it may be stated approximately as follows : — THE COMPONENTS OF PLANTS AND ANIMALS 19 MELTING POINTS Olein . . -4° to -<' 3 r Oleic acid 14° C Palmitin 63° to 65' 3 r Palmitic acid 62.6° C. Stearin . 71.6° C. Stearic acid 71.5° C. A distinction is commonly made between fats and oils, the fats being solid at ordinary temperatures and the oils liquid. The difference depends largely upon the proportion in which the various simple fats are present. Olein and other fats con- taining unsaturated acids are usually liquid at room temper- ature and their presence increases the softness of the fat. The fatty acids of higher molecular weight are volatile only at comparatively high temperatures and at reduced pressure. Those of lower molecular weight, notably those contained in but- ter fat, can be readily distilled in a current of steam and their proportion serves to distinguish butter fat from other animal fats. An important physical property of the fats, which, however, is by no means peculiar to them, is that of forming what is known as an emulsion. Fat is said to be emulsified when, in the liquid state, it is distributed in minute droplets or globules throughout some other liquid ; for example, if fat be violently shaken with water an emulsion is formed. Such an emulsion is not permanent, however, the fat droplets very soon coalescing and rising to the surface. The presence of small amounts of certain other substances dissolved in the water, however, will prevent this separation and give rise to a permanent emulsion. The most common substance producing this effect is soap. Certain gums and also proteins likewise serve to retain fat in the emulsified state. The most familiar example of such an emulsion is milk, the fat being held in suspension in this case by the action of the proteins of the mUk. This effect of various substances in retaining fat in the emulsified form depends upon their effect upon the surface tension of the contact layer be- tween fat and water, but a full discussion of this point would be out of place in this connection. 33. Native fats. — As has already been stated, the reserve fats of the animal body are triglycerids, chiefly of stearic, oleic 20 NUTRITION OF FARM ANIMALS and palmitic acids, although small quantities of esters of lauric, myristic and arachnic acids and frequently free fatty acids are also found, as well as minute amounts of esters of the higher alcohols, coloring matter, etc. Since stearin and palmitin are solid at ordinary temperatures, while olein is liquid, the con- sistency of a fat depends largely upon the proportion of olein which it contains and varies not only between different species of animals but often in different parts of the body .of the same animal. The fats of cold-blooded animals contain more olein than those of warm-blooded animals and therefore remain liquid at lower temperatures. The vegetable fats contain a greater variety of fatty acids than the animal fats, notably unsaturated acids like linoleic and eruic, as well as oxy-acids and esters of the higher alcohols (waxes), while the so-called crude fat, or ether extract (108) of vegetable materials contains a great variety of ether-soluble substances, including waxes, resins, chlorophyl, etc., some of which are but remotely related to the true fats. 34. Elementary composition. — The three principal triglyc- erids, stearin, palmitin and olein, while differing in formula and molecular weight, differ but little in their elementary com- position, as the following table shows : — TABLE i. — COMPOSITION OF TRIGLYCERIDS TRISTEARIN % TRIPALMITIN % TRIOLEIN % Carbon Hydrogen 76.77 1 2 4.^ 75.86 I 2 2A 77-31 ii 84. Oxygen 10.78 11.90 10.85 Total . ... . . TOO OO TOO OO IOO.OO Naturally, therefore, the composition of the ordinary mixed animal fats varies but little, either in different individuals or in different species of animals. The classic investigations of Schulze and Reinecke1 upon the composition of animal fats gave the following results. 1 Landw. Vers. Stat., 9 (1867), 97. THE COMPONENTS OF PLANTS AND ANIMALS 21 TABLE 2. — COMPOSITION OF ANIMAL FATS CARBON HYDROGEN OXYGEN No OF SAMPLES Aver- Maxi- Mini- Aver- Maxi- Mini- Aver- Maxi- Mini- age mum mum 0 age mum mum age mum mum Beef fat 10 76.50 76.74 76.27 11.91 12. II 11.76 "•59 11.86 ii. 15 Pork fat . . Mutton fat . 6 12 76.54 76.61 76.78 76.85 76.29 76.27 11-95 ,12.03 I2.O7 12. l6 11.86 11.87 11.52 11.36 ".83 11.56 ".15 11.00 Average 2~8 76.50 12.00 11.50 Dog. . . . 76.63 I2.O5 11.32 Cat .... 76.56 11.90 11.44 Horse . . . 76.07 11.69 11.24 Man . . . 77.62 11.94 11.44 Benedict and Osterberg1 obtained the following for the com- position of human fat : — TABLE 3. — COMPOSITION OF HUMAN FAT CARBON % HYDROGEN % Sample No. i 76 20 II 80 Sample No. 2 76 *6 Sample No. 3 7S 8l II 87 Sample No. 4 7c QC; II 85 Sample No. 5 7? QA Sample No. 6 ?6 O7 1 1 60 76 I? II 84 Sample No. 8 .... 76 o^ ii 81 Average 76 08 ii 78 The average carbon content of animal fat is commonly con- sidered to be 76.5 per cent. Waxes 35. In popular usage, the distinction between fats and waxes is based upon their obvious physical properties, substances having the well-known greasy feel being called fats or oils according to their consistency at ordinary temperatures while the waxes are solid, can be kneaded and lack largely or wholly the greasy feel. 1 Amer. Jour. Physiol., 4 (1901), 69. 22 NUTRITION OF FARM ANIMALS Chemically, waxes are defined as fatty acid esters of alcohols other than glycerol, while the fats have already been denned as the fatty esters of glycerol. This distinction is far from according with com- mon usage. Under it many substances popularly known as waxes are technically fats, as for example, Japan wax and in part beeswax. On the other hand, numerous materials ordinarily regarded as oils or fats must be designated as waxes. One of the most familiar bodies of this class is spermaceti, commonly regarded as a fat, which consists chiefly of the palmitic ester of cetyl alcohol, CH3(CH2)i4CH2OH, and sperm oil, which contains no glycerids, would also be regarded as a liquid wax. Similarly wool fat is chemically a mixture of waxes, in- cluding the stearic esters of cholesterin and isocholesterin. Beeswax is likewise in part a true wax, containing the palmitic ester of myricyl alcohol, CH3(CH2)28CH2OH. The secretion of the anal glands of certain birds contains esters of octodeckyl alcohol, CisHsyOH. Cholesterins 36. Substances of this group are found in the nonsaponifiable resi- due of various fats. In the animal organism they are found widely distributed through the tissues in small amounts and are appar- ently normal constituents of protoplasm. As just noted, they are especially abundant in wool fats in combination with stearic acid. They are also widely distributed in plants. Their exact constitution is still unknown, but they contain a single alcohol hydroxyl and ap- parently belong to the terpene group. Their formula is C27H440H, or C2?H460H, more probably the latter. From the chemical point of view, they are entirely unrelated to the other groups classified as lipoids, but biologically their functions appear to be closely related to those of the other ether-soluble cell constituents. . Phosphatids or Phospholipins 37. Lecithins. — Quite closely related to the fats are the substances known as lecithins, which are sometimes, although inexactly, called phosphorized fats. Like the fats, the leci- thins are esters of glycerol. They differ from the fats in that only two of the hydroxyls of the glycerol are replaced by fatty acid radicles, the third being replaced by phosphoric acid which is also in combination with the nitrogenous base cholin, a derivative of glycol. The lecithins, therefore, contain, in addition to carbon, hydrogen and oxygen, both phosphorus and nitrogen. THE COMPONENTS OF PLANTS AND ANIMALS 23 The molecular structure of the lecithins is illustrated by the fol- lowing formula for distearyl lecithin : — CH2-0-C18H36O I CH -O-Ci8H36O I CH2-0 HO-PO / CH2-0 I CH2-N = (CH3)3 OH The lecithins resemble fats in their general properties. They are soluble in ether but, unlike the fats, readily form permanent emulsions or colloidal solutions with water. On hydrolysis, they yield fatty acids, glycero-phosphoric acid and cholin. They are found widely distributed both in animals and plants and appear to be essential constituents of protoplasm. 38. Other phosphatids. — A variety of other lipoids of the type of the lecithins, but differing in both the fatty acid and the nitrogenous base which they contain and likewise in the ratio of phosphorus to nitrogen, have been described, but the chemistry of this group is still in a very unsatisfactory state. The various phosphatid preparations obtained from vegetable materials, especially seeds, by E. Schulze and his associates and designated as lecithins are held by other authors to be such only in a generic sense and in some cases are re- garded as more analogous to the cerebrosids or galactolipins of the succeeding paragraph. Cerebrosids or Galactolipins 39. This group of substances, found especially in the brain and in nerve tissue in general, belongs chemically to the lipoids, since its members yield fatty acids on hydrolysis. The other products of hydrolysis are galactose and nitrogenous substances but no phosphoric acid, but the constitution of these compounds is still unknown. 24 NUTRITION OF FARM ANIMALS § 4. THE PROTEINS 40. Importance. — By far the larger share of the organic matter of the animal body, aside from fat, consists of sub- stances belonging to the well-defined group of the proteins, these compounds, according to the results of analyses recorded on subsequent pages (98), making up from 17.5 to 21 per cent of the fat-free body. These substances are characteristic of the animal body, as the carbohydrates are of plants. Biologi- cally, they are of prime importance to both plants and animals, since they form the basis of the cytoplasm and nucleus of every living cell. 41. Nomenclature. — The chemical structure of the pro- tein molecule has until quite recently been almost entirely un- known and even yet has been but very partially unraveled. Accordingly, the basis for a scientific classification of these substances has been lacking. As a matter of necessity, there- fore, the nomenclature hitherto followed has been based chiefly on their physical properties, more particularly their solubilities and coagulation temperatures. Naturally, such a classification has been far from satisfactory and this has been the more true on account of the difficulty of accurately separating the differ- ent proteins either by precipitation or crystallization. Accordingly, there has existed a great and confusing diversity in the nomenclature of the proteins, and uniformity is still far from having been reached. For the present, it seems desirable to follow the classification and nomenclature which has been adopted provisionally by the American Physiological Society1 and the American Society of Biological Chemists.2 This nomenclature rejects entirely the term proteid as ambiguous on account of the wide diversity in its use, and employs protein as a general term to signify the group of substances which, according to the nomenclature adopted by the Association of American Agricultural Colleges and Experiment Stations in 1 8g8,3 was called proteids. In other words, protein under the new plan excludes altogether the non-protein nitrogenous substances of plants and animals. 1 Proceedings, Amer. Physiol. Soc., Amer. Jour. Physiol., 21 (1908), xxvii. 2 Proceedings, Amer. Soc. Biol. Chemists, 1, 142. 3 U. S. Dept. Agr., Office of Expt. Stas., Bui. 65, pp. 117-123. THE COMPONENTS OF PLANTS AND ANIMALS 25 The proteins in this sense are subdivided into : — 1. Simple proteins 2. Conjugated proteins 3. Derived proteins Simple proteins are denned as those yielding only a amino acids or their derivatives upon hydrolysis. Conju- gated proteins are those which contain the protein mole- cule united to some other molecule or molecules otherwise than as a salt. Derived proteins are the products of the hydrolytic cleavage of the protein molecule and include a wide range of substances, from slightly altered protein to the peptids. 42. Physical properties. — In the dry state, the proteins are in general white or slightly tinted substances. They are usually amorphous, but a number of them have also been obtained in the crystalline form and some are found crystallized in nature. Some of the proteins are soluble in water, others only in salt solutions or in acids or alkalies. They are insoluble in most other ordinary solvents. The proteins belong to the class of colloids, i.e., they do not diffuse through membranes and are claimed to have no osmotic pressure when free from electrolytes. Colloids in general exist in two forms, a liquid form, technically known as a sol, and a solid form called a gel, the difference being well illustrated by the familiar substance gelatin. When a colloid is distributed through water so as to form an apparent solution the latter is known as a hydrosol. Whether the proteins are to be regarded as soluble in water, or whether their apparent solution is in reality a suspension, has been much discussed. It has been shown, however, that these solutions are conductors of electricity and it has been concluded that they are true solutions. It may be said, however, that no sharp boundary exists between a true solution and a suspension but that an indefinite number of intermediate stages is possible. As a matter of convenience, however, we may speak of solutions of the proteins. Different proteins may be precipitated from their solutions by various reagents, particularly acids, alkalies and metallic salts. Ammonium sulphate, especially, has been largely used for the purpose of separating different proteins by means of fractional precipitation. 26 NUTRITION OF FARM ANIMALS 43. Coagulation. — An important property of the proteins is that of coagulation. For instance, if a solution of ordinary egg albumin be heated to 55° C. the albumin begins to separate in an insoluble form and at about 6'o° C. the precipitation is complete. This change differs from the change in the case of gelatin solutions from liquid to solid in being irreversible, i.e., coagulated protein cannot be changed back to the soluble form. It should be noted that this change is entirely distinct from the precipitation of proteins by means of ammonium sulphate for example. The exact nature of the change is unknown, but it would seem to be in part chemical in character. All forms of protein appear to be subject to coagulation in the chemical sense of the word. Thus the precipitated proteins obtained from solutions are at first in the colloidal form but on standing pass more or less rapidly into the coagulated or " de- natured " form. The same is true of the solid proteins like fibrin, etc. The coagulated proteins are insoluble in water and salt solutions, but may be dissolved in acids or alkalies. The simple proteins 44. Composition. — The simple proteins differ from the com- pounds considered in the previous sections in containing, in addition to carbon, hydrogen and oxygen, the elements nitro- gen and sulphur. Notwithstanding the considerable variation in the properties of the different simple proteins and the notable differences which have been shown to exist in their chemical structure, their elementary composition differs but little. Cohnheim 1 quotes the following figures from Michel for the composition of serum albumin, which is in many respects a typical animal protein. Carbon 53-o8 Hydrogen 7.10 Nitrogen 15.93 Sulphur 1.90 Oxygen 21.99 100.00 The variations in the percentages of the principal elements as stated by Cohnheim 1 and by Plimmer 2 are : — 1 Chemie der Eiweisskorper, 2d Ed., p. 151. * The Chemical Constitution of the Proteins, Part I, p. 2. THE COMPONENTS OF PLANTS AND ANIMALS 27 COHNHEIM PLIMMER Carbon r2-^q% ei-ec% Nitrogen Hydrogen 15-19% 15-17% n®7 Sulphur .... ... o 4—2.0% 1 O.A—2.=;% As a rule, the vegetable proteins contain a higher percentage of nitrogen than do the animal proteins. 45. Structure of the proteins. — The molecular structure of the proteins is very complex and their molecular weights are very large, but as yet no very satisfactory determinations of the latter magnitude have been made. Determinations of the molecular structure of haemoglobin (a conjugated protein) by two methods have given concordant results indicating a mini- mum molecular weight of 16,666, from which has been computed the formula C758H12o3Ni95FeS2. Confirmation of this result has been reported as the result of determinations of its osmotic pressure.2 For the globin of haemoglobin, a minimum molecu- lar weight of between 5000 and 8000 has been obtained. For serum albumin, the figure 10,166 is reported, for egg albumin, 5378, and for edestin 14,500. These figures are of value, how- ever, chiefly as showing the complex nature of the protein mole- cule. Up to within a comparatively few years, general statements like those just made marked the limits of our knowledge of the chemical nature of the proteins. The masterly researches of Emil Fischer, however, and especially his creation of new experimental methods, have resulted in a very great advance in knowledge, and to-day, thanks to his labors and those of a large number of investigators in applying and improving his methods, we possess a fairly definite general conception of the structure of the protein molecule. As in the investigation of chemical compounds in general, two lines of attack have been followed, viz., a study of the products resulting from the splitting up of the molecule and attempts to synthesize the compound from simpler substances of known composition and structure. 1 Four to five per cent in keratins. 2 Zentbl. Physiol., 21, 730. 28 NUTRITION OF FARM ANIMALS 46. Hydrolysis of proteins. — The simple proteins readily undergo hydrolysis when acted upon by strong acids or alkalies, or by various enzyms such as the pepsin of the gastric juice, the trypsin of the pancreatic juice, etc. These various agents effect a succession of cleavages and hydrations resulting in a series of products of decreasing molecular complexity and in- creasing solubility, ranging from very slightly modified proteins through the so-called proteoses and peptones to still simpler substances. 47. Cleavage products of proteins. — When the hydrolysis, especially acid hydrolysis, of the simple proteins is pushed as far as possible, there result a number of comparatively simple crystalline substances which are qualitatively the same for all proteins with a few exceptions, although the proportions of the various products obtained from different proteins vary ma- terially. It is believed, therefore, that the protein molecule is built up of these final products of hydrolysis, the so-called " building stones." These primary cleavage products of the simple proteins are all a amino acids. One of the first of them to be isolated was glycin or aminoacetic acid, represented by .the following for- mula : — CH3 CH2 • NH2 I I COOH COOH Acetic acid Glycin The other cleavage products of the simple proteins may be regarded as derived from glycin by the replacement of one atom of hydrogen in the CH2 group by various atomic group- ings. In all of them the NH2 group occupies the same position in the molecule relative to the group COOH as in glycin, the so-called a position. The atomic grouping CH - NH2 I CO -OH is therefore common to all of these bodies and determines their general chemical behavior as well as that of the proteins from which they are derived. THE COMPONENTS OF PLANTS AND ANIMALS 29 The amino acids derived from the proteins may be divided into two classes ; the monamino acids, of which glycin is typi- cal, containing one NH2 group, and the diamino acids, contain- ing two NH2 groups. To these there are to be added certain heterocyclic compounds. Plimmer 1 gives the following list of the amino acids which have been identified with certainty among the cleavage products of the proteins. The presence of others has been claimed by several investigators. A. Monoaminomonocarboxylic acids 1. Glycin, C2H5NO2, or aminoacetic acid. CH2 • (NH2) • COOH 2. Alanin, C3H7NO2, or a-aminopropionic acid. CH3 • CH(NH2) • COOH 3. Valin, C5HnNO2, or a-aminoisovalerianic acid. CH3\ ^CH • CH(NH2) • COOH CH3/ 4. Leucin, C6Hi3NO2 or a-aminoisocaproic acid. CH3\ CH • CH2 • CH(NH2) • COOH CH3/ 5. Isoleucin, C6Hi3NO2, or a-amino-/3-methyl-/3-ethyl-propionic acid. CH3\ CH • CH(NH2) - COOH C2H5/ 6. Phenylalanin, C9HnNO2, or /3-phenyl-a-aminopropionic acid. C6H5 • CH2 • CH(NH2) • COOH 7. Ty rosin, C9HnNO3, or 0-parahydroxyphenyl-a-aminopropionic acid. HO • C6H4 • CH2 • CH(NH2) • COOH 8. Serin, C3H7NO3, or 0-hydroxy-a-aminopropiomc acid. CH2(OH) • CH(NH2) • COOH 9. Cystin, C6Hi2N2O4S2, or dicysteine, or di- (/3-thio-a-aminopro- pionicacid) HOOC • CH(NH2) • CH2 • S — S • CH2 • CH(NH2) • COOH B. Monoaminodicarboxylic acids 10. Aspartic acid, C^rNO^ or a aminosuccinic acid. HOOC • CH2 • CH(NH2) • COOH 11. Glutamic acid, C5H9NO4, or a-aminoglutaric acid. HOOC • CH2 - CH2 • CH(NH2) • COOH 1The Chemical Constitution of the Proteins, Part I, ad Ed., 1912. 30 NUTRITION OF FARM ANIMALS C. Diaminomonocarboxylic acids 12. Arginin, C6Hi4N4O2, or a-amino-y-guanidin valerianic acid. HN=C/NH2 NH • CH2 • CH2 • CH2 • CH(NH2) • COOH 13. Lysin, CoHi4N2O2 or a, e-diaminocaproic acid. H2N • CH2 • CH2 • CH2 • CH2 • CH(NH2) • COOH D. Heterocyclic compounds 14. Histidin, CeHgNsO^ or /S-imidazol-a-aminopropionic acid. CH ^ \ N NH I I CH = C — CH2 • CH(NH2) • COOH. 15. Prolin, C5H9NO2, or -pyrrolidin carboxylic acid CH2 — CH2 I I CH2 CH • COOH \ / NH 1 6. Oxyprolin, or oxypyrrolidine carboxylic acid. C5H9N03 17. Tryptophan, CnHt2N2O2, or jS-indol-a-aminopropionic acid. C — CH2 • CH(NH2) • COOH /\ CCH4 CH \ / NH 48. Synthesis of proteins. — Peptids. — Fischer and others have shown that the amino acids which result from the cleavage of the simple proteins may combine with each other, the NH2 of one uniting with the COOH group of the other with the elimination of one molecule of water. As many as 18 molecules of amino acids have been combined in this way, although the exact structure of the resulting compounds is still more or less uncertain. The compounds of the amino acids which have been prepared artificially have received the general name of peptids, the pre- fixes di-, tri-, etc., being used to indicate the number of amino acid molecules entering into the compound. The term poly- THE COMPONENTS OF PLANTS AND ANIMALS 31 peptids is also commonly used as a general term for the more complex substances of this group. The latter show many of the reactions of the proteins or of their less modified deriva- tives. For example, many of them give the biuret reaction characteristic of the proteins, are precipitated by phospho- tungstic acid and undergo cleavage by appropriate proteolytic ferments. Moreover, some of the artificial polypeptids of known composition have also been isolated from the mixture of products resulting from the action of ferments upon the proteins. 49. Conclusions. — Since, therefore, the same comparatively simple crystalline products are obtained as the final result of the complete hydrolysis of all the simple proteins, viz., the various amino acids enumerated in a previous paragraph (47), and since, on the other hand, these cleavage products may be synthesized to form substances closely resembling the proteins, it is believed that the protein molecule is built up of these amino acids, united in substantially the same way as in the artificially prepared polypeptids. In other words, it is be- lieved that the latter are the first steps toward the synthesis of proteins, or indeed that they may, from a systematic point of view, be regarded as the simplest of the proteins. It should be noted, however, that while the foregoing method of combination of the amino acids appears to be characteristic of the protein molecule, it is not the only form of combination in which nitrogen enters into it. For example, arginin, apparently a constit- uent of all proteins, contains an atom of imid nitrogen, HN. The proteins also contain amid nitrogen (i.e., NH2 substituted for the OH of the carboxyl group) which yields ammonia on hydrolysis. Further- more, the proteins are capable of acting as polyacid bases and there- fore the molecule must contain numerous NH2 end-groups such as that of the amids just mentioned or those of the diamino-acids like lysin and arginin. 50. Proportions of cleavage products in different proteins. — While all the simple proteins yield, with a few exceptions, qualitatively the same cleavage products, the relative pro- portions of these " building stones " vary widely in proteins from different sources. This is strikingly illustrated by the following tabulation of the percentages of the various amino acids yielded by a number of proteins according to the researches of Osborne and his associates. 32 NUTRITION OF FARM ANIMALS TABLE 4. — CLEAVAGE PRODUCTS OF PROTEINS * 1 1 a 1 1 g p § M ^ o ^ M PQ ^ *S U II ll I 1-1 o ^ s - w S w P s M EJ O en • sS 3 5 o£ 35 %l 1 * < 0 00 fi ss Glycin 0.00 0.89 o.oo 0.38 0.0 o.o 0.00 2.06 0.68 Alanin 2.OO 4.65 1 3 30 2.08 2.22 2 ?O 1.50 3-72 2.28 Valin 3-34 0.24 1.88 2.50 o'oo 7.20 0.81 Leucin 6.62 C.QC 19. cc 8.00 IO.7I IQ 40 11.65 I I.IQ Phenylalanin .... 2-35 0 "VO 1.97 V 0 O 6.55 3-75 5-07 2.40 3.20 3-53 Tyrosin 1.50 4.25 3-55 1-55 1.77 2. 2O 4.50 2.20 2.16 Serin 0.13 0.74 i. 02 0^3 •J) -j> o ^o •^ p Cystin w- *O 0.45 O.O2 J_ ^> p J> Prolin 13.22 4-23 9.04 3.22 3-56 4.00 6.70 5-82 4-74 Aspartic acid .... 0.58 0.91 1.71 5-30 2.20 I.OO i-39 4.51 3-21 Glutamic acid . . . 43-66 23.42 26.17 13.80 9.10 10.10 15-55 15-49 16.48 Tryptophan .... I.OO + o.oo + + + 1.50 + + Arginin « 16 4.72 i.cc IO.I2 A OI „ o_ „ gy 7 47 6.50 Lysin J> *T- / I.Q2 00 o.oo 4.98 3.76 8.10 7.61 / •** / 7* S9 7 24, Histidin 1.49 -y 1.76 0.82 *T*y w 2.42 Of 1.71 i-53 / * 2.50 1.76 / ••£ir 2-47 Ammonia 5-22 4 OI - £.. I no i. 24 I 12 1.61 I O7 I 67 Total *!•• •$• 4 vv "« J..V-* / i ,VJ ^ 84.72 59-68 88.87 58.12 48.85 56.46 66.92 67.29 62.15 The results shown in the foregoing table are typical. In a few proteins, certain amino acids have not been found at all. For example, no glycin has been found among the products of the hydrolysis of gliadin, zein, albumin or casein and no lysin among those of gliadin or zein. Furthermore, the proportion of the various cleavage products is quite variable in the different proteins, one of the most striking instances being that of glu- tamic acid which ranges from nearly 44 per cent in the gliadin of wheat to a little over 9 per cent in egg albumin, and is no- tably more abundant in vegetable than in animal proteins. 51. Classification. — For the present purpose, it seems super- fluous to enter into a full description of the various simple pro- 1 The sign + signifies that the substance was present but was not quantitatively determined. A blank simply indicates that the ingredient in question was not determined but does not show that it was not present. THE COMPONENTS OF PLANTS AND ANIMALS 33 teins. The principal groups into which they are subdivided are designated as follows : — a. Albumins. — These are simple proteins soluble in pure water and coagulable by heat. Besides the familiar egg al- bumin, they include the albumins of blood serum and of milk serum. Albumins have also been found in small amounts in a great variety of seeds, including those of wheat, rye, barley, pea, vetch, soybean and cowpea. b. Globulins. — The globulins are simple proteins insoluble in pure water but soluble in neutral solutions of salts of strong bases with strong acids. Globulins are found in the lymph and the blood serum and in the muscles and other organs, but they appear to be especially characteristic of the vegetable kingdom, occurring in considerable amounts in a large number of seeds. Osborne 1 gives a list of 1 5 globulins occurring in 24 different species of seeds and enumerates 12 additional species which contain globulins to which no distinctive names have yet been given. c. Glutelins. — These are defined as simple proteins insoluble in all neutral solvents but readily soluble in very dilute acids and alkalies. The only well-defined members of this group at present known are the glutenin of wheat and the oryzenin of rice, although there seems reason to believe that similar pro- teins exist in the seeds of other cereals. d. Prolamins, or alcohol-soluble proteins. — The typical mem- ber of this group is the gliadin of wheat and the name has been applied by some authors to the entire group, but the term prolamins, proposed by Osborne, seems preferable. The prolamins are soluble in relatively strong alcohol (70-80 percent) but insoluble in water, absolute alcohol and other neutral sol- vents. They are characteristic of the seeds of the cereals, the principal prolamins being the gliadin of wheat and rye, the hordein of barley, the zein of maize and the bynin of malt. e. Albuminoids. — This name, formerly used to a consider- able extent as practically synonymous with proteins, is now applied to two groups of nitrogenous substances which have been otherwise designated as the collagens, or gelatinoids, and the keratins. Albuminoids are defined as simple proteins which possess essentially the same chemical structure as the other 1 The Vegetable Proteins, p. 78. 34 NUTRITION OF FARM ANIMALS proteins but are characterized by great insolubility in all neutral solvents. They form the principal organic constituents of the skeletal structures of animals and of their external cover- ing and its appendages and hence have also been called sclero- proteins. This definition does not provide for gelatin, which is, however, an artificial derivative of collagen. Besides gela- tin the more important members of this group are chondrin, or collagen, which constitutes the organic basis of cartilage and bone; elastin, the characteristic component of the ligaments; and the keratins of the epidermal tissues such as hair, wool, feathers, horns, hoofs, etc. The conjugated proteins 52. Nucleoproteins. — In the scheme of classification here followed (41), the nucleoproteins are defined as follows : " These proteins are especially characteristic of the nucleus of the vegetable and animal cell (74). They consist of protein mole- cules united with one or more of the compounds known as nucleic acids. These are complex organic compounds contain- ing a phosphoric acid radicle and also a xanthin group." The simple proteins of the nucleoproteins apparently may be of quite diverse nature and belong to various groups of the simple proteins. The special interest of the nucleoproteins attaches to the nucleic acids entering into their composition. 53. Nucleic acids. — These compounds contain in addition to carbon, hydrogen, nitrogen and oxygen the element phos- phorus. Their constitution has not yet been fully worked out, but their cleavage yields four classes of products, viz., 1. Xanthin, or purin, bases 2. Pyrimidin bases 3. A pentose carbohydrate 4. Phosphoric acid According to the recent investigations of Levene and others, the nucleic acid molecule may be regarded as built up from nucleosids, or glucosid-like combinations of a pentose carbohy- drate with a purin or pyrimidin base. By the union of such a nucleosid with phosphoric acid there is formed a nucleotid. Finally, the most common nucleic acids are tetranucleotids. THE COMPONENTS OF PLANTS AND ANIMALS 35 which seem always to contain both purin and pyrimidin nucleo- sids. 54. Glycoproteins. — The glycoproteins are denned as " Com- pounds of the protein molecule with a substance or substances containing a carbohydrate group other than a nucleic acid. The principal compounds of this group are the mucins and the mucoids." 55. Phosphoproteins. — These are denned as compounds of the protein molecule with some, as yet undefined, phosphorus- containing substance other than a nucleic acid or lecithin. The casein, or rather caseinogen, of milk is one of the most familiar and important of this group. 56. Haemoglobins. — The haemoglobins are compounds of the protein molecule with hsematin or some similar substance, and constitute the red coloring matter of the blood. 57. Lecithoproteins. — Compounds of the protein molecule with lecithins. The derived proteins 58. Primary protein derivatives. — Derivatives of protein ap- parently formed through hydrolytic changes which involve only slight alterations of the molecule. Proteans. — Insoluble products which apparently result from the incipient action of water, very dilute acids or enzyms. Metaproteins. — Products of the further action of acids and alkalies whereby the molecule is so far altered as to form products soluble in very weak acids and alkalies but insoluble in neutral fluids. This group will thus include the familiar "acid proteins" and " alkali pro- teins," not the salts of proteins with acids. Coagulated proteins. — Insoluble products which result from (i) the action of heat on their solutions, or (2) the action of alcohols on the protein. 59. Secondary protein derivatives. — Products of the further hydrolytic cleavage of the protein molecule. Proteases. — Soluble in water, uncoagulated by heat, and pre- cipitated by saturating their solutions with ammonium or zinc sulphate. Peptones. — Soluble in water, uncoagulated by heat but not pre- cipitated by saturating their solutions with ammonium sulphate. Peptids. — Definitely characterized combinations of two or more amino acids, the carboxyl group of one being united with the ammo group of the other with the elimination of a molecule of water (48). 36 NUTRITION OF FARM ANIMALS § 5. THE NON-PROTEINS 60. Occurrence. — In addition to the proteins, both plants and animals contain a great variety and sometimes relatively considerable amounts of nitrogenous compounds of the most diverse nature. While the occurrence of such compounds, es- pecially in feeding stuffs, was known from an early day, it was long assumed that the amounts present were relatively insignifi- cant and that no material error was involved in regarding all the nitrogen of a feeding stuff as existing in the form of protein. Accordingly, the total nitrogen multiplied by the conventional factor 6.25 and designated as " crude protein " was taken as representing the true protein content of the material. The researches of Scheibler, E. Schulze and Kellner in the seventies, however, showed that this was far from being the case. It was found that nitrogenous substances other than protein were very widely distributed and that sometimes as much as one-third or even one-half of the total nitrogen of feeding stuffs existed in these non-protein compounds. These results have been fully confirmed by subsequent investigations and it has therefore become necessary to distinguish between these substances and the true proteins. 61. Definition. General properties. — While these nitrog- enous compounds other than protein are of the most varied nature, they all differ from the proteins in having a much less complex molecular structure. Many are comparatively simple, crystalline substances, most of them readily soluble in water and diffusible, and they appear distinctly inferior in nutritive value to the proteins. It is a matter of practical convenience, therefore, to have a group name by which to distinguish them and for this purpose the term non-proteins has been proposed. It is, of course, a contraction for non-protein nitrogenous sub- stances and means simply substances which contain nitrogen but are not proteins. It therefore includes a great variety of compounds and may be considered as in a sense a cover for our ignorance of their exact nature. The more important groups of non-proteins are : — The nitrogenous muscle extractives The nitrogenous lipoids THE COMPONENTS OF PLANTS AND ANIMALS 37 The nitrogenous glucosids Alkaloids and organic bases Amino acids and amids Nitrates and ammonium salts 62. The muscle extractives. — The more important nitrog- enous muscle extractives are creatin, creatinin and the purin bases xanthin and hypoxanthin. 63. Nitrogenous lipoids. — As noted (37-39), the lipoid group includes a number of compounds, classed as phosphatids and cerebrosids, which contain a nitrogenous group in combi- nation with fatty acid radicles. The most familiar members of this group are the lecithins. The actual amounts of these sub- stances contained either in the animal or plant are small and their nitrogen does not constitute any important fraction of the total nitrogen of the body or of the feed. 64. Alkaloids and organic bases. — Alkaloids are compara- tively rare in agricultural plants, the seeds of the lupine forming the principal exception. The organic bases, on the other hand, appear to be somewhat widely distributed. In addition to the so-called " hexon bases " arginin, lysin and histidin, de- rived from the proteins and nucleo-proteins, the bases cholin, betain, trigonellin and stachydrin have been found in a variety of plants. 65. Nitrogenous glucosids. — The substances of this group are characteristic of the vegetable kingdom. They contain a variety of nitrogenous compounds coupled with simple sugars. The nitrogenous glucosids do not appear to be especially abun- dant in the ordinary feeding stuffs of domestic animals and where they do occur are distinguished rather by their specific physiological effects than by their nutritive value in the or- dinary sense. E. Schulze l mentions seven bodies of this class which have been found in various plants. 66. Amino acids and amids. — These substances are by far the most abundant forms of non-protein in vegetable materials. The first one to be discovered was asparagin, in 1805, in aspar- agus shoots, and this substance has since been found in a large number of plants or parts of plants. Glutamin, a second amid, is also of frequent occurrence in plants. 1 Jour. Landw., 52 (1904), 305. 38 NUTRITION OF FARM ANIMALS Asparagin and glutamin are respectively the amids of aspartic and glutamic acids, both of which are constituents of the protein molecule. COOH COOH COOH COOH I I I I CH2 CH2 CH2 CH2 CH • NH2 CH • NH2 CH, CH2 1 1 1 1 COOH CO • NH2 CH • NH2 CH- Aspartic acid Asparagin | | COOH CO- Glutamic acid Glut It has thus come about that the term amids has been more or less commonly used as a general designation for the non-pro- tein nitrogenous substances found in feeding stuffs. The usage, however, is unfortunate. The word amid denotes a distinct class of chemical substances of which only asparagin and glu- tamin appear to be especially common in plants, while the latter contain a variety of nitrogenous substances which are not amids at all. The general term non-protein proposed above, therefore, seems preferable. In addition to asparagin and glutamin there have been found in feeding stuffs a large number of the cleavage products of the proteins. E. Schulze 1>2)3 enumerates ten amino acids, viz., valin, leucin, isoleucin, phenylalanin, tyrosin, prolin, tryptophan, arginin, lysin and histidin, besides the purin bases xanthin, hy- poxanthin, adenin and guanin, as well as guanidin, allantoin and carnin, as having been isolated from various vegetable ma- terials. Hart and Bentley 4 found that from 50 to 70 per cent of the water-soluble nitrogen of a variety of feeding stuffs ex- isted as amino acids or peptids, while the amid nitrogen proper amounted to only 10 to 20 per cent. Occurrence. — These substances evidently stand in a close relation to the protein metabolism of the plant. They appear to be in part intermediate products in the synthesis of protein from nitrates and ammonium salts and in part to be formed in the cleavage of proteins necessary for their translocation and resynthesis during the processes of growth. They are especially 1 Jour. f. Landw., 52 (1904), 305. 2 Ztschr. Physiol. Chem., 45 (1905), 38. 3 Ztschr. Physiol. Chem., 47 (1906), 507. 4 Jour. Biol. Chem., 22 (1915), 477- THE COMPONENTS OF PLANTS AND ANIMALS 39 abundant, therefore, where growth is going on most rapidly. Young and succulent feeding stuffs, such as pasture grass, green soiling crops and the like, accordingly contain a considerable proportion of their nitrogen in the non-protein form. As plants approach ripeness, the proportion of non-protein to protein nitrogen becomes less, so that mature hay, straw and the like are relatively poor in non-proteins. This is especially true of seeds, whose nitrogen is contained chiefly in the reserve pro- teins, although small amounts of various non-proteins are also found. One class of feeding stuffs relatively rich in non-protein is the roots and tubers, in which the conversion of inorganic nitrogen into protein seems to be incomplete and in which the non-protein serves as a nitrogenous reserve for the growth of the succeeding year. Finally, feeding stuffs which have under- gone fermentation, such as silage, show a relative increase of the non-protein nitrogen over that of the original material. 67. Nitrates and ammonium salts. — Occasionally somewhat considerable amounts of nitrates or of ammonium compounds are found in vegetable material, especially when the supply of soluble nitrogen compounds in the soil is abundant. In such cases they are to be regarded as materials taken up in excess of the immediate needs of the plant. § 6. SUNDRY INGREDIENTS 68. Number and significance. — In the foregoing sections the groups of substances which constitute the animal body or, in the form of feed, supply the matter and energy for its growth and maintenance have been considered. It is hardly necessary to say, however, that these four groups, the carbohydrates, fats, proteins and non-proteins, are very far from comprising all the constituents of animals or plants. In the animal body the physiological chemist has recognized relatively small amounts of a vast number of substances of the most varied nature. Some of these are derived quite di- rectly from the proteins, fats or carbohydrates and these will be considered to a greater or less extent in studying the changes which these substances undergo in the body. Others, while of great physiological importance, have little direct relation to the processes of nutrition. 40 NUTRITION OF FARM ANIMALS Similarly, plants contain a great variety of ingredients not strictly belonging to any of the four main groups. In the aggregate, these substances do not often add greatly to the po- tential food value of feeding stuffs, but, on the other hand, they may in some cases considerably modify their palatability or the activity of the various processes of nutrition and so affect the actual results of feeding. Until recently these secondary in- gredients of feeding stuffs have received comparatively little attention. 69. Organic acids. — Aside from the small amounts of free fatty acids occurring in most native fats, both animal and vege- table (29, 33), the acids of this series are seldom or never found in native feeding stuffs. In those feeding stuffs which have undergone bacterial fermentation, however, notably in the case of silage, more or less acetic and butyric acids occur, but the principal acid product of such fermentations is lactic acid, C3H6O3. The same acids, along with formic and propionic acids and minute amounts of ethyl aldehyde, likewise result from the bacterial fermentation of the carbohydrates of the feed in the paunch of ruminants and thus constitute a not un- important portion of the non-nitrogenous material resorbed from the feed (128-132). The principal organic acids found in native feeding stuffs are malic, tartaric, citric and oxalic, usually as the potassium, sodium or calcium salts. 70. Ethereal oils. — The so-called ethereal oils are substances of complex molecular nature, somewhat resembling the true oils in their physical properties but which can readily be dis- tilled in a current of steam. Familiar examples are the so-called oils of peppermint, lemon, anise, and the like. It is not known that they have any direct nutritive value themselves but they add to the flavor and aroma of feeds and in some cases are be- lieved to stimulate the digestive processes. The agreeable odor of good hay, for example, and doubtless in part its fa- vorable dietetic effect, is due to substances resembling in prop- erties the ethereal oils. To the same class of ethereal oils belong the oils of mustard, onion and garlic, whose deleterious effect upon the flavor of dairy products is so well known. 71. Flavoring substances in general. — What is called the flavor of a food or feeding stuff depends largely upon the action on the sense of smell of a great variety of substances either con- THE COMPONENTS OF PLANTS AND ANIMALS 41 tained in the material originally or, especially in the case of human foods, artificially added or developed during cooking. Besides ethereal oils, stock -feeds contain a great variety of bitter or astringent substances, gums, waxes, resins, etc., etc., of whose properties and physiological effects little or nothing is known. The flavor and palatability of feed, as already indi- cated, are usually dependent upon these accessory ingredients, while the fact that palatability is an important factor in nu- trition aside from any direct nutritive effect will appear in later discussions. 72. Vitamins: Growth substances. — Much attention has been devoted during the past few years to an important but as yet rather ill-defined group of food constituents called by some investigators vitamins and by others growth substances. These substances, however, are known by their effects rather than by their chemical properties and may therefore be more appropri- ately considered in their relations to the requirements for maintenance and growth (438, 498, 738). CHAPTER II THE COMPOSITION OF ANIMALS AND OF FEEDING STUFFS § i. THE CELL 73. Definition. — The cell may be defined as the biological unit of all life. It is the simplest form in which living matter can exist. It might be regarded as bearing somewhat the same relation to the animal or plant that the atom does to a complex organic molecule such as that of one of the proteins for example. It is seen in its simplest form in unicellular organisms (protozoa) in which all the functions of life are performed by a single cell. As we ascend in the scale of organization a number of cells are united to form one individual, the various vital functions being to a greater or less extent distributed among different cells or cell groups. In the higher organisms the cells are numbered by myriads, while the physiological division of labor and the corresponding differentiation of form reach an extreme. The organization of such an individual has been likened to that of a state or nation, in which the functions of the single citizen are highly specialized. A few of the diverse forms of animal cells are represented in Fig. i. 74. Structure of cells. — The typical cell consists of the cell body, or cytoplasm, within which is the nucleus. The peripheral portion of the cytoplasm is often somewhat more compact than the remainder and serves to separate the cell from its surroundings. Sometimes a distinct membrane, or cell wall, is developed, especially in plants, although this is not a necessary part of the cell. The name protoplasm is often applied to the entire active part of the cell, i.e., to cytoplasm plus nucleus. All forms of life, vegetable as well as animal, are in- dissolubly associated with and manifested through the activities of protoplasm, which was called by Huxley the physical basis of life. It should be understood, however, that the word pro- toplasm is not a chemical but a biological term. It is a struc- 42 COMPOSITION OF ANIMALS AND OF FEEDING STUFFS 43 ture rather than a substance. Moreover, there is not one pro- toplasm, common to all cells, but as many protoplasms as there are kinds of cells. There is a more or less sharp differentiation between the functions of the nucleus and those of the cytoplasm. The nucleus appears to be especially concerned in cell reproduction, FIG. i. — Different types of cell composing the body. (Hadley, The Horse in Health and Disease.) the formation of a new cell beginning with a division of the nucleus of an existing cell and being followed by a division of its cytoplasm. The main function of the cytoplasm, on the other hand, seems to be the nutrition of the cell, and the presence of at least a minimum amount of it is essential to the continued existence of the nucleus. For the present purpose, it is unnecessary to attempt a further discussion of those finer details of the structure of the cell which have been worked out by the labors of the histologist and physiologist. 44 NUTRITION OF FARM ANIMALS 75. Composition of protoplasm. — The chemical constitution of living protoplasm is unknown, partly because it is undoubtedly very complex but chiefly because of its instability and the im- possibility of isolating it without at the same time destroying its life. Moreover, it doubtless varies materially in cells of different types. The proteins, perhaps combined with each other into " giant molecules," undoubtedly constitute the basis and predominating ingredient of protoplasm, but certain lipoids (lecithins and cholesterins), ash ingredients (electrolytes), and perhaps glycogen and other carbohydrates, in addition, of course, to water, appear to be also essential constituents. In the cytoplasm, the simple proteins (41) seem to predominate, while the nucleus is especially characterized by the presence of the nucleoproteins (52). 76. The cell wall. — As already indicated, the protoplasm often develops a cell wall. So far as concerns the species of plants which serve as feed for farm animals, it may be said that a vegetable cell is always surrounded by a cell wall the basic ingredient of which is the carbohydrate cellulose, a sub- stance not found in the bodies of the higher animals. In the young and growing parts of plants, the cell wall is thin and consists substantially of cellulose only. In certain parts of plants, such as the cotyledons and endosperms of seeds or the tissues of succulent roots and tubers, the cell wall remains comparatively thin even in mature tissue. In other parts of the plant, on the contrary, it becomes very much thickened by the deposition of additional cellulose and especially of substances other than cellulose. These other substances, which appear to be essentially carbohydrates or their derivatives, are of two general kinds. The first of these is the hemicelluloses (18), which are more readily attacked by hydrolyzing agents than pure cellulose and which constitute to a large extent a deposit of reserve material and include both hexosans and pentosans. The second consists of substances belonging to the lignin and cutin groups (19, 20), which serve to impart strength and rigidity along with more or less impermeability to the cell wall. They are, therefore, particularly abundant in older plants as com- pared with younger ones and in those organs which serve to support the plant, such as the stem. The extreme form of the thickened cell wall is seen in wood. A few of the numerous COMPOSITION OF ANIMALS AND OF FEEDING STUFFS 45 forms of vegetable cells are illustrated in Figs. 43-45 of Chapter XV. 77. Cell enclosures. — In addition to the essential constitu- ents of the cytoplasm and nucleus there are observed in cells a variety of other substances designated as subsidiary ingredi- ents or cell enclosures. These may consist of food material which has entered the cell and is on its way to being incor- porated into the molecules of protoplasm, or, on the other hand, of waste products of cell activity on their way to being ex- creted from the cell into the surrounding medium. Moreover, many cells have the power of storing up surplus food, especially non-nitrogenous substances, as reserve material. Such material is not usually regarded as constituting a part of the protoplasm but as being simply included in it mechanically. The most common cell enclosure in the animal is fat, which is contained in large quantity in certain connective tissue cells and constitutes the reserve fuel material of the animal body, the storage of carbohydrates (glycogen) being much more limited in amount. While some important groups of plants also store up large amounts of fat in their seeds, nevertheless the predominating reserve materials in the vegetable kingdom are carbohydrates, including the reserve carbohydrates of the cell wall and, as a cell enclosure, starch. Starch is found in all parts of plants, but is especially abundant in seeds and in the starchy roots and tubers, where large amounts of this sub- stance are stored up. Illustrations of plant cells containing starch are afforded by Figs. 43-45 of Chapter XV. Both because of the chemical composition of the cell wall and the nature of the cell enclosures, carbohydrates are quan- titatively the predominating ingredients of most plants, while animal cells and tissues are chiefly proteid or fatty in character. § 2. ANIMAL TISSUES AND ORGANS 78. Classification. — Not only do the cells of higher animals show extreme differentiation of form and function, but cells having the same general nature and office are associated together to form what are called tissues, such as nerve tissue, muscular tissue, connective tissue and the like, each serving its own specific purpose. These tissues, again, are grouped together 46 NUTRITION OF FARM AMIMALS to form organs, such as the muscles, heart, lungs, stomach, liver and the like, each performing its special part in the com- plex interplay of activities necessary for the life of the organism as a whole. Since this is not a treatise on anatomy, it is unnecessary to consider in detail all the diverse types of tissue or all the various organs making up the body. It is desirable, however, that the student of nutrition should acquire some notion of the chemical make-up of the various parts of the body. For this purpose it will be convenient to use the following scheme, based chiefly on the functions performed by the different groups of tissues, which ignores to some extent the distinction between tissues and organs and which does not pretend to be an exact or ex- haustive classification. First : The supporting tissues, including bone, tendon, carti- lage, ligament, elastic tissue, etc. Second : The tissues of motion, including the muscular tissues and the nerve tissues or the nervous system. Third : The tissues of alimentation, including the tissues and organs concerned in digestion, resorption, circulation, respiration and excretion. Fourth : The epidermal' tissues. Fifth : The reserve tissues, including, besides adipose tissue, those tissues in which glycogen is more or less abun- dantly stored. The supporting tissues 79. Intercellular substance. — In the bodies of the higher animals certain tissues show an enormous development of the so-called intercellular substance, so that the cells, instead of closely adjoining each other, are imbedded in a mass of non- cellular material which may vary greatly in consistency. Some- times this intercellular substance is entirely homogeneous but it usually contains a greater or less number of fibers imbedded in a homogeneous basis. By virtue of the special properties of the intercellular substance, tissues of this sort perform pri- marily mechanical functions, maintaining the form of the body or serving to connect and support other tissues, while the cells themselves serve principally to produce and maintain the inter- cellular substance. The organic basis of the latter is the group COMPOSITION OF ANIMALS AND OF FEEDING STUFFS 47 of proteins called in Chapter I the albuminoids or the sclero- proteins (51 e) accompanied by varying amounts of mineral matter, and, of course, a considerable proportion of water. 80. Bone. — Bone is the most familiar example of the sup- porting tissues of the animal body. In the young embryo the bones first appear as cartilaginous structures consisting of rounded cells imbedded in a homogeneous intercellular substance containing also fibers and consisting mainly of collagen (51 e). As development advances, the process of ossification begins, the homogeneous substance of the cartilage taking up inorganic salts, chiefly calcium phosphate, while the fibers of the cartilage are stated not to take part in this process. In addition to mineral matter, the bones store up also a variable amount of fat. Ma- ture bone, therefore, aside from its fat, consists of a basis of organic matter largely impregnated with mineral matter. The presence of these two classes of constituents is readily demon- strated by the familiar experiments in which, on the one hand, the mineral matter is removed by immersion in dilute acid leaving behind the flexible cartilage, or, on the other hand, the organic basis of the bone is destroyed by heating, leaving the so-called bone ash. Ossification has not been completed at birth but continues to a greater or less extent up to full maturity Moreover, it is not carried to the same extent in all bones nor in different parts of the same bone. Consequently, both the percentages of ash and of fat and the propor- tion of water to dry matter in bones may vary within wide limits, so that it is impossible to state an average composition. The extremes of 1 5 per cent and 44 per cent have been found for the average water content of the entire skeleton of the dog and even wider variations have been reported in the case of man. Compact bones contain less water than more spongy ones. In general it may be said that from one-half to two-thirds of the dry, fat-free bone consists of ash. About three-fourths of the remainder is stated to consist essentially of albuminoids, or collagens, yielding gelatin when treated with hot water, es- pecially under pressure. It is evident, therefore, that the skeleton of an animal contains not only a large share of the total ash of the body but a not inconsiderable portion of its nitrogenous constituents as well. On the average of the ten animals analyzed by Lawes and Gilbert (97), 77.78 per cent of 48 NUTRITION OF FARM ANIMALS the total ash of the entire animal and 83.01 per cent of the ash of the carcass was contained in the bones. Of the total nitro- gen of the carcasses of eight of these animals 18.04 Per cent was contained in the bones. Corresponding data for the entire animal are not recorded. 81. Bone ash. — But while the composition of bone itself is quite variable, that of the bone ash is notably constant even in different species. The predominant ingredient is tri-calcic phosphate but it contains, also, calcium carbonate as well as phosphates and carbonates of magnesium and other bases. The average composition given by Zalesky 1 is as follows : — TABLE 5. — COMPOSITION OF BONE ASH OF DIFFERENT SPECIES MAN CATTLE GUINEA PIG TURTLE Calcium phosphate .... 83-89 86.09 87.32 85.98 Magnesium phosphate . . . 1.04 1. 02 1.05 1.36 Calcium combined with CO2, Cl, Fl 7 6< 7 36 7 O3 6 32 Carbon dioxid . 573 6 20 507 More detailed analyses by Gabriel2 yielded the following results : — TABLE 6. — COMPOSITION OF BONE ASH TEETH OF CATTLE BONES OF MAN BONES OF CATTLE BONES OF GEESE CaO MgO . % 50.76 I C2 % 5L31 O 77 % 51.28 I OZ % 51.01 I 27 K2O Na2O O.2O I l6 0.32 I OA 0.18 I OQ O.I9 I II H2O . 221 2 4.6 2 33 3.O1? P2O5 3888 36 6s 37.46 38.19 CO2 A OQ 5 86 e 06 4. II Cl 0.05 O.OI O.O4 0.06 98.87 98.43 98.49 98.99 1 Neumeister; Lehrbuch der Physiologischen Chemie, 1897, p. 456. 2Ztschr. Physiol. Chem., 18 (1894), 257. COMPOSITION OF ANIMALS AND OF FEEDING STUFFS 49 The small amounts of magnesium, sodium, potassium, carbon dioxid and chlorin appear to be as essential ingredients of bone ash as its calcium or phosphorus. 82. Cartilage, ligament, tendon, elastic tissue. — Not all of the cartilaginous ground work of the skeleton as laid down in the embryo is converted into bone. In particular, the end surfaces of bones at a joint consist of cartilage, which in other cases forms a connecting link between adjoining bones, as, for example, the cartilage connecting the ribs with the breast bone, thus allowing a limited degree of motion. At the joints proper, the bones are held in place, and the direction and extent of their motions limited, by the ligaments, while the muscles which serve to impart motion to the various parts of the body are attached to the bones by means of tendons. In many cases the intercellular substance of the supporting tissue contains fibers of elastin. When these fibers are abundant the tissue is elastic in contrast to the ligaments and tendons of the joints, which are almost inextensible. A striking instance is afforded by the elastic band (Ligamentum mtchce) which runs along the back of the neck of quadrupeds and supports the weight of the head. Another example is furnished by the layer of elastic tissue contained in the walls of the arteries which gives them a certain degree of resilience to the pressure of the blood pumped by the heart. The " organic " portion of all these forms of supporting tis- sue, like the organic portion of the bones, consists essentially of different proteins belonging to the group of albuminoids. 83. Connective tissue. — This name is sometimes applied to all the various forms of supporting tissue, since they also serve to connect the various organs of the body. In a more ordinary and limited sense, however, it is used to designate a form of supporting tissue of which the most familiar example is the tissue lying between the skin and the underlying muscles, or lean meat, and serving to connect them together. A more careful examination shows that this subcutaneous connective tissue is continuous with other similar tissue which extends between the single muscles and serves at the same time to de- limit them and connect them. Not only so, but this sheath of connective tissue extends into the muscle itself, dividing it into muscular bundles or fasciculi and these again into secondary 50 NUTRITION OF FARM ANIMALS fasciculi. The connective tissue of the interior of the muscle unites at the ends and is continuous with a form of connective tissue already mentioned, viz., the tendons, by means of which the muscles are attached to the bones (Fig. 2). A similar sheath of connective tissue surrounds the internal organs of the body and extends into them, forming a framework which supports the active tissues of these organs as well as the blood vessels, nerves, lymphatics, etc., so that it may be said in a broad general way that the body of a higher animal consists of a variety of active tissues and organs contained in and supported by connec- One end of a tive tissue and the other forms of supporting tissue d already described. s e d g w i c k, Like other forms of supporting tissue, the connec- The Human tive tissue consists of cells which have produced a relatively large amount of intercellular substance, which in connective tissue consists chiefly of fibers. Chem- ically, it is composed of collagen. Cells of connective tissue, however, may also store up within themselves large amounts of fat (94). Tissues of motion 84. The muscles. — Both the external movements of an animal and those of the internal organs are effected by means of the muscles, and the muscular tissue is preeminently the tissue of motion. Moreover, the muscles make up a large part of the entire mass of the body of a lean animal and furnish nearly all the protein contained in the edible portion of the carcass. The composition of muscle and muscular tissue, there- fore, is of special interest. 85. Structure of muscles. — The smallest anatomical element of muscular tissue is the single muscle fiber. This is a highly specialized and greatly elongated, thread-like cell one to one and a half inches long and having a diameter of from .0004 to .004 inch. It is en- closed in a very thin transparent membrane and contains many nuclei. A large number of these fibers — hundreds or even thousands — are bound together to form a fasciculus, the fibers running length- wise and overlapping each other, being generally shorter than the COMPOSITION OF ANIMALS AND OF FEEDING STUFFS 51 fasciculus. These fasciculi, as stated in a previous paragraph (83), are surrounded by connective tissue and united into larger fasciculi, or bundles, each with its envelope of connective tissue, these bundles again being united to form the individual muscles. The connective tissue serves also to carry the blood vessels, nerves and lymphatics with which the muscle is abundantly supplied, and, moreover, may contain larger or smaller accumulations of fat. Evidently, then, the muscle as a whole, and even more the collective muscles making up the lean meat of an animal, are far from consti- tuting a homogeneous material. 86. Composition of muscles. — If the term muscular tissue be limited to the ultimate muscular fibers which are the ac- tive agents in producing motion, consider- ing the other structural elements of the muscle as accessory, it may probably be said in a general way that it consists essen- tially of water, protein, meat extractives and the various lipoids and electrolytes found in greater or less amounts in all protoplasm. But such a limitation of the term muscular tissue, however rational from an anatomical standpoint, is little suited to the present purpose. In the nutrition muscle fiber. of the animal, material is required to build up the entire muscular system, with all its accessory structures, and not merely for the production of the muscle fibers, and we are concerned, therefore, with the composition of the muscles as a whole — i.e., of the lean meat — rather than with that of the ultimate muscle-fibers. Since, however, the lean meat contains a variety of tissues aside from muscular tissue in the narrower sense — connective tissue, nerves, blood and lymph vessels, etc. (85), with more or less of the fluid contents of the latter — it is evident that its composition is likely to be variable. Moreover, the lean meat, especially of fat animals, contains a considerable and variable amount of fat even after all the fat tissue which it is practicable to separate mechanically has been removed. This fat, how- ever, forms no part of the muscle proper but is simply a deposit FIG. 3. — Part of a (Hough NUTRITION OF FARM ANIMALS of reserve material. It is contained in minute masses of adipose tissue (94) developed between the muscle bundles or even between the individual muscular fibers and differing only in size from the larger masses which may be trimmed off or re- moved with the scalpel. It is necessary to distinguish, there- fore, between lean meat in the commercial sense, with its vary- ing content of fat, and lean meat in the stricter scientific sense, i.e., the fat-free muscle. The com- position of the latter may be ascer- tained either by actually removing the fat from the ordinary trimmed meat by means of a solvent and analyzing the residue or, more con- veniently, by analyzing the fresh meat and removing the fat arithmet- ically, i.e., by calculating the com- position of the fat-free muscle. 87. Composition of fat-free muscle. -The composition of the fat-free lean meat of butchers' cuts has been determined by Henneberg, Kern and Wattenberg1 for two old sheep and FIG. 4. — Fat cells in muscle, six younger ones ranging from 6| to (Bailey's Cyclopedia of Ameri- 2g months old, and Jordan2 has de- can Agriculture.) , ,, ... ,. , , , termmed the composition of the lean meat of the entire carcasses of four steers. TABLE 7. — AVERAGE COMPOSITION OF FAT-FREE LEAN MEAT OF SHEEP OLD SHEEP LAMBS AVERAGE OF ALL No. 8 Lean No. 8 Very fat 6| mos. old 13 mos. old 22 mos. old 28 mos. old Fat 13 mos. old Fat 18 mos. old Fat Water Insoluble protein Soluble protein . . Meat extractives . . Ash 79.41 15-85 1.29 2.18 1.27 79.02 15-73 1-93 2.17 I-I5 81.01 14.89 1.56 1.44 1. 10 80.35 15-12 1.72 1.74 1.07 79-35 15-74 1-63 2.14 1.14 78.60 15.90 1.90 2.40 i. 20 80.21 14.86 2.16 1.66 i. ii 79.17 15-65 2.16 1.84 1.18 79.64 I5-47-) 1.79 U9-2I 1-95 J i. IS 100.00 100.00 100.00 IOO.OO 100.00 100.00 100.00 100.00 IOO.OO 1 Jour. Landw., 26 (1878), 549; 28 (1881), 289. 2 Maine Expt. Sta. Rpt. 1895, II, 36. COMPOSITION OF ANIMALS AND OF FEEDING STUFFS 53 TABLE 8. — AVERAGE COMPOSITION OF FAT-FREE LEAN MEAT OF STEERS 22 Mos. OLD 32 Mos. OLD AVERAGE OF ALL No. i No. 4 No. 2 No. 3 Water 77.61 21.37 1. 02 76.60 22.30 I.IO 78.01 20.94 1.05 77.18 21.77 1.05 77-35 21.60 1.05 Total nitrogenous matter (by difference) Ash IOO.OO 100.00 100.00 IOO.OO IOO.OO The figures of the foregoing tables indicate but very slight differences in the composition of the fat-free lean meat of the different animals, aside from a slightly greater water content in that of the sheep. An approximate average is 95 per cent total nitrogenous matter and 5 per cent ash in the dry, fat-free sub- stance. In the course of investigations upon human nutrition, nu- merous analyses have been made of the various commercial cuts of meat which in general confirm the foregoing figures and show relatively small differences in this respect between the different cuts. 88. Elementary composition of fat-free meat. — The fol- lowing analyses by Rubner, Stohmann and Langbein, and Ar- gutinsky show the ultimate composition of ash-free muscular tissue after prolonged extraction with ether : — TABLE 9. — COMPOSITION OF FAT- AND ASH-FREE MUSCULAR TISSUE CARBON HYDRO- GEN NITRO- GEN OXYGEN HEAT OF COMBUSTION &f (y PER GRAM. % % CALS. Rubner e -i AQ 16 30 6 61 Stohmann and Langbein . 52.02 7-30 16.36 24.32 5.6409 Argutinsky r 2 it 7 "?O 16 15 24 22 Kohler J has investigated the elementary composition of the muscular tissue of cattle, sheep, swine, horses, rabbits and hens. The material was prepared with much care, the fat being 1 Ztschr. Physiol. Chem., 31 (1901), 479. 54 NUTRITION OF FARM ANIMALS removed as fully as possible by prolonged extraction with ether. The residual fat which cannot be removed in this way was determined by Dornmeyer's digestion method and a cor- responding correction made in the analytical results. The fol- lowing are his averages for the fat- and ash-free substance: — TABLE 10. — COMPOSITION or FAT- AND ASH-FREE LEAN MEAT HEAT OF No. OF SAM- CARBON HYDRO- GEN NITRO- GEN SULPHUR OXYGEN COMBUS- TION PER PLES % °7 cy % % GRAM. CALS. Cattle .... 4 7 14. 16 67 0^2 22 12 c 6776 Sheep 52.53 7.19 16.64 0.69 22.96 5-6387 Swine 2 r 2 71 717 1 6 60 o ^o 22 95 r 67^8 Horse 3 52.64 7.10 15-55 0.64 24.08 5-5990 Rabbit .... 2 52.83 7.10 16.90 — — 5.6l66 Hen . . 2 <2 36 6 QO 16 88 o 50 23 28 5 6173 All the samples were tested for glycogen, but only traces were found, except in the horseflesh, for the two samples of which an average of 3.65 per cent was obtained, a result which accounts for the low figure for nitrogen. The tissues of alimentation 89. Definition. — Under this heading may be grouped the organs and tissues directly concerned with supplying food to the organism, with its distribution through the body, and with the removal of waste products of cell activity. That is, it in- cludes the organs of digestion, resorption, circulation, respira- tion and excretion, which constitute what are ordinarily spoken of as the entrails of slaughtered animals. So far as most of the familiar internal organs of the animal are concerned they may be considered as made up to a large extent of the classes of tis- sues already considered. In addition, however, the internal organs include a somewhat distinct type of tissue, viz., glandu- lar tissue, which plays an especially important part in the di- gestive processes, while it is also of the highest significance for other bodily functions. COMPOSITION OF ANIMALS AND OF FEEDING STUFFS 55 Glands, like many other organs, have as their basis a rather loose and soft framework of connective tissue serving to support cells whose function it is to prepare certain fluids or chemical substances required in the body. The largest gland is the liver, which secretes the bile and has other important functions. Other examples are the pancreas, spleen, salivary glands, etc. Less conspicuous but equally important are the smaller glands imbedded in the walls of the stomach and intestines which secrete such important fluids as the gastric juice, intestinal juices, etc. 90. Chemical composition. — From the standpoint of hu- man nutrition, the tissues of alimentation of farm animals, as here broadly denned, are largely waste products. While cer- tain organs, like the liver, kidneys, heart, etc., are utilized as food, the larger portion of the entrails passes into the offal and the feed consumed in its growth and maintenance is a part of the necessary cost of production of animal foods. An idea of the composition of the offal and of the proportion of total protein, fat and ash of the body which it contains is afforded by Lawes and Gilbert's analyses of entire animals (97), although the offal in their experiments included, in the case of cattle and sheep (but not of pigs) , the head, feet and skin, while the kidneys and kidney fat were in all cases included in the carcass. On the average of the ten animals the percentage composition of the carcass and of the offal was : — TABLE n. — COMPOSITION OF CARCASS AND OFFAL CARCASS % OFFAL % In the fresh state Water ... 4.8 4. 58 8 Ash ? 7 -i o I? er 17.2 Fat -3A A 21 O In the fat-free dry matter Ash IOO.O 21 C IOO.O 14. 0 Nitrogenous matter 78.S 8S.I IOO.O IOO.O NUTRITION OF FARM ANIMALS TABLE 12. — PERCENTAGE DISTRIBUTION OF ASH, PROTEIN AND FAT BETWEEN CARCASS AND OFFAL ASH PROTEIN (NX 6.25) FAT Fat calf In carcass 73.2 6^.0 70. c In offal 26.8 34-i 29-5 Half -fat ox In carcass In offal 100.0 77-3 22.7 IOO.O 66.8 33.2 IOO.O 78.1 21.9 Fat ox In carcass In offal IOO.O 77.0 23 O IOO.O 67.2 32 8 IOO.O 77.0 23 O Fat lamb In carcass IOO.O 7-1 o IOO.O ^2 I IOO.O 78 I In offal 26 o 4.7 O 21 O Store sheep In carcass IOO.O 73 ^ IOO.O f\ A R 400 4.09 21.59 2O.4O 73-52 500 4.88 22.20 27.08 72.92 Haecker .... , 600 5-35 22-34 27.69 72.31 e if\ 700 .5-3° 22.39 27.70 72.24 800 5-20 23.14 28.34 71.66 o r Oo - . /• 900 5-4 23.70 25.74 71.20 IOOO I IOO 5.12 5-70 23-59 23.78 28.71 29.48 71.29 70.52 , OA n& . . V» 1 2OO 5-33 24.00 29.41 7°-59 Sheep Lawes and Gilbert . — 6 mos. 4.60 19.60 24.IO 75-90 Lawes and Gilbert . — 1-2 yrs. 5-oo 21. 2O 25.90 74.10 Lawes and Gilbert . — 3l y^. 4.70 21.10 25-50 74-50 Swine Wilson .... — New born 6.38 12.39 18.77 81.23 Wilson .... — 1 6 days 4.27 14.69 18.96 81.04 Tschirwinsky . . — 9-10 wks. 4.27 16.45 2O.72 79.28 Tschirwinsky . . — 23-28 wks. 4.92 17-47 22.39 77.61 Lawes and Gilbert — 11-12 mos. 3-40 19.90 23.10 76.90 Soxhlet .... — 17-19 mos. 4-04 20.37 25-34 74-66 Geese B. Schulze . . . — 9-10 mos. 4-54 26.06 30.60 69.40 Chaniewski . . . — Mature 4.14 25.85 29-93 70.07 COMPOSITION OF ANIMALS AND OF FEEDING STUFFS 65 body and may be stored up in large quantities, reaching in one instance 48 per cent, or, on the other hand, may be almost lack- ing in the insufficiently fed or fasted animal. 98. Composition of fat-free body. — Since the adipose tissue of the animal body represents substantially a storage of reserve material (93, 94) temporarily set aside from the physiological activities of the organism, a better idea of the composition of the working machinery of the body is obtained by computing its composition fat-free as in Table 18. When this is done, it appears that the composition of the fat- free body is much less variable than that of the body as a whole, the chief difference being due to variations in the water content, which in turn depends chiefly upon the age of the animal, as the preceding table shows. So far as can be concluded from these few cases, however, the fat-free bodies of mature cattle would appear to contain three to four per cent less water than those of mature sheep or swine. In the case of geese, the per- centage of water is probably low on account of the relatively small amount in the feathers. 99. Composition of fat- and ash-free dry matter. — In some of the foregoing investigations, viz., in Lawes and Gilbert's, Sox- hlet's and three of Chaniewski's, the total nitrogen was deter- mined and the protein has been calculated by multiplying by the factor 6.25. These experiments permit a computation of the percentage of nitrogen contained in the fat-free dry matter which in the other experiments has been regarded as protein. For example, in the case of Lawes and Gilbert's fat calf the figures are as follows : — Per cent Total dry matter 34.9 Ash 3.9 Fat 15.3 19.2 Fat- and ash-free dry matter . . . 15.7 Total nitrogen 2.537 2-537 •*• J5-7 = 16.16% nitrogen in ash- and fat-free dry matter. The results of such a computation for all of the experiments in which the published data permit it are contained in Table 19. 66 NUTRITION OF FARM ANIMALS TABLE 19. — NITROGEN IN FAT- AND ASH-FREE DRY MATTER EMPTY WEIGHT NITROGEN IN FAT- AND Fat- and Ash-free Dry Matter % Nitrogen % ASH-FREE DRY MATTER % Fat calf IS-7 2-537 16.16 Half-fat ox 18.1 2.950 16.30 Fat ox 15-4 2.466 16.01 Fat lamb 13-4 2.150 16.05 Store sheep 15-7 2-525 16.08 Lawes and Gilbert . . Half-fat sheep 15-4 2.486 16.14 Fat sheep 13.0 2.085 16.04 Extra-fat sheep ii. 5 1-857 16.15 Store pig 14-5 2.342 16.15 Fat pig 11.4 1.830 16.05 Average i6.n Swine 15-54 2.033 13.08 Soxhlet Swine 15-87 2.068 13-03 Swine 14.00 1.741 12.44 Average 12.85 Geese 26.84 4-309 16.06 Chaniewski .... Geese Geese 23.80 21.68 3.824 3-479 16.07 16.05 Average 16.06 With the exception of Soxhlet's experiments, the percentage of nitrogen approximates closely to that of animal proteins. If account be taken of the fact that the ether-extraction method used in these investigations does not completely remove the fat from dried animal tissue, the conclusion appears justified that the organic matter other than fat contained in the animal body has substantially the composition of protein. § 4. THE COMPOSITION or FEEDING STUFFS 100. Groups of ingredients. — As in the case of the animal body, the vast number of single chemical compounds found in the plant, as well as the lack of accurate quantitative methods for the determination of many of them, renders it practically COMPOSITION OF ANIMALS AND OF FEEDING STUFFS 67 necessary to be content in most cases with a separation of the plant substances into a few major groups or sub-groups of in- gredients. As ordinarily carried out, feeding stuffs analysis recognizes seven of these categories, viz., water, ash, protein, non-protein, ether extract, crude fiber and nitrogen-free ex- tract. 101. Water. — The amount of water in a feeding stuff is commonly inferred from the loss of weight which the substance undergoes at a temperature above the boiling point of water. There is also a possibility, however, of a loss of other volatile matter besides water, while, on the other hand, some substances tend to absorb oxygen and thus increase in weight, especially when dried in air at a high temperature. The exact deter- mination of water and dry matter, therefore, is by no means an easy problem, but the results obtained by the ordinary methods are sufficiently exact for almost all purposes related to stock feeding. Commonly, the residue is weighed and regarded as dry matter, the amount of water being obtained by difference. 102. Ash. — In the ordinary feeding stuffs analysis, ash is equivalent to the residue left after complete incineration of the substance in air or oxygen, the process being carried out at as low a temperature as practicable in order to avoid volatilization of part of the alkalies present. That this method fails entirely to distinguish between those ele- ments which were originally present as electrolytes and those which were in organic combination has already been pointed out (5) , as has also the fact that certain elements, notably sulphur and phosphorus, are only partially recovered in the ash by the ordinary method of preparation. As the study of the functions of the ash ingredients pro- gresses, it may be anticipated that we shall come to determine the several elements involved in the way most appropriate to each rather than simply to determine the ash as a whole. 103. Nitrogenous constituents. — As yet no methods exist for the quantitative separation of the nitrogenous constituents from the other ingredients of plants. While much labor has been expended upon a study of the individual proteins of a com- paratively few vegetable materials, and while in some instances it is possible to state with approximate accuracy the amounts of the several proteins present, nevertheless the only available methods for the determination of the nitrogenous compounds 68 NUTRITION OF FARM ANIMALS of feeding stuffs in general are indirect ones based upon a determination of their characteristic element nitrogen. 104. Crude protein. — In the method of feeding stuffs anal- ysis inherited from the early investigations of Henneberg and Stohmann, the protein is estimated from the amount of total nitrogen upon two assumptions: first, that all the proteins contain 16 per cent of nitrogen and, second, that all the nitro- gen of feeding stuffs exists in the protein form. On the basis of these assumptions, the protein is, of course, equal to total nitrogen multiplied by 6.25. The protein as thus determined is designated as crude protein to indicate the approximate na- ture of the determination. Subsequent investigations by Scheibler, E. Schulze, Kellner and others have shown the presence in many feeding stuffs of relatively large amounts of non-protein nitrogenous com- pounds, so that it is desirable to distinguish at least between the nitrogen present as true protein and that present in the simpler compounds grouped under the general term non-protein (60-67) , and all analyses of feeding stuffs for scientific purposes should at least make this distinction. Logically, too, the term crude protein should be dropped altogether, but when, as in the case of the older analyses, this is impracticable, care should be taken to retain the adjective, reserving the term " protein " for use in the sense given it in the next paragraph. 105. True protein. — As a means of effecting an approximate separation of the true protein from the other nitrogenous compounds present in plants, advantage is taken of the fact that most of the latter class of substances are soluble in water. An aqueous extract of a feeding stuff, therefore, contains by far the larger share of its non-protein. Such an extract, however, contains also any water-soluble proteins existing in the substance. These are removed in part by coagulation by heating, i.e., by boiling the solution, and in part by the addition of some reagent with which they form insoluble compounds. Various substances have been used for this purpose but the present official method of analysis, based upon Stutzer's inves- tigations, uses copper hydrate as the precipitant. In practice, the feeding stuff is boiled with water, the precipitant added and the soluble matter filtered off. The nitrogen of the in- soluble residue is regarded as being protein nitrogen and from COMPOSITION OF ANIMALS AND OF FEEDING STUFFS 69 it by multiplication by 6.25 (or some other agreed factor) the amount of protein is calculated. It is obvious that this method of determining protein is sub- stantially a conventional method and that the adjective true is employed in a somewhat Pickwickian sense. The result probably includes all of the proteins of the feed but may also include other insoluble nitrogenous compounds. 106. Non-protein. — The non-protein in feeding stuffs analy- sis includes all the nitrogenous compounds which remain in solution when the material is treated in the manner just de- scribed for the determination of protein. The nitrogen may be determined in the solution but ordinarily it is obtained by subtracting the protein nitrogen from the total nitrogen. The difference, multiplied by some conventional factor, equals the non-protein. Obviously, the non-protein is a heterogeneous mixture, varying as between different feeding stuffs and even in the same feeding stuff grown or harvested under different conditions. 107. Nitrogen factors. — Evidently the accuracy with which the protein and the non-protein in a feeding stuff are determined depends not only upon the accuracy with which the protein and non-protein nitrogen can be separated and determined but also on the correctness of the factors used for converting nitro- gen into protein or non-protein respectively. For protein the usual factor has been 6.25 as already stated, based upon the assumption of 16 per cent of nitrogen in average protein. As was stated in Chapter I (44), however, different proteins vary in their nitrogen content, and in particular the vegetable proteins run higher in nitrogen than the animal pro- teins, which is, of course, equivalent to a smaller conversion factor. But while it is easily shown that the present factor is incorrect in many cases, it is not so easy to find a substitute. There is a rather wide range in the nitrogen content of the individual vegetable proteins, while most feeding stuffs con- tain two or more proteins in unknown proportions. Moreover, the proteins of the majority of feeding stuffs, especially of the roughages, have not yet been separated and studied. Ritthausen 1 has suggested the use of the factor 5.7 for the majority of cereal grains and leguminous seeds, 5.5 for the oil 1 Landw. Vers. Stat., 47 (1896), 391. 70 NUTRITION OF FARM ANIMALS seeds and for lupines, and 6.0 for barley, maize, buckwheat, soybean, white bean, and rape and other brassicas. For various classes of human foods, Atwater and Bryant l have proposed the following factors for the computation of protein from protein nitrogen : — Animal foods 6.25 Wheat, rye, barley and their manufactured products . . . . 5.70 Maize, oats, buckwheat and rice, and their manufactured products 6.00 Dried seeds of legumes 6.25 Vegetables • 5.65 Fruits 5.80 For feeding stuffs whose proteins have not yet been studied, there seems to be no reason for changing from the present usage. With the non-proteins the case is even more perplexing in view of the greater variety of substances included under this term and the wide range of their nitrogen content. The writer has used tentatively 4.7, the factor for asparagin (66), one of the most widely distributed substances of this class, but the choice of this factor is substantially arbitrary. 108. Crude fat. Ether extract. — The methods for de- termining the fat content of feeds are based upon its extrac- tion by means of some solvent which dissolves as little as possible of the other ingredients. A variety of solvents has been used for this purpose, such as carbon disulphid, carbon tetrachlorid, petroleum ether and the like, but the one most commonly employed is ethyl ether, or the so-called " sulphuric " ether commonly used as an anaesthetic. All the various solvents used, however, remove other sub- stances besides neutral fats and fatty acids, including more or less of the more complex lipoids. In particular the ether ex- tract obtained from coarse fodders contains a variety of waxes, resins, etc., as well as the chlorophyl of the leaves, and a rela- tively small proportion of true fats. It is customary, there- fore, to designate the extracted material as " crude fat " or, since ether is the reagent ordinarily used, as " ether extract." 1 Storrs (Conn.) Agr. Expt. Sta., Rpt., 12, 79. COMPOSITION OF ANIMALS AND OF FEEDING STUFFS 71 If a different solvent is used, this should be specified in the state- ment of the analysis. 109. Crude fiber. — The so-called crude fiber of feeding stuffs is determined by boiling them first with dilute acid and then with dilute alkali under strictly defined conditions of concen- tration and time, and washing the undissolved residue with alcohol and ether. The residue, after deducting the small amount of ash remaining in it, constitutes the crude fiber. Crude fiber as thus obtained contains most of the cellulose, lignin and cutin of the feeding stuff, along with more or less of the more difficultly soluble hemicelluloses, particularly those containing pentosans. The proportion of pentosans contained in the crude fiber naturally varies according to the nature of the feeding stuff. Tollens, l for example, obtained the fol- lowing figures for the crude fiber of meadow hay and of brewers' grains : — PENTOSANS IN CRUDE FIBER PER CENT OF TOTAL PENTOSANS OF FEED RETAINED m CRUDE FIBER Meadow hay 16 89 2^63 Brewers' grains 1 1 61 8 56 110. Nitrogen-free extract. — All the ingredients of feeding stuffs which are not included in the foregoing six categories, viz., water, ash, protein, non-protein, ether extract and crude fiber, are usually grouped together under the collective name of nitrogen-free extract. The significance of the name is evident. By definition the nitrogen-free extract includes all those non- nitrogenous organic constituents, other than fat, which are extracted from the feeding stuff in the process of determining the crude fiber. The amount of nitrogen-free extract in a feeding stuff is not ascertained by any process of direct deter- mination but simply by subtracting the sum of the other six groups from 100 per cent. Such a residual group naturally in- cludes a great variety of substances of very diverse nature, 2 1 Jour. Landw., 45 (1897), 103. 2 For an enumeration of the principal ingredients of the nitrogen-free extract, compare Tollens, Jour. Landw., 45 (1897), 295. 72 NUTRITION OF FARM ANIMALS but as a rule the nitrogen-free extract consists to a considerable extent of carbohydrates of one sort or another. Indeed, it has sometimes been designated by the latter name, but the use of the word in this sense is misleading and undesirable. The nitrogen-free extract includes not only hexose but also pentose carbohydrates, these latter substances being, therefore, by the ordinary method of feeding stuffs analysis, divided be- tween the crude fiber and nitrogen-free extract. Some of these various carbohydrates can be determined separately with a reasonable degree of accuracy, while others, including unfor- tunately starch, can be determined only more or less approxi- mately. That the nitrogen-free extract is far from consisting exclusively of carbohydrates has been strikingly shown, by Stone.1 He determined the content of the various classes of carbohydrates in samples of wheat and maize as accurately as possible and found that the sum in both cases was considerably less than the amount of nitrogen-free extract as determined by the conventional method. Much greater differences in this respect have been shown to exist in roughages. 111. Classes of feeding stuffs. — • The composition and char- acteristics of the principal classes of feeding stuffs are considered in Chapter XV, but it seems desirable to anticipate that dis- cussion here to the extent of indicating the three major classes into which the feeding stuffs are commonly divided. This classification is based primarily on botanical characteristics with which, however, are associated corresponding differences in chemical composition. Concentrates or concentrated feeds. — As the name implies, these are feeding stuffs which contain much nutriment in a small bulk. They include primarily the grains and other seeds and, secondarily, a wide range of technical by-products de- rived from them as well as certain by-products of animal origin. Chemically, they are characterized by their relatively low con- tent of crude fiber, ranging from practically zero in certain by- products to perhaps 10 or 12 per cent in grains having a con- siderable proportion of hulls, like oats or buckwheat, and in certain by-products. Coarse fodders or roughage. — • Botanically, these consist of the vegetative organs of the plant, i. e., substantially of stalks 1 Jour. Amer. Chem. Soc., 19 (1897), 183. COMPOSITION OF ANIMALS AND OF FEEDING STUFFS 73 and leaves. They include hay, straw and other forms of for- age either fresh, ensiled or dried. Chemically, they are char- acterized by their relatively high percentage of crude fiber, which, however, naturally varies within quite wide limits. As compared with the concentrates they are bulky feeds and contain a larger proportion of difficultly soluble ingredients. Roots and tubers. — These feeding stuffs contain a large per- centage of water, resembling in this respect the fresh or ensiled roughages. Their dry matter, on the other hand, resembles that of the concentrates in containing relatively little crude fiber and a large proportion of ingredients which are easily soluble. They might be briefly characterized as dilute con- centrates. PART II THE PROCESSES OF NUTRITION CHAPTER III DIGESTION AND RESORPTION 112. The first step in nutrition. — The facts considered in Part I have served incidentally to show some particulars of those differences between the feed of herbivora and the animal body which it serves to nourish which are, in a general way, familiar to every one. The former contains many ingredients not found in the latter, and it is plain that, for example, sub- stances like starch and cellulose must undergo considerable modification before they can be used in the animal organism. One need not be a chemist, however, to reach this conclusion. A simple comparison of the feeds given farm animals with the products which they manufacture out of them convinces one that profound changes are necessary to convert hay and grain into meat or milk. The first step in this process is the diges- tion of the feed. In all but the lowest animals, special tissues are set apart for this work, together constituting the organs of digestion, or the alimentary canal with its appendages. § i. THE ORGANS OF DIGESTION 113. General plan. — The process of digestion is seen in its simplest form in unicellular animals like the ameba. When the ameba comes in contact with a particle of feed, a depres- sion forms in its outer surface which finally closes around the particle, forming a cavity which serves as a temporary digestive organ. Undigested residues are rejected by the reverse pro- cess. In animals slightly higher in the scale, this temporary cavity becomes a permanent one, the same opening serving for the entrance of feed and the exit of waste. The next step in the evolution is the provision of a separate exit for the waste matter, thus giving the typical form of digestive apparatus, of which that of the higher animals is a development, consisting 77 7 8 NUTRITION OF FARM ANIMALS of a cavity or cavities communicating with the external world by two openings, one for the reception of feed and the other for the rejection of waste. In domestic animals, the digestive tract is large and of very complex structure, but in all cases it is built upon the general plan just outlined. -Always, from the ameba up to man, the inner surface of the digestive cavity is morphologically simply a continuation of the external surface of the body, turned in as one might a glove finger. Conse- quently, the material contained in the digestive cavity, strictly speaking, is still outside the body.1 Finally, as an essential part of the digestive apparatus, there must be such organs as the cilia, tentacles, proboscis, lips, etc., by which feed is grasped and introduced into the digestive cavity, and likewise means by which it may be mechanically ground to fit it for the process of digestion, as, for example, the teeth of mammals, the bills and gizzards of birds, etc. For the present purpose, it is unnecessary to enter into any elaborate consideration of the anatomy of the digestive organs, since we are concerned chiefly with the chemical rather than the physical processes of digestion, and this section may be confined to a very general description of the digestive organs of domestic animals. In these animals, the digestive apparatus may be described briefly as a tube having various enlargements, folds and diverticula. 114. Digestive fluids and enzyms. — In the ameba, what- ever changes are effected in the substances which it takes as feed are accomplished by the cells of the introverted surface or by their secretions. As the digestive apparatus becomes more complicated, however, a division of cellular labor takes place and certain groups of cells are set apart to produce the digestive juices which act upon the feed. In the higher animals, these cells become the numerous secreting glands which are an essential part of the organs of digestion. The principal active agents in digestion are certain enzyms secreted by these glands, the more important digestive enzyms in the higher animals being : — • i. The amylases, ptyalin (in 'the saliva) and amylopsin (in the pancreatic juice), acting upon starch. 1 For a more complete discussion of the development of the digestive apparatus see R. Meade Smith, The Physiology of the Domestic Animals, pp. 203-226. DIGESTION AND RESORPTION 79 2. The invertases, sucrase, maltase and lactase (in the in- testinal juice), acting upon di-saccharids. 3. The proteases, pepsin (in the gastric juice), trypsin (in the pancreatic juice) and erepsin (in the intestinal juice), act- ing upon proteins. 4. The lipase, steapsin (in the pancreatic juice), acting upon the lipoids, or specifically the fats. 115. The mouth. — The mouth is the organ of prehension and mastication. Chiefly by means of the tongue, lips and teeth, feed is seized and introduced into the digestive cavity, while the teeth serve also to grind it up, rendering it capable of being swallowed and also exposing more surface to the action of the digestive fluids. The mouth also receives the secretion of three pairs of glands called the salivary glands whose product is known as the saliva. These three pairs are called, respec- tively, the parotid, the submaxillary and the sublingual glands. The mixed saliva, consisting of the secretion of all three pairs of salivary glands, together with the comparatively insignificant amounts secreted by the various smaller glands of the mouth, is a thin, colorless, watery, slightly viscid liquid .of alkaline reaction. The organic matter of the saliva includes a trace of albumin, more or less mucus, and the enzym ptyalin,1 which is its active constituent. The saliva has both physical and chemical functions. The presence of feed in the mouth, its taste, odor or sometimes even sight, causes active secretion of saliva, which is mixed with the feed in the act of mastication and moistens and lubricates it so that it can be swallowed. With dry feeding stuffs, the amount of saliva required for this purpose is very large. The total secretion has been estimated at about 84 pounds per day for the horse and at least 112 pounds per day for the ox, although varying greatly with the dryness of the feed. Besides moisten- ing and lubricating the feed, the saliva has also a chemical action upon it. In a slightly alkaline medium and at body tempera- ture, the ptyalin acts upon the starch of the feed, converting it ultimately into maltose. 116. The stomach. — From the mouth, the feed in the act of swallowing passes through the esophagus, or gullet, to the stomach which, except in fowls, is the first enlargement of the 1 Not present in the saliva of carnivora. 8o NUTRITION OF FARM ANIMALS alimentary canal. The horse 'and hog, as well as carnivorous animals like the dog and cat, have a single stomach cavity, while ruminants, such as cattle, sheep and goats, have a so- called compound stomach consisting, in the farm animals, of four divisions, called respectively the rumen, or paunch, the FIG. 9. — Sheep's stomach. (Hagemann, Anatomie und Physiologic der • . Haus-Saugetiere.) i, Rumen. 2,.Reticulum. 3, Omasum. 4, Abomasum. 5, Duodenum. 6, Esophagus. reticulum, the omasum, or manifolds, and the abomasum, or true stomach. In reality the first three divisions of the ruminant stomach are to be regarded as dilatations of the esophagus in which the feed undergoes a softening and fermentation preliminary to true gastric digestion, while only the fourth division is a stomach in the -strict sense. In domestic fowls a similar dilatation of the esophagus at the base of the neck constitutes the crop. Moreover, even the so-called single stomachs of the horse and hog, while they have but a single cavity, are in reality compound stomachs! In the. case of the horse three quite distinct parts may be distinguished, viz., the left or cardiac portion, the fundus region and the pyloric region, the two latter having the functions of the true stomach. In the case of the DIGESTION AND RESORPTION 8l FIG. 10. — r Stomach and duodenum of horse. (Haglmann, Anatomic und Physiologic der Haus-Saugetiere.) Sch., Esophagus. C, Cardiac portion. M, Fundus. F, Pyloric region. D, Duodenum. hog the cardiac portion is comparatively small and the remainder of the organ is to be regarded as constituting the stomach proper. 117. Rumination. — In the ruminant, water and liquid feeds may pass quite directly to the aboma- sum, although as a matter of fact, they seem to reach all four divisions of the stomach. The more bulky feeds, however, fail to pass through the esophageal canal but enter the rumen and reticulum. This is especially the case because the rumi- nant masticates its feed FlG> — Stomach of hog. (Hagemann, Ana- tomie und 1-4, Fundus. 2, Cardiac portion, s, Pylorus, 8, Duodenum. .very imperfectly at the time of eating. In the reticulum and especially in the capacious rumen, the partially masticated feed re- mains for some time in contact with the saliva and such. 82 NUTRITION OF FARM ANIMALS portion of the drink as reaches this stomach and is thoroughly softened and prepared for further action. The rumen is so large that it always contains a considerable amount of material and the new feed when swallowed is more or less completely mixed with that already in the rumen by the peri- staltic action of the latter, thus tending to prolong its stay. The liquid or finely comminuted portions probably pass on directly to the omasum, or manifolds, and the abomasum, but the bulk of the feed undergoes the process of rumination. After the animal has completed feeding, and if it is left in quiet, small portions of the feed are raised again to the mouth from the rumen and reticulum by contraction of these organs, aided by the action of the abdominal muscles and of the dia- phragm, thoroughly chewed and swallowed a second time. This time they pass to a considerable extent, though not entirely, the esophageal . canal and enter the third stomach, the manifolds, and from this pass into the fourth or true stomach. The ruminants are animals which in the wild state depend on speed and cunning to escape from their enemies. Hence it is an advantage to them to be able to consume hastily large amounts of feed and then to retire to some safe concealment to remasticate and prepare it for digestion. Rumination also enables these animals to utilize more thoroughly coarse rough fodders, the long stay in the paunch softening and fer- menting the material and helping especially to destroy or dis- solve the carbohydrates of the cell walls and thus render the cell contents accessible to the digestive fluids. 118. The gastric juice. — The mucous membrane lining the true stomach contains numerous simple glands (tubular glands) differing in appearance in different portions of the stomach. Those of the fundus region contain two kinds of secreting cells, commonly designated as " chief " and " parietal " cells. The glands of the pyloric end contain " chief " cells similar to those of the fundus glands, but only an occasional " parietal " cell. The parietal cells secrete as their essential product hydrochloric acid. The " chief " cells produce the enzym pepsin, or rather a precursor of pepsin called pepsinogen. The mixed secretion of these different glands constitutes the gastric juice, which is a thin, clear acid liquid having a DIGESTION AND RESORPTION 83 specific gravity of 1.005 to I-°I and containing a maximum of about 2.5 per cent of solids. The combined action of the pepsin and hydrochloric acid of the gastric juice splits the proteins of the feed into derived proteins, especially proteoses and peptones, and to some extent into polypeptids.1 The hydro- chloric acid of the gastric juice has also an important anti- septic action and likewise serves to dissolve more or less of the ash of the feed. In addition to its digestive action on proteins, the gastric juice contains an enzym which brings about the coagulation of the caseinogen of milk — the rennet ferment, or chymosin. According to some investigators, chymosin is identical with pepsin, but the weight of opinion seems to be in favor of its independent existence. 119. The passage of feed from the stomach. — The lower or posterior end of the stomach is closed by a sphincter muscle called the pylorus, which prevents the ingested feed from pass- ing into the next division of the alimentary canal, the duodenum, or being forced into it by the contractions of the stomach. When in the course of gastric digestion, however; the difference between the acid reaction of the stomach contents and the al- kaline reaction which normally prevails in the duodenum reaches a certain level, the pylorus relaxes and allows the per- istaltic contraction of the stomach to press a portion of its acid contents into the duodenum. The partial neutralization of the duodenal contents which results causes the pylorus to close again until the alkaline reaction is restored, when the cycle may be repeated. The mechanism of this process has been especially studied by Cannon in carnivora, but it may be presumed that what is true of these animals is also substantially true of herbivora, although experimental proof of this is lacking. While both protein and carbohydrates undergo considerable digestion in the stomach, it is evident that one important function which the stomach performs is that of a receptacle which prevents too rapid passage of the feed into the duodenum and at the same time initiates chemical changes and prepares 1 By prolonged peptic digestion in vitro amino acids may also be produced but it is not believed that this occurs in natural digestion. 84 NUTRITION OF FARM ANIMALS the feed for the more vigorous action of the intestinal enzyms. Moreover, the setting free of cell contents by the fermentation of the cell walls of vegetable feeds, as well as the liberation of the fat of animal feeds by the solution of the protein of the adipose tissue, render these materials more accessible to the action of the digestive juices. 120. The small intestine. — On leaving the stomach through the pylorus, the feed enters the small intestine, which may briefly FIG. 12. — Intestines of cattle. (Leisering, Die Rindviehzucht.) be described as a long, comparatively narrow tube. Its average length is, according to Colin, about nine times that of the body in the horse, sixteen times in the ox and sheep and eleven times in the hog. It is suspended in the abdominal cavity by a re- flection of the peritoneum called the mesentery, and as shown in Fig. 1 2 is much convoluted. It is commonly subdivided into duodenum, jejunum and ileum. 121. The coecum. — From the small intestine the contents of the digestive tract pass, through the ileo-ccecal valve,, in to the ccecum, which is a diverticulum of the digestive canal, the point DIGESTION AND RESORPTION of entrance from the small intestine and that of exit into the colon being near together and in the upper part of the ccecum. Anatomically, it might almost be called a second stomach. Its functions, however, resemble those of the first stomach of ruminants and not those of the true stomach, the feed stagnating, so to speak, in the ccecum and under- going extensive f er- mentation and putrefaction. The size of the ccecum, in a general way, varies inversely as that of the stom- ach. Thus in the horse it is very large, having about 1 6 per cent of the total capacity of the digestive canal. In the ox, on the other hand, it has only about 3 per cent and in the sheep less than 2.5 per cent of the total capacity and in the hog about 5.5 per cent. 122. The large intestine. -- The alimentary canal is continued from the ccecum as the large intestine, which, as its name implies, is gen- erally of greater diameter than the small intestine but also shorter. It is subdivided into the colon and the rectum and serves rather as a resorbent than as a digestive organ. The colon FIG. 13. Coecum of horse. (Colin, Physiologic comparee des Animaux.) 86 NUTRITION OF FARM ANIMALS is enormously developed in the horse, having about 45 per cent of the total capacity of the digestive tract, and serves, like the ccecum, to continue the digestion of the less soluble portions of the feed. 123. The pancreas. — In the stomach, the glands which secrete the gastric juice are located in the mucous lining of the organ. In the case of the intestines, the glands which supply the various digestive juices, like the salivary glands of the mouth, lie in part entirely outside the alimentary canal proper. One of the most important of these is the pancreas. This is a large gland located near the stomach, liver and duodenum, its duct opening into the latter either by a common exit with that of the bile duct (horse, sheep), or somewhat lower down (cattle, swine). The secretory action of the pancreas, like that of the salivary and gastric glands, is intermittent, the gland being active only when feed is present in the duodenum. The pancreatic juice is a clear, viscid liquid, having an al- kaline reaction due to its content of sodium salts. It contains in the neighborhood of eight to ten per cent of solid matter and has a specific gravity of approximately 1.030. It differs from other digestive juices in containing a relatively large amount of protein. As in the case of all the other digestive fluids, the essential active ingredients are enzyms, of which the pancreatic juice contains three in particular, viz., a protease, trypsin, acting upon the proteins, an amylase, amylopsin, acting upon starch, and a lipase, steapsin, acting upon fats. Small amounts of chymosin and of a lactase have also been found. In the juice as secreted by the pancreas, the trypsin at least, if not the other enzyms, exists in the form of a pro-enzym, trypsinogen, which is converted into trypsin (" activated ") after the secretion reaches the duodenum. 124. The liver. — This, the largest gland in the body, is located immediately below the diaphragm and lies chiefly on the right side of the body. It is relatively small in ruminants and large in the hog. The liver has other important functions in nutrition, as will appear in Chapter V, but as related to digestion it secretes the bile. This fluid, produced by the hepatic cells, passes from them into the bile capillaries, which unite to form small ducts, the latter finally coalescing and constituting the bile duct. In DIGESTION AND RESORPTION 87 the horse, this empties directly into the duodenum a short distance from the stomach. In cattle, sheep and swine, the bile is stored up in the gall bladder, a reservoir from which a duct leads to the duodenum. The chief action of the bile is upon fats of the feed. To a small extent, it saponifies them and it also assists in emulsifying them. Its digestive action may, however, be more conveniently considered along with that of the pancreatic juice (126, 135). In addition to its action upon the fats, an antiseptic effect and also a stimulating effect upon peristalsis have been ascribed to the bile. 125. The intestinal juice. — In addition to the external glands (pancreas and liver), already mentioned, the walls of the small intestine contain a large number of small glands of two kinds, Brunner's and Lieberkiihn's glands, which yield an intestinal juice containing a number of enzyms. Prominent among these are the invertases maltase, sucrase and lactase, which act upon the corresponding disaccharids, the maltose re- sulting from the digestion of starch being converted into dextrose, sucrose into a mixture of dextrose and levulose, and lactose, in suckling animals at least, into dextrose and galactose. There may also be extracted from the mucous membrane of the small intestine a protease called erepsin. This enzym does not act upon the native proteins, with the exception of casein, but acts powerfully upon the derived proteins (proteoses and peptones), hydrolyzing them and breaking them down very completely to their constituent amino acids. The presence of erepsin has also been demonstrated in the intestinal juice, but its action in this case was weaker than in the extracts of the intesti- nal wall and it may be that a portion of its action in the living animal takes place within the cells in which it is produced. The presence in the intestinal juice of an amylase, a lipase and of ferments (nucleinases and nucleotidases) , which act upon the nucleic acids has also been demonstrated. 126. Intestinal digestion. — In the duodenum the neutraliza- tion of the acid material coming from the stomach is effected by the alkalies of the bile and pancreatic juice, while the bile also precipitates proteins and partly digested proteins in com- bination with the bile acids and this precipitate carries down with it mechanically the pepsin which is present. In these 88 NUTRITION OF FARM ANIMALS two ways, namely by neutralization and precipitation, the pep- sin is prevented from digesting the enzyms of the pancreatic juice and bile, an action which would otherwise take place, since these enzyms appear to be substantially protein in their nature.1 In the small intestine, the neutralized contents of the stomach are subjected to the combined action of the pancreatic juice, the bile and the intestinal juice, while they are moved along through the successive divisions of the small and large intestines by means of the peristaltic movements of the latter. These movements serve also to mix the contents of the intestines and to bring them into intimate contact with the intestinal walls. The fats of the feed, under the action of the steapsin of the pancreatic juice, undergo a cleavage into glycerol and fatty acids and this change is considerably accelerated by the bile, which also aids in emulsifying the fats and so exposing vastly more surface to the action of the enzyms. The fatty acids thus set free unite to a greater or less extent with the alkali of the pancreatic juice and bile, forming soaps, while both soaps and free fatty acids are soluble in bile in the presence of sodium carbonate. The presence of soaps in solution also aids, as was pointed out in Chapter I, in producing a permanent emulsion of the fats. Starch, if any escapes digestion in the stomach, is acted upon by the pancreatic amylopsin substantially in the same manner as by the ptyalin of the saliva but much more energetically, yielding maltose, while both maltose and any other disaccharid present in the feed are acted upon by the invertases of the in- testinal juice, yielding monosaccharids. Any proteins which escape digestion in the stomach, and likewise the proteoses and peptones resulting from peptic di- gestion, are hydrolyzed by trypsin and erepsin much more energetically than by pepsin and yield not only proteoses and peptones, but a whole series of progressively simpler poly- peptids and finally are largely or wholly split up into their constituent amino acids. 1 The foregoing statements describe what takes place when the materials are mixed in the laboratory. The actual importance of the precipitation of the pepsin in the intestine is somewhat in doubt. DIGESTION AND RESORPTION 89 § 2. THE CHEMISTRY OF DIGESTION 127. Digestion a chemical process. — The foregoing para- graphs have dealt chiefly with those more general facts regard- ing the organs of digestion which are necessary for an under- standing of their functions and only incidentally and in outline with the chemical processes involved. It is now time to revert to the statement made at the beginning of the chapter, namely, that digestion is the first step in the conversion of feed sub- stances into body substances, and specifically in the case of farm animals the conversion of vegetable into animal substances. These, however, are chemical transformations and from this point of view a knowledge of the structure of the digestive apparatus is of significance chiefly as an aid to the understanding of these processes. Tn taking up this aspect of the subject, it will be convenient to consider the three chief groups of nutrients separately. The digestion of carbohydrates By far the larger proportion of the carbohydrates contained in the feed of farm animals consists of polysaccharids, especially starch, cellulose and the various pentosans and hexo-pentosans. The disaccharids, especially sucrose and lactose, probably stand next in importance, while comparatively small amounts of monosaccharids are consumed. 128. Cellulose. — The cellulose of feeding stuffs was long assumed to be indigestible. Haubner was the first to show the incorrectness of this assumption and to prove that even the cellulose of such substances as paper pulp and sawdust, as well as that of ordinary feeds, was digested by cattle. The subse- quent investigations of Henneberg and Stohmann (158, 707) showed that the crude fiber of feeding stuffs was digested to a considerable extent by cattle, and sheep, and later digestion ex- periments have proved this to be true not only of ruminants but to a varying degree of other animals, both herbivora and omnivora, including domestic fowls. Even man is capable of digesting the tenderer forms of cellulose to a considerable extent. None of the digestive enzyms of the higher animals, however, have been shown to have any action upon cellulose and the small go NUTRITION OF FARM ANIMALS amounts of cellulose-dissolving enzyms (cytases) found in some feeds appear quite inadequate to account for its solution, so that the manner of its digestion was long a puzzle. The in- vestigations of Wildt l in 1874 upon the digestive process in sheep, however, showed, as Zuntz 2 subsequently pointed out, that the solution of cellulose occurs chiefly in those portions of the alimentary canal where the feed stagnates, — that is, in the paunch of the ruminant and in the ccecum and colon. This fact tended to confirm the view already advanced that the solution of cellulose in the digestive tract was due to a process of fermentation, and this hypothesis also served to explain the presence of methane and hydrogen in the digestive tract. Tap- peiner,3 however, seems to have been the first to show ex- perimentally that the disappearance of cellulose in the digestive tract is effected by a fermentation brought 'about by the micro- organisms inhabiting the alimentary canal. Tappeiner's conclusions have been fully confirmed by more recent investigations, notably those of Markoff 4 in Zuntz's laboratory, while Kellner 5 has shown that the consumption of crude fiber (straw pulp) by cattle causes a marked increase in the amount of methane eliminated. In the light of these results it may be regarded as established that the disappearance of cellulose during its passage through the alimentary canal of herbivora is not due to a digestion in the sense of a simple hydro- lytic cleavage, like that of starch or protein, but to a destructive fermentation. The products of this fermentation are large quan- tities of carbon dioxid and methane and small amounts of hydro- gen, which are excreted, and various organic acids of the aliphatic series which combine with the alkalies of the saliva or other digestive fluids. The salts thus formed are resorbed and consti- tute the sole contribution which cellulose makes to the nutrition of the body. The principal acids formed appear to be acetic and butyric, although others are also . present. In ruminants, the chief seat of this fermentation is the capacious first stomach, while in the horse, with his relatively small, simple stomach, it takes place principally or wholly in the enormous ccecum and colon. 1 Jour. Landw., 22 (1874), i. 2 Landw. Jahrb., 8 (1879), 101. 3Ztschr. Biol., 20 (1884), 52. 4 Biochem. Ztschr., 34 (IQII), 211 ; 57 (1913), i. 5 Landw. Vers. Stat., 53 (1900), 193, 300. DIGESTION AND RESORPTION 91 129. Pentosans. — The pentosans are widely distributed in the vegetable kingdom and appear to be contained chiefly or wholly in the cell walls of plants, probably in combination to a greater or less extent with hexosans. If the ordinary methods of feeding stuffs analysis are followed, both the crude fiber and nitrogen-free extract contain them (109, 110). Stone,1 who was the first to show that they were digestible, found a percentage digestibility of about 60 for the pentosans in the ordinary feed of the rabbit. Later,2 in conjunction with Jones, he showed that in 14 different samples of roughages from 48 to 90 per cent of the pentosans were digested by sheep, while in mixed rations the corresponding figures were from 46 to 71 per cent. Weiske3 about the same time obtained similar results in experiments with sheep and rabbits. The digesti- bility of pentosans has been fully confirmed by later experiments. But while pentosans are digestible, or at least disappear in the digestive tract, the manner of their digestion is not cer- tainly known. Up to the present time no enzyms have been discovered either in the digestive organs or elsewhere, which have been proved to be capable of hydrolyzing them. On the other hand, however, the pentosans are attacked by bacteria much like other carbohydrates and yield similar products, especially the acids of the aliphatic series. That the pentosans are to a considerable extent subject to the methane fermenta- tion in the digestive tract seems clear from Kellner's investi- gations upon straw pulp (128) , in which over one- third of the di- gested organic matter consisted of pentosans, so that it is difficult to resist the conclusion that these, as well as the cellulose, under- went fermentation. Moreover, in a large number of similar experiments, the methane fermentation has been found in a general way to be proportional to the total digestible crude fiber and nitrogen-free extract, including the pentosans. Of course these results do not preclude the possibility of a hy- drolysis of the pentosans in the digestive tract, converting them into pentose sugars, but as yet there is no direct evidence that such a process takes place. If it does not, then the products of the digestion of the pentosans are substantially the same as those from cellulose. 1 Amer. Chem. Jour., 14 (1892), 9. 2 Agricultural Science, 7 (1893), 6. 3Ztschr. Physiol. Chem., 20 (1895), 489, ' 92 NUTRITION OF FARM ANIMALS 130. Hemicelluloses. — What is true specifically of the pentosans appears to hold also for the reserve carbohydrates of the cell wall — the so-called hemicelluloses (18) — as a whole. No animal enzyms are known which hydrolyze the galactans, levulans, etc., or which break up their union, if it exists, with the pentosans, but nevertheless these substances disappear in part in the digestive tract of herbivora. Pending more exact knowledge on this point, the assumption seems warranted that they as well as the pentosans undergo bacterial fermentation and yield substantially the same products. 131. Starch. — The first agent to act upon starch is the ptyalin of the saliva (115). As is the case with the other enzyms, ptyalin has never been isolated, but its effects and the conditions governing its action have been extensively studied, in part owing to the ease with which saliva can be procured. The most important of these conditions are that ptyalin acts most efficiently in the neighborhood of 40° C., that is, at about blood temperature, in a neutral solution, while more than a trace of free acid or alkali inhibits its action. To acids or al- kalies combined with proteins, even though they show an acid or alkaline reaction to ordinary indicators, ptyalin is much less sensitive and it is also less sensitive to organic than to inorganic acids. In brief, the action of ptyalin is inhibited by a very low concentration of either hydrogen or hydroxyl ions. The action of ptyalin on starch consists of a succession of cleavages and hydrations resulting in the formation of the various dextrins (24) and finally of sugar. With cooked starch, the action is fairly rapid ; upon raw starch ptyalin acts more slowly, the rate varying somewhat with the kind of starch and being apparently determined by the degree of resistance of the cellulose envelope of the starch grains. Chemically, the action is analo- gous to that of acids, but is less vigorous and is not carried so far. The action of acids yields dextrose as a final product ; that of ptyalin is usually stated to stop with the production of maltose.1 The action of ptyalin in the mouth is necessarily very brief. In the stomach the feed comes into contact with the gastric juice containing free hydrochloric acid. At first, this acid combines with the proteins contained in the feed, but as soon 1 Carlson and Luckhart (Amer. Jour. Physiol., 23 (1908-9), 149) state that both ptyalin and amylopsin produce dextrose from starch. DIGESTION AND RESORPTION 93 as more than a trace of free acid accumulates, or to speak more exactly, as soon as the concentration of the hydrogen ions ex- ceeds a certain small limit, the action of the ferment is not only inhibited, but the ptyalin is digested by the pepsin. This, however, is far from happening immediately upon the entry of the feed into the stomach. The secretion of the gastric juice requires a certain length of time. Moreover, the contents of the stomach are semi-solid rather than liquid and while the muscular contractions of the stomach serve to mix the feed to some extent, this effect is less than is often assumed. Frozen sections of animals to which variously colored feeds have been given show the contents of the stomach to be distinctly stratified some time after the ingestion of feed. Furthermore, the gastric juice is secreted only in the pyloric portion of the stomach (116). Time is required, therefore, for the acid to penetrate and acidify the whole mass and conse- quently the action of the ptyalin may continue for a con- siderable period. Extensive investigations, especially by Ellenberger and Hofmeister, upon gastric digestion in the horse and hog have demonstrated that in these animals the action of the saliva in the stomach upon the starch of the feed plays an important part in digestion. In the horse (116), the left end of the stomach is really a dilation of the esophagus. In the hog, while nearly the entire surface of the stomach is lined with mucous membrane, the left-hand end contains no peptic glands. When the stomach is filled with feed, therefore, it is evident that the action of the hydrochloric acid will begin along the walls of the fundus of the stomach and only gradually spread to the rest of the con- tents. It is true that lactic fermentation usually sets in during this period, aiding to acidify the stomach contents but, as already stated, ptyalin is less sensitive to organic than to inorganic acids. It has been found that the solution of starch may continue to a greater or less extent for as much as four or five hours both in the horse and hog. • In ruminants, the con- ditions are even more favorable for salivary action, since the feed remains in contact with the saliva in the rumen for a con- siderable time, the contents of this stomach being maintained slightly alkaline by the large amount of saliva secreted by these animals (115). It may be assumed, therefore, in spite of the 94 NUTRITION OF FARM ANIMALS fact that the saliva of ruminants contains but little ptyalin, that a considerable digestion of starch is effected. In the duodenum, any starch not digested in the stomach, as well as any dextrins, etc., produced by the action of the ptyalin, are subjected to the action of the amylopsin of the pancreatic juice. This enzym, if not identical with ptyalin, is very similar to it but appears to act more energetically. As in the case of ptyalin, the final product of its action is maltose.1 The further fate of the maltose resulting from the digestion of starch is more conveniently considered along with that of other disaccharids in a succeeding paragraph. 132. Fermentation of starch. — The organisms producing the methane fermentation in the digestive tract were believed by Tappeiner to attack cellulose specifically and not to act upon other carbohydrates. As regards ruminants, however, this has been shown to *be an error. In G. Klihn's 2 extensive respiration experiments with cattle upon the formation of fats from carbohydrates, considerable amounts of starch were added to basal rations. Invariably this resulted in an increased ex- cretion of methane. Moreover, there was no increase, but on the other hand, more or less decrease in the amount of crude fiber digested, showing that the additional methane must have had its source in the starch consumed. This conclusion is confirmed by the fact that the total excretion of methane was quite closely proportional to the sum of the digested crude fiber and nitrogen-free extract. On the average four parts of methane were produced for each one hundred parts of starch digested. Kellner's subsequent investigations 3 have fully confirmed these results, although giving a lower average, viz., 3.07 parts of methane per one hundred parts of digestible starch. Moreover, Kellner's investigations have shown that the methane fermentation is not confined to cellulose and starch but that, as already indicated, the complex of compounds grouped under the head of nitrogen-free extract, including the sugars, is subject to this process. His experiments also show that the proportion of methane produced is somewhat variable, depending upon conditions not yet fully investigated. As already stated (128), the chief seat of fermentation in the 1 See footnote on p. 92. 2 Kellner; Landw. Vers. Stat, 44 (1894), 257. 3Landw. Vers. Stat., 53 (1900), 423. DIGESTION AND RESORPTION 95 horse is the coecum and colon. Before the feed reaches these, however, it has been acted upon by the amylases of the saliva and the pancreatic juice and its starch and soluble carbohy- drates pretty thoroughly extracted. Consequently, the meth- ane production of the horse is substantially at the expense of the crude fiber of his feed, although if starch for any reason escapes digestion and reaches the ccecum it is doubtless also attacked by the bacteria. 133. The disaccharids. — At first thought, it would seem that the carbohydrates of this group need no digestive change, since they are already soluble and diffusible and seemingly ready to pass into the circulation. But while this is true, they are not assimilable by the organism. Disaccharids are nowhere found in the normal body fluids and if injected into the circu- lation in any considerable amount are voided in the urine. In other words, the disaccharids are treated in the organism as foreign substances which the cells cannot use. In the small intestine the disaccharids are inverted, that is, hydrolyzed to monosaccharids. Cane sugar taken in the food appears to be inverted to some extent by the acid of the gastric juice, but the principal action is by the inverting enzym sucrase of the intestinal juice, which splits up the cane sugar into dex- trose and levulose. Similarly, the maltose resulting from the digestion of starch is split up by the maltase of the intestinal juice, yielding dextrose, while lactose, at least in suckling animals, is split up by lactase into dextrose and galactose. These in- versions appear to take place to a considerable extent in the epithelial cells lining the intestines, and this seems to be the normal method of assimilation of lactose in many mature ani- mals. The epithelial cells are also stated to convert levulose and galactose into dextrose. Finally it should be added that the sugars, like other carbo- hydrates, may undergo the methane fermentation in the first stomach of ruminants. The digestion of fats 134. Emulsification. — As already indicated, the digestion of fats includes two processes, namely, emulsification and saponification, effected chiefly by the action of the bile and 96 NUTRITION OF FARM ANIMALS pancreatic juice. The two processes go hand in hand. As explained in Chapter I, the presence of free fatty acids favors the formation of a permanent emulsion. As there noted, most native fats contain small amounts of such acids which exist dissolved in the natural fat. Furthermore, there seems to be good evidence that some cleavage of fat into fatty acids and glycerol takes place in the stomach of carnivora, while the di- gestion of protein in the stomach helps to liberate any enclosed fat. When the acid fats come in contact with the alkaline pancreatic juice, the molecules of the free acid in solution are saponified and in this way the mass of fat is broken up into an emulsion. The action of the steapsin of the pancreatic juice, which splits fat into glycerol and ^fatty acids, would obviously tend to aid in the emulsification, while, on the other hand, the latter, by vastly increasing the amount of surface exposed by the fats, tends to aid the action of the enzyms. 135. Saponification. — The saponification of fat is accom- plished essentially by the lipase steapsin of the pancreatic juice. As just noted, the saponification is facilitated by the previous emulsification, while the presence of the bile is also an important factor. It is claimed that the presence of bile is necessary to activate the steapsin, while it has also been shown that the cleavage of the fats is much accelerated by the presence of bile, the effect being ascribed to the lecithins which it contains. The presence of bile also assists in the process of digestion by its power of dissolving large quantities of fatty acids and of their calcium and magnesium soaps. It appears also that the bile aids in some way in the resorption of the fat, but just how is not clear. Fats do not seem to be fermented to any extent in the diges- tive tract and when administered to cattle in the form of emul- sions have been found to produce no effect upon the excretion of methane. When given in substance, they have in some in- stances had the effect of diminishing the excretion of that gas. The digestion of the proteins and non-proteins 136. Peptic digestion. — In digestion the proteins are first subjected in the stomach to the action of the pepsin and hy- drochloric acid of the gastric juice. DIGESTION AND RESORPTION 97 The products of peptic digestion are usually substances be- longing to the group of derived proteins (58, 59). The first product or products are substances called metaproteins, or, according to the older terminology, syntonin or acid proteins. By still further action there is formed a succession of proteoses and from these, by subsequent cleavage, peptones. Undoubt- edly the products resulting from peptic digestion contain a large number of chemical individuals but for the present pur- pose it. is sufficient to say that the action of pepsin and hydro- chloric acid gives rise to the formation of a series of progres- sively simpler, more soluble and more diffusible substances. In natural digestion, the action extends in the main only as far as the production of peptones, although polypeptids seem to be also formed to some extent. Amino acids are not found among the products of natural peptic digestion, although they may be produced by the long continued action of pepsin-hy- drochloric acid in artificial digestion. The conjugated proteins are split into their two constituents and the protein portion is then acted upon like other proteins. The gastric juice has no action upon the nucleic acids of the nucleoproteins. 137. Tryptic digestion. — In the duodenum, the proteins and the products of their peptic digestion are subjected to the action of the trypsin of the pancreatic juice. This is produced in the pancreas in the form of a pro-ferment or zymogen, called trypsinogen. The presence of pancreatic juice in the duodenum stimulates the glands of the latter to the production of the in- testinal juice which Pawlow has found to contain a substance, enterokinase, which activates the trypsinogen, or converts it into trypsin, in some unknown manner. The action of trypsin, like that of pepsin, has been largely studied in laboratory experiments either with extracts of the pancreas or with its secretion as obtained from fistulae. Tryp- sin, especially in a neutral or alkaline solution, acts upon pro- teins substantially in the same manner as pepsin, causing a hydrolytic cleavage and producing at first proteoses and pep- tones. It acts much "more energetically than pepsin, however, and carries the cleavage much further. The action of pepsin substantially stops with the production of peptones. Trypsin, on the other hand, produces a relatively large amount of the 98 NUTRITION OF FARM ANIMALS simple amino acids out of which the protein molecule is built up. Even the most prolonged action of trypsin, however, leaves a considerable residue in which no free amino acids are found but which on hydrolysis with strong mineral acids yields them in abundance. Conjugated proteins seem to be acted upon by trypsin in the same manner as by pepsin but much more energetically. 138. Erepsin. — The presence of a proteolytic enzym in the intestinal juice and in the epithelial cells of the small intes- tine has already been noted (135) . This enzym does not act on unaltered proteins, with the exception of casein, but it hy- drolyzes proteoses and peptones energetically, yielding crystal- line cleavage products. It is of special interest to note that, according to Cohnheim,1 erepsin is capable of effecting the cleavage of that part of the protein molecule which is not at- tacked by pepsin and trypsin and that in artificial digestion experiments almost complete conversion into comparatively simple crystalline products may be obtained in a relatively short time. 139. Extent of protein cleavage in natural digestion. — When it was first shown by Kiihne and Chittenden that the action of trypsin upon proteins yielded among other products such simple crystalline substances as leucin and tyrosin, com- paratively little importance was attached to the fact from the physiological standpoint. While the fact was interesting as throwing light upon the chemical structure of the proteins, it was believed that in natural digestion the soluble proteoses and peptones were promptly removed from the digestive tract by resorption and that at most but a small proportion of the feed protein underwent any profound cleavage. This belief was the stronger because it was believed that only proteins or their slightly altered derivatives, the proteoses and peptones, could supply the demands of the organism for proteins. Whatever protein was broken down into crystalline products was looked upon as wasted. With the progress of investigation, however, it has become increasingly clear that the processes of hy- drolytic cleavage go further and play a much more important part than was formerly supposed. While it is admitted that peptones, or even soluble proteins, may be resorbed, the weight 3 Ztschr. Physiol. Chem. 49 (1906), 64; 51 (1907), 415. DIGESTION AND RESORPTION 99 of opinion is that, as a matter of fact, proteins are largely re- sorbed in the form of comparatively simple cleavage products ; not necessarily in every case as simple amino acids but at least in the form of comparatively simple peptids. The nucleic acids derived from the peptic or tryptic diges- tion of the nucleoproteins are split by the nucleases of the in- testinal juice into mononucleotids and these again by the nucleotidases of the same secretion into nucleosids (53). No digestive enzyms attacking the latter class of compounds are known, but they are split to some extent by intestinal bacteria into pentoses and purin or pyrimidin bases. Furthermore, it has been found that extracts of the intestinal mucous membrane (epithelial cells) possess the power of bringing about the same cleavages which are accomplished by the enzyms of the in- testinal juice, and in addition are able to split the resulting nucleosids into pentose and base. It appears, then, that the final digestive products of the nucleic acids are, as in the case of the simple proteins, relatively simple substances, viz., phos- phoric acid, pentoses, and purin and pyrimidin bases. 140. Putrefaction of proteins. — Attention has already been called, in connection with the digestion of the carbohydrates, to the bacterial flora of the digestive tract. The carbohy- drates, as was shown, are acted upon chiefly by the organisms producing the methane fermentation. Proteins and their de- rivatives, on the other hand, have been shown by Kellner to contribute practically nothing to this fermentation in the case of cattle. They are, however, especially subject to the action of the organisms producing putrefaction. The action of such organisms is prevented in the stomach by the hydrochloric acid of the gastric juice. In the small intestine they become more active, especially as the feed reaches the lower portion, while their activity lessens again as the lower portion of the large in- testine is reached, owing to the progressive resorption of water from the intestinal contents. The characteristic products of the putrefaction are ammonia and certain aromatic compounds derived from the heterocyclic components of the proteins (47). The aromatic products of putrefaction (indols and phenols) are found in part in the feces but are in large part resorbed. They cannot, however, be utilized by the organism but, on the contrary, are poisonous and are therefore combined with other 100 NUTRITION OF FARM ANIMALS substances which render them innocuous. In particular, they unite with sulphates to form the so-called ether-sulphates which are excreted in the urine. The amount of these substances in the urine furnishes a convenient index to the extent of intestinal putrefaction. 141. The non-proteins. — As ordinarily determined (61, 106), the non-proteins constitute a group of nitrogenous sub- stances soluble in water, many of which are identical with or closely related to the final products of the digestion of the proteins. Accordingly, they have generally been assumed to be ready for resorption without further action by the digestive juices and therefore to be entirely digestible. It has been shown, however, that, in ruminants at least, the matter is by no means so simple as the mere resorption of water- soluble substances. In the capacious first stomach of these animals, the non-proteins play an important role as a supply of nitrogenous food for the organisms which are so abundant there. This has several consequences. In the first place, it appears that these soluble compounds are more readily attacked and utilized by the organisms than are the true proteins of the feed. The presence of non-proteins, therefore, tends to protect the proteins from bacterial decom- position. In the second place, an abundant supply of soluble nitrogenous matter stimulates the multiplication and activity of the or- ganisms and so brings about a more extensive fermentation of the carbohydrates of the feed, as is evidenced by an increase in the methane given off and in the proportion of the carbo- hydrates apparently digested. Third, it seems to be fairly well made out that the nitrogen which these organisms assimilate is utilized to build up their protoplasm and thus, by a sort of symbiosis, becomes a source of protein to their host. It has been claimed that this bacterial protein is indigestible, but the evidence on which this claim is based is capable of a different interpretation and there seems to be good reason for believing that it may be acted on in the stomach and intestines by the digestive enzyms like other pro- teins and serve as a source of protein to the body. Some of the evidence in favor of this view is presented in a subsequent discussion of the nutritive value of the non-proteins (786-789). DIGESTION AND RESORPTJON^ ' IOI The digestion of ash The various digestive enzyms whose action has been con- sidered in the foregoing pages bring about extensive chemical changes in the organic nutrients of feeding stuffs by means of which they are prepared to enter into the nutritive processes in the tissues. At the same time, the so-called " inorganic " ingredients of feed are also prepared for resorption, but the di- gestion of these substances has been less extensively studied than that of the organic nutrients. 142. Sulphur and phosphorus. — As regards the sulphur of the proteins, it does not appear that this element is separated from its union with carbon, nitrogen and hydrogen in the pro- cesses of protein digestion. The sulphur of the proteins is con- tained in the amino-acid cystin, which, so far as known, is resorbed without further change. As regards the phosphorus of the nucleo-proteins, opinions still differ as to whether it is split off as phosphoric acid in the course of digestion or resorbed, still in "organic" combination, as a nucleosid. To what extent other ash ingredients are taken up, like sulphur and phosphorus, in organic combination, it is difficult to say, but that such re- sorption takes place is to be regarded as probable. 143. Electrolytes. — As regards those ash ingredients of feeds which are present as electrolytes, it may be assumed that they are brought into solution to a greater or less extent by the hydrochloric acid of the gastric juice, but how much reprecipi- tation may occur in the more or less alkaline contents of the intestine it is difficult to say. The whole subject of the diges- tion, of the ash ingredients of feeding stuffs, however, is so in- timately related to the question of the paths of excretion and to the problems of ash metabolism that it can be more profitably considered in that connection. Summary of changes in digestion 144. Solution of nutrients. — The substances which make up the larger part of the feed of domestic animals (and of man as well) are comparatively insoluble in water. Some of them, such as cellulose and the fats, may be regarded as practically entirely insoluble. Others, like starch and the proteins, are 102 i^TiTLDN OF FARM ANIMALS sparingly s-X : While small amounts of soluble proteins and somewhat larger quantities of soluble carbohydrates occur, they ordinarily play but a subordinate role in nutrition. One obvious result of the chemical changes brought about by the enzyms and organized ferments of the digestive tract is to convert these insoluble substances into soluble ones. Thus starch yields sugar, cellulose the organic acids, fats form soaps and protein yields peptones and amino acids. It was natural, therefore, that digestion should be looked upon as a process of solution and compared to the preparation of extracts in a pharmaceutical laboratory by means of various solvents. The solvent action of the digestive juices is important, since the animal, like the plant, absorbs its real food substances substantially in aqueous solution. The mere dissolving of the ingredients of the feeds, however, is far from being the only or even the chief function of the digestive juices, as is clearly indicated, for example, by the existence of a coagulating enzym like chymosin, which precipitates the soluble casein, or the pres- ence of the various invertases, which attack substances already soluble. 145. Colloids converted into crystalloids. — The principal nutrients belong to the class of substances called colloids. Gelatin is a typical colloid as are, indeed, all the proteins and the more abundant carbohydrates, while the sugars, organic acids, etc., are classed as crystalloids. As related to digestion, the most important distinction be- tween colloids and crystalloids is the difference in the osmotic pressures of their solutions by virtue of which crystalloids diffuse readily through permeable membranes. This diffusi- bility plays an important part in the resorption of the digested material into the blood and lymph current, as will appear in the next section, although it is by no means the only factor con- cerned. A review of the chemical changes which take place in diges- tion shows that they are all in the direction of molecular simplifi- cation. They are substantially processes of cleavage by which large molecules are split into two or more smaller ones. Such a change, however, is in the direction from the colloid to the crystalloid condition. The final products of digestion are DIGESTION AND RESORPTION 103 mostly substances of comparatively low molecular weight, readily soluble in water and having a relatively high osmotic pressure and therefore readily diffusible. This difference is most marked in some of the more simple cleavage products of the proteins and least so in the case of the digestive products of the fats. 146. Uniformity in nutritive material. — The feed consumed, especially by herbivora, is of a very heterogeneous character. The proteins and carbohydrates in particular are present in great variety, so that the nature and proportions of the sub- stances out of which the body must draw the material for the construction and maintenance of its tissues may vary greatly at different times. Under the action of the digestive enzyms, however, these diverse substances all yield substantially the same products so that the nutritive solution supplied to the body proper is qualitatively of a very uniform composition, contain- ing chiefly monosaccharids, various acids of the aliphatic series, amino acids derived from the proteins, and the soluble ash in- gredients. By this preliminary action upon the feed in the digestive canal, — i.e., practically outside the body, — the organism is rendered independent of the particular kinds of feed available, its cells being constantly supplied with uniform nutritive material. 147. Molecular simplification. — It has just been pointed out (145) that digestion from the chemical standpoint consists substantially of a series of hydrolytic cleavages of the nutrients, yielding compounds of lower molecular weight and greater solu- bility and diffusibility. This molecular simplification has, however, a more important aspect which is most strikingly illustrated in the case of the proteins. It was shown in Chap- ter I that the proteins, although very similar in many physical properties, may differ widely from each other as regards molecu- lar structure. This is shown in the first place by the wide variations in the proportions of the constituent amino acids which they yield on hydrolysis (50). Moreover, even were these cleavage products present in the same proportions, the existence of optical isomers and the possible variations in the order and manner of linkage of the amino acids afford almost endless possibilities of isomerism. Studies in immunity have in fact revealed striking specific differences between proteins 104 NUTRITION OF FARM ANIMALS bearing the same name but derived from different species or different individuals of the same species, the proteins of one animal often being toxic to another. The body proteins, then, are specific both as to composition and structure and differ in both respects from those of the feed. In order that the latter may nourish the organism they must be converted into the specific proteins of the body. This is accom- plished through their cleavage in the digestive tract into their constituent " building stones " which the body may then reassemble to form proteins constructed according to its own specific pattern. Not only so, but the proteins of different tissues or even cells must be regarded as specific. The body proteins are built not after a single pattern but after numerous ones. It is only by a very extensive, even if not complete (139), breaking down of the structure of the feed proteins that it becomes possible for the body to build up out of the fragments the various proteins which it requires. "Its protein mole- cules have a different architecture from those of the plant." This fact throws an interesting light upon the coagulation of the soluble casein of milk in the stomach. Although present in soluble form, it is not a body protein and its coagulation serves to retain it in the digestive tract and give the proteolytic enzyms an opportunity to break it up into its constituent amino acids. What is so strikingly true of the proteins is likewise true of other nutrients. The digestive cleavages serve not merely, or perhaps not chiefly, to render them soluble and diffusible but to reduce the molecular complexes to forms which the body cells can assimilate. The carbohydrates, e.g., are converted into monosaccharids, even the somewhat larger molecules of the disaccharids appearing to be too large or to have an un- suitable molecular structure for the body cells to use. In general the complex compounds of the feed are split up by the enzyms of the digestive fluids into their constituent atomic groupings or " building stones " which supply the material out of which the body by selection and rearrangement builds up the proteins, carbohydrates and fats peculiar to itself, and the value of a feed depends upon the nature and amounts of the cleavage products which it yields in digestion rather than upon the specific substances which it contains. DIGESTION AND RESORPTION 105 § 3. RESORPTION — THE FECES 148. Definition. — As was 'stated at the beginning of this chapter (113), the digested feed contained in the alimentary canal is really outside the body, just as in the case of the ameba. In order to enter the body, the digested material must pass through or be taken up by the cells surrounding the digestive cavity. The process by which the products of the digestion of the feed are transferred from the digestive organs to the circulat- ing media (blood and lymph) of the body is called resorption. 149. Epithelium. Villi. — The inner, or mucous, membrane of the digestive tract bears on its surface a layer of epi- FIG. 14. — Section of villi. (Bohm, Davidorf, Huber, Text Book of Histology.) thelial cells, more or less resembling those lining the mouth, which is closely underlaid with a network of blood capillaries and lymph vessels. It is these epithelial cells which are the active agents in resorption. 106 NUTRITION OF FARM ANIMALS In the higher animals the extent of resorbing surface is greatly increased by certain projections of the interior surface of the small intestine known as the mill. Those are round or club- shaped protuberances of the inner surface of the intestine. They are covered, like all parts of the intestinal surface, with the epithelial cells just described, which are underlaid by a deli- cate membrane, beneath which are found numerous minute capillary blood-vessels, a layer of smooth (involuntary) mus- cular fibers and a network of nerves. In the center of each villus ends a vessel called a lacteal, belonging to the lymphatic system. Figure 14 shows a longitudinal section of three villi. The villi are absent in the stomach and in the large intestine. Although some resorption takes place in the stomach, and while a considerable amount of water and more or less of the fermen- tation products are resorbed in the large intestine, the small intestine is the special resorptive organ. 150. Mechanism of resorption. — Since the processes of digestion are apparently directed toward the conversion of feed substances into soluble and diffusible forms, it was quite natural that resorption should be regarded as an osmotic process. In this conception of it, the epithelial cells and other tissues be- tween the cavity of the digestive organs and the blood and lymph vessels constituted a membrane through which osmosis took place. On the one side of this membrane were the contents of the digestive tract, containing the soluble products of, diges- tion, while on the other side were the blood and lymph, contain- ing little or none of these products. Under these conditions, osmosis was assumed to set in and transfer the digested nutrients to the blood and lymph. Undoubtedly osmosis plays an important part in resorption, but its effects are profoundly modified by the properties of the resorbing cells of the intestinal epithelium in ways which as yet are but very partially understood, and resorption can by no means be explained by a simple analogy with the parchment dialyzing -tube of the laboratory. Differences in the permeability of the epithelial cells and of the intercellular substance for the various dissolved substances in the digestive tract doubtless play their part in bringing about the phe- nomena of selective resorption, while variations in the affinity of the cell colloids for water may be assumed to influence the resorp- DIGESTION AND RESORPTION 107 tion of that substance as well as of salts. There are other facts, however, for which it is difficult at present to offer any physico- chemical explanation. Notable among these is the predominant per- meability of the intestinal epithelium in one direction, viz., from the intestinal lumen towards the blood and lymph vessels. For the present, it is necessary to be content with the state- ment that resorption is a function of the living epithelial cells, although such a statement, of course, explains nothing but simply means that it is impossible at present to form an adequate conception of the intimate mechanism of the process. Resorption might be characterized briefly as a reverse se- cretion. In secretion the active cells of a gland take materials from the blood or lymph, transform them into the specific substances characteristic of the cells, and then eject the latter into the duct of the gland. The epithelial cells of the digestive tract, on the other hand, take up digested materials from the contents of the alimentary canal, modify them more or less chemically and transmit the products to the blood or lymph. 151. Paths of resorption. — Most of the resorbed substances seem to pass from the epithelial cells to the blood in the capil- laries and thus finally through the portal vein (182) to the liver. This is true of the cleavage products of the proteins, of carbo- hydrates, organic acids, salts and water. Fats, on the other hand, enter the circulation chiefly or wholly through the lymph in the form of minute droplets which are secreted by the epi- thelium into the lacteals of the villi, whence they pass through the lymphatics to the thoracic duct (186). 152. Chemical changes in resorption. — It is somewhat generally believed that the products of digestion, especially of the proteins and fats, undergo rather extensive chemical changes in the epithelial cells during the process of resorption. This question is considered more particularly in Chapter V but may be briefly referred to here for the sake of completeness. Proteins. — In digestion the proteins yield comparatively simple cleavage products. It has been maintained, especially by Abderhalden and his school, that these cleavage products are resynthesized in the epithelial cells into serum albumin, which is regarded as the common source of all the body proteins. This view has been based chiefly on the failure to detect amino acids or other protein cleavage products in the blood coming I08 NUTRITION OF FARM ANIMALS from the intestines even during the height of protein resorption. Folin and Denis and also Van Slyke and Meyer have recently demonstrated, however, that sufficiently delicate tests show the presence of such products in the portal blood in amounts as large as could be expected in view of the gradual nature of digestion and resorption and of the large volume of blood pass- ing through the intestinal capillaries. The prevailing opinion seems to be at present that the digestive products of the pro- teins undergo relatively little modification before entering into the circulation. Fats. — The mechanism of fat resorption has been the subject of heated controversy. Some physiologists have maintained that it is chiefly a physical process ; that globules of emulsified feed fat are taken up bodily by the epithelial cells and excreted again by them into the lacteals. This view is based largely on microscopical observations which show the presence of ap- parently unaltered fat globules of the intestinal emulsion in the protoplasm of the epithelial cells and in the lymph of the lacteals after the ingestion of fat. Other no less eminent physi- ologists, however, have as stoutly held that fats are not resorbed unaltered but only after cleavage into glycerol and fatty acids (or their salts), which are soluble in bile (135), and that the fat globules observed in the epithelial cells are the product of a resynthesis. At present the weight of scientific opinion is strongly in favor of this latter view. It is perhaps true that unaltered fat droplets may be taken up by the epithelial cells but that any considerable amount of fat is resorbed in this fashion is to say the least very questionable. At any rate, the digested fat reaches the lacteals almost en- tirely in the form of fat, so that after a meal containing much fat the lacteals are filled with a milky fluid and the lymph is found to contain relatively large amounts of neutral fats. It is clear, then, that the resorbed soaps and fatty acids are speedily synthesized to fat again. This synthetic power is still further and strikingly demonstrated by the fact that free fatty acids are readily digested but are transmitted to the lacteals in the form of the corresponding neutral fats, having evidently been combined with glycerol in the process of resorption, although the source from which the body derives its glycerol is still un- certain. Evidently, then, from this point of view, nothing DIGESTION AND RESORPTION 1 09 stands in the way of the supposition that the digested fats are completely split up into glycerol and fatty acids in the process of digestion and synthesized again in the epithelial cells, although, on the other hand, of course, it does not prove that such is the case. 153. The feces. — As the contents of the digestive tract move forward through the small and large intestines they become progressively more and more impoverished in digestible material and also, in the lower portion of the large intestine, are deprived of part of their water, so that there accumulates in the rectum a more or less solid residue which is voided at intervals as the feces. The feces are to be regarded as both an excretory product (198) and a feed residue. 154. The feces as an excretory product. — The fact that the feces are an excretory product is most obvious in the carnivora, whose normal feed consists of substances almost wholly diges- tible, but it is evident also in man. On a pure meat diet, for example, feces continue to be produced in which undigested feed residues are either absent entirely or present in minimal amounts only. Even a fasting animal continues to produce feces, while an empty loop of the intestine, separated from the remainder of the digestive tract, soon fills up with fecal-like material. The excretory ingredients of the feces include unresorbed digestive juices and their decomposition products, intestinal mucus, worn-out epithelial cells and cell fragments, leucocytes and excretions of the intestinal mucosa. Especially notable among the latter are salts of calcium and of iron and in herbivora the phosphates of calcium and magnesium. The feces also include a not inconsiderable proportion of intestinal micro- organisms. 155. The feces as a feed residue. — The ordinary mixed diet of man, and to a much more marked degree the ordinary feed of herbivorous animals, contains relatively considerable amounts of materials which are either indigestible or which for one reason or another escape digestion and therefore reappear in the feces. Among these, some, like lignin, cutin, the waxes, chlorophyl and other non-fatty ingredients of the " ether extract," and the insoluble ash ingredients, may be regarded 110 NUTRITION OF FARM ANIMALS as wholly indigestible. Of more importance, however, are such carbohydrates as cellulose and the various hemicelluloses, the levulans, galactans, mannans, pentosans, etc., which may be said to be practically only partially digestible. By this is not meant, of course, that one molecule of cellulose, e.g., is any less digestible per se than another, but only that part of the cellulose of ordinary feeds does as a matter of fact escape digestion, largely because the length of time during which it is exposed to the action of the organisms which attack it is insufficient to allow of its complete solution. The feces of herbivorous animals, therefore, contain amounts of these di- verse carbohydrates varying with the character of the feed and the activity of the fermentation processes in the digestive tract. Since these compounds are especially abundant in the roughages, the feces from these feeds are bulky and especially rich in un- digested " crude fiber." Other ingredients, particularly of vegetable feeding stuffs, partially escape digestion not on account of any lack of the ap- propriate digestive enzyms but because they are mechanically protected from the action of the latter. If granules of starch, e.g., are contained within a cell which has not been ruptured during the mastication of the feed, the cell wall tends to protect them from the action of the digestive juices, and they may escape digestion although per se entirely digestible. The extent to which such a nutrient will actually be digested, therefore, will depend to a considerable degree upon whether the cellulose of the cell wall is attacked and destroyed by the organisms of the alimentary canal. What is true of starch in this respect is obviously true of all cell enclosures, and explains why more or less matter intrinsically digestible may be rejected in the feces. For a like reason, seeds which escape mastication are but im- perfectly digested, being protected by the relatively insoluble seed coats. Similarly, cellulose itself may be so impregnated with lignin and cutin substances that the " crude fiber " may be attacked only with difficulty or not at all by the methane fermentation. Finally, there is to be considered the possibility of a mis- proportion between digestion and resorption. In heavy rations, especially, substances which are actually digested may per- haps escape resorption through insufficient contact with the DIGESTION AND RESORPTION III intestinal walls or from lack of time, and so be found in the feces. 156. Composition of feces. — Evidently the feces are a very complex and variable mixture, including, on the one hand, the various excretory products just enumerated and, on the other hand, indigestible feed substances 'and digestible materials which have for one reason or another escaped actual digestion or which, having been digested, have failed of resorption. Among the latter may be included unresorbed products of the putrefaction of the proteins, especially skatol, which impart to the feces their offensive odor. The proportions of these two groups — the excretory products and the feed residues — in the feces vary widely with the nature of the feed consumed. In the carnivora the body wastes predominate, so that the feces of these animals are to be regarded as primarily an excretory product. To a considerable degree the same thing is true of man, especially when living on a con- centrated diet. With the herbivora, on the contrary, the in- digestible or undigested feed residues constitute the bulk of the feces, although the amount of true excretory products is by no means insignificant. Omnivora like the hog occupy an intermediate position in this respect. § 4. THE DETERMINATION or DIGESTIBILITY 157. Definition of digestibility. — The words digestible and digestibility are used in more than one sense. Sometimes, for example, a food is said to be digestible because it is easily di- gested — that is, causes no unpleasant sensation after it is eaten — while by an indigestible food is meant one that is apt to cause gastric or intestinal disturbances. Again, it is not un- common to judge of the digestibility of stock feeds by their effects and to regard that one as the more digestible which causes or seems to cause the greater gain in weight. The word digestibility as used in the study of animal nutrition, however, has a definite and limited meaning. It denotes the percentage of the feed, or of any single ingredient of the feed, which is dissolved or otherwise acted on in the digestive canal so that it can be resorbed and thus put at the disposal of the body cells. For example, the digestion experiment with a steer 112 NUTRITION OF FARM ANIMALS described in a subsequent paragraph (160) showed that out of each 100 grams of protein in the clover hay eaten by the animal 53 grams were apparently digested and resorbed. The digesti- bility of the protein in this case, therefore, is said to be 53 per cent. Digestibility in this sense is obviously a conception entirely distinct from that of rapidity or ease of digestion. A feed may have no injurious nor disagreeable effects and yet may have a low percentage digestibility — straw, for example. Neither does the percentage digestibility alone determine the effect produced by a feed. Two feeds may be equally di- gestible and yet one may be more valuable than the other be- cause its digested matter can be used to better advantage by the body. Nevertheless, it is clear that the indigestible portion of the feed can make no contribution to the nutrition of the body. The first step, therefore, although by no means the last one, in comparing the values of different feeds or rations is to determine as accurately as possible what proportion of each ingredient is capable of digestion. In other words, we shall seek to add to the qualitative knowledge of the processes of digestion and resorption outlined in the preceding sections a quantitative knowledge of the extent of digestion. 158. Method of digestion experiments. — The percentage digestibility of feeding stuffs can, as a rule, be determined only by trial with an animal. Such trials are called digestion ex- periments, and a brief outline of the way in which they are made will aid in understanding just what is meant by digestibility. The method is substantially that originated by Henneberg and Stohmann in their early investigations (707). Since it is obviously impossible to collect and measure the substances digested and resorbed from the feed, it is necessary to have recourse to an indirect method, viz., to determine what portions of the feed are not digested and compute by difference the amounts digested. As already stated (155), the undigested portions of the feed are excreted in the feces. A digestion experiment really consists in determining as exactly as may be how much of the feed is thus rejected, any portions of it which disappear during its passage through the alimentary canal being regarded as digested. If the feces consisted only of undigested feed residues, the matter would be very simple, but they also include a greater or DIGESTION AND RESORPTION less proportion of excretory products (154). In the case herbiyora, the proportion of the latter is relatively small and in the digestion experiment as ordinarily conducted is neglected, it being assumed, in other words, that the feces are equivalent to undigested feed residues. The same method is pursued in digestion experiments with swine, although in these animals £-- the proportion of excretory products in the feces is larger. The error thus introduced into digestion experiments is not negligible, especially as regards certain ingredients. It will be convenient, however, to take up first the methods of digestion experiments as ordinarily conducted and to consider later the nature and extent of the errors introduced by neglecting the presence of the excretory products. 159. Time required for digestion experiments. — It is es- sential that a digestion experiment be preceded by a prelimi- FIG. 15. — Steer in digestion stall. (Bailey's Cyclopedia of American Agriculture.) nary period in which the feed to be investigated is fed in the same weighed amounts daily as in the actual experiment. This is for the purpose of removing from the digestive tract residues 114 NUTRITION OF FARM ANIMALS of previous feeds and also of establishing as uniform a rate of excretion of feces as practicable. In the case of ruminants, such a preliminary period should extend over one or two weeks, while with swine it may be made somewhat less. In the succeeding digestion experiment proper, the same feeding is continued and the feces are collected quantitatively for a number of days (seven to ten or more), in order to eliminate the error due to the irregularity of the excretion from day to day. From the weights of feeds and feces and their composition as determined by analysis, the digestibility is computed as illustrated in the following paragraphs. 160. Example of a digestion experiment. — A steer was fed 3.7 kilograms of clover hay per day for three weeks. During the last ten days of this time, the average weight of the daily feces was 5.662 kilograms. Samples of each were analyzed and found to contain the following percentages of dry matter. Clover hay 84.97 Per cent Feces 22. 36 per cent The 3.7 kilograms of hay, therefore, contained 3.144 kilograms of dry matter while the 5.662 kilograms of feces excreted contained only 1.267 kilograms of dry matter. The difference, 1.877 kilograms, which did not appear in the feces, is regarded as having been digested by the steer. This amount is 59.7 per cent of the 3.144 kilograms eaten; we say, then, that the percentage digestibility of the dry matter of this hay was 59.7 and this number is sometimes called its "digestion coefficient." In precisely the same way the percentage digestibility of each in- gredient may be computed from the results of analyses of the hay and of the feces, which in this case gave the following results : — HAY FECES % % Water 15.03 77.64 Ash 5.49 1.92 Protein 10.24 3.13 l Non-protein 1.36 Crude fiber 28.61 9.29 Nitrogen-free extract 36.98 7.50 Ether extract 2.29 0.52 100.00 100.00 1 All the nitrogen of the feces is here assumed to exist in the form of protein, an assumption which, as will appear later, is far from being true (166), but which does not affect the method of computation. DIGESTION AND RESORPTION These figures, together with the weights of hay eaten and feces excreted per day, yield the following results : — TABLE 20. — RESULTS OF A DIGESTION EXPERIMENT NITRO- DRY MATTER ASH PRO- TEIN NON- PROTEIN CRUDE FIBER GEN FREE I EX- ETHER EX- TRACT TRACT Kgs. Kgs. Kgs. Kgs. Kgs. Kgs. Kgs. In hay eaten . . . 3-J44 0.203 0-379 0.050 1.059 1.368 0.085 In feces excreted . . 1.267 0.109 0.177 — 0.526 0.425 0.030 Difference = digested 1.877 0.094 O.2O2 0.050 0-533 0.943 0-055 Percentage digesti- bility CTQ.7O 4.6.4.8 c? 19 IOO.OO 50.27 68 04. 65.02 161. Digestibility of concentrates. — The method just outlined for determining the digestibility of a roughage or of a total ration is in conception very simple. The determination of the digestibility of concentrates by herbivora is somewhat more complicated, since they cannot be made the sole feed of these animals. They must, therefore, be fed along with a known amount of a roughage whose digestibility by the same animal is likewise determined in a preced- ing or following period. From the digestibility of the total ration and the known digestibility of the roughage that of the concentrate is obtained by means of a second calculation by difference. Thus, the same steer used in the experiment of the preceding para- graph received per day in a subsequent period the same amount, 3.7 kilograms, of clover hay and in addition 4 kilograms of maize meal. The average daily excretion of feces on this mixed ration was 8.715 kilograms. An analysis of the clover hay used in this period showed but slight variations from that of the preceding period. The com- position of the maize meal and of the feces was : — Water Ash Protein .... Non-protein . . . Crude fiber . . . Nitrogen-free extract Ether extract FECES % 81.91 1.77 3.66 6.51 5-71 0.44 Il6 NUTRITION OF FARM ANIMALS The digestible matter contained in the total ration, computed exactly as in the previous example, was as shown in the first part of Table 21. If, now, it be assumed that the digestibility of the clover hay was unaltered by the addition of the maize meal, it is possible to compute how much of each kind of digestible matter (pro- tein, crude fiber, nitrogen-free extract, etc.) in the total ration was derived from the hay; the remainder, therefore, must have come from the maize meal and by comparison with the total amounts present in the latter the percentage digestibility is computed. It is evident that the determination of the digestibility of a con- centrate in this way is less accurate than that of a feed which can be given by itself. The assumption that the digestibility of the roughage is not changed is unproved and probably not strictly correct. More- over, any errors arising from this source and likewise all the errors in weighing and analysis are, by the method of calculation, assigned to the concentrate. The writer has shown 1 that the range of uncer- tainty thus introduced may be very wide. It will evidently be greatest when the proportion of concentrate to roughage is least and will affect most those ingredients of the concentrate which are present in the smallest proportion, such as crude fiber and often ether extract. In extreme cases, absurd results are sometimes obtained, such as negative digestibility or a digestibility greater than 100 per cent. 162. Laboratory determination of digestibility. — Actual digestion experiments upon animals according to the method just outlined, while simple in principle, require special facilities and a considerable expenditure of time. It would obviously be very desirable to possess methods by which the action of the digestive fluids of the body could be imitated in the laboratory and the digestibility of feeds thus determined in a simpler and more expeditious manner. Numerous attempts have been made to solve this problem, but as yet a satisfactory method has been worked out only for protein, while attempts to devise similar methods for the non-nitrogenous ingredients of feeding stuffs have not yet proven successful. The method for protein is based upon suggestions made long ago by Stockhardt and by Hofmeister, but was first put into practical form by Stutzer.2 It consists in treating the feed with a solution of pepsin and hydrochloric acid under specified conditions and determining the undissolved nitrogen in the residue. The difference between this and the 1 Amer. Jour. Sci., 29 (1885), 35-5- 2 Jour. Landw., 28 (1880), 195 and 435. TABLE 21. — COM DIGESTIO PUTATION C h N AND RESORPTION F THE DIGESTIBILITY OF \O GO rt OO vO H 1000 II7 A CONCENTRATE r-» to rf JO CN oi 0 M CO ON CM M 00 CN O 00 & CO M II 3« O r- ts.«0 H H 0 0 COOO to r^ •<3- to H M M O co O O t- co M CN t^ CO - CN covO ]]l rj- vq ON Tj- O CN t^- CO CO co 00 •"*• CO* ON ON IO M t? ON IO CO ON M §u& . §SS3 OO O -t t- tr^ co to oo r- ON M COCO H CS co t^ M CO < w w ^00 CM •rh 00 co O to to l>- H 10 ON H co vO O CO to M rt- cO co ro t-- vO 10 O M to O 0 ON IO M • Digested from maize meal Percentage digestibility . . g ' ^ • • • --81 . . . . ».a x| | S 1 1 Il8 NUTRITION OF FARM ANIMALS total nitrogen of the feed represents, of course, the amount of nitrogenous matter which has been dissolved and which, there- fore, is regarded as digestible. Comparisons by Kellner,1 Pfeiffer,2 G. Kiihn 3 and others between the natural and artificial digestion of protein have shown that the former method gives lower results on account of the presence in the feces of nitrogenous excretory products (154, 158), but that when a correction is made for the latter in the manner indicated on a subse- quent page (166) the results of the two methods show a substan- tial agreement. In other words, the method of artificial digestion shows with a good degree of accuracy the true as compared with the apparent digestibility (163, 167) of the protein. 163. Influence of excretory products on apparent digesti- bility. — Since the digestion experiment as ordinarily conducted ignores the presence in the feces of excretory products, the re- sults obtained by its use will necessarily be too low, since sub- stances are reckoned as undigested ingredients which really are not such. Obviously, the ingredients most affected by this error will be those which, on the one hand, are contained in the feed in the smallest proportion and which, on the other hand, are relatively most abundant among the excretory products in the feces. These ingredients are, when the or- dinary scheme of feeding stuffs analysis is followed, ash, ether extract and 'nitrogenous substances. As regards the crude fiber, on the other hand, this error is absent, since obviously no crude fiber is included among the excretory products, and it seems probable that substantially the same thing is true of the nitrogen-free extract. 164. Digestibility of ash ingredients. — Certain ash in- gredients, particularly iron, calcium, magnesium and phos- phorus, are largely or wholly excreted from the body in the feces (199). Furthermore, the resorption of the ash ingredients of the digestive juices may not be complete and these residues may be added to the ash content of the feces. The ordinary digestion experiment, therefore, affords little information as to the extent to which the ash ingredients of the feed are actually 1 Centbl. Agr. Chem., 9 (1880), 763. 2 Jour. Landw., 33 (1885), 149; 34 (1886), 425. "Landw. Vers. Stat., 44 (1894), 188. DIGESTION AND RESORPTION 1 19 digested and resorbed and this fact constitutes a serious dif- ficulty in the study of the ash metabolism. 165. Digestibility of ether extract. — Among the excretory products contained in the feces are included ether-soluble substances, especially those derived from unresorbed bile con- stituents. While their total amount is small, the feed of farm animals is also usually poor in ether extract and consequently the error in the computation of the percentage digestibility may be relatively large. Indeed, not a few instances are on record in which the ether extract of the feces has exceeded that of the feed. Little definite knowledge is available, however, as to the actual extent of the error thus introduced, but it is of relatively less importance in view of the small role which fat plays in the ordinary rations of farm animals. 166. Digestibility of nitrogenous substances. — Most of the excretory products in the feces (154) are nitrogenous substances and it is particularly with reference to their influence upon the determination of the digestibility of the nitrogenous constituents of feeding stuffs that investigation has been active. That they may seriously affect it is evident from the results obtained in numerous experiments upon feeding stuffs poor in protein, such as straw, in which a negative digestibility of the crude protein has been observed, — i.e., in which the feces have contained more nitrogen than the feed. Moreover, experiments upon rations containing no nitrogen at all have shown that under these conditions nitrogen continues to be excreted in the feces. Various methods for distinguishing between the nitrogen of feed residues and the nitrogen of excretory products have been proposed at different times, but the one which has proved most satisfactory and which is generally accepted at present is based upon the solubility of the nitrogenous excretory products in the solution of pepsin and hydrochloric acid employed in Stut- zer's method for the laboratory determination of the digesti- bility of protein described in a previous paragraph (162). By treatment of a sample of the fresh feces with such a solu- tion under proper conditions the excretory nitrogenous products are dissolved, and it has been shown that very close agreement can be obtained between the artificial and natural digestion of protein if the comparison in the latter case be made upon the pepsin-insoluble nitrogen of the feces. In other words, the 120 NUTRITION OF FARM ANIMALS pepsin-insoluble nitrogen of the feeds appears quantitatively in the feces, where it may be regarded as representing indigesti- ble feed protein, while the pepsin-soluble nitrogen of the feces is contained in the excretory products, part of which are protein (mucus, epithelium, etc.) and part non-protein (residues of digestive fluids, etc.). An approximate correction for the amount of nitrogenous excretory products may also be com- puted by the use of Pfeiffer's factor of 0.4 gram nitrogen per 100 grams digested dry matter. 167. Apparent digestibility. — When the results of the or- dinary digestion experiment are corrected, in the manner just outlined, for the nitrogenous excretory products in the feces we get an approximation to the true percentage digestibility of the protein, while, as regards the carbohydrates, the error, as has been shown, is probably not serious, at least for herbivora. There is another way of looking at the matter, however. The intestinal products found in the feces are, in effect, part of the cost of digesting the feed. They represent the " wear and tear " of the digestive organs. The difference, then, be- tween feed and feces will show the net gain to the animal from the digestion of the feed, that is, it will show how much more proteins, carbohydrates, etc., the body has at its disposal than it would have had if the feed had not been given. From this point of view, we may speak of the digestibility as ordinarily determined as the apparent digestibility, and regard it as a measure (approximately at least) of the matter gained by the body from the consumption of the feed. For many purposes, therefore, the apparent digestibility gives a better basis for com- paring the values of feeding stuffs than does the real digestibility. It was from this point of view that Atwater 1 proposed the use of the term availability as the equivalent of what is here called apparent digestibility. 168. Composition of digested crude fiber. — The crude fiber (109) consists of the cellulose of the plant together with varying amounts of pentosans and of lignin and other incrusting substances, the ratio of which to the cellulose increases with the maturity of the plant. Cellulose itself seems to be attacked and dissolved with comparative ease by the organisms of the rumen and the coecum, and the same is probably true of the 1 Rpt. Conn. (Storrs) Expt. Sta., 1897, p. 156. DIGESTION AND RESORPTION 1 21 pentosans, but lignin appears to be much less readily digested and some of the other incrusting materials not at all. As a consequence, a computation based on the elementary composi- tion of the crude fiber of the feed and of the feces respectively and on the percentage of the former which is digestible shows the digested portion to have approximately the ultimate compo- sition and heat of combustion of cellulose. This is by no means equivalent to saying that the digested crude fiber consists only of cellulose. The variations between the results in individual experiments show clearly that this cannot be the case and doubtless more or less of the pentosans and other ingredients of the crude fiber are attacked to some extent, but it is nevertheless evident that the cellulose is the chief constituent digested. Neither is the heat of combustion of the digested portion in any sense a measure of the energy which it can supply for the bodily activities, as will appear more clearly later. 169. Composition of digested nitrogen-free extract. — By a difference calculation identical in principle with that em- ployed for crude fiber but somewhat more complicated in its details and involving certain assumptions, it has been shown that the digested portion of the nitrogen-free extract has also approximately the composition and heat of combustion of starch or cellulose. Even less than in the case of crude fiber does this fact serve to fix with any definiteness the nutritive value of the digested portion. We know that the nitrogen- free extract of feeding stuffs includes a great variety of sub- stances (110), some of which, like starch, are digested in the narrower sense of the word while many others, like the hemi- celluloses, pentosans, etc., are fermented rather than digested. The data as to the composition of the digested portion indicate, it is true, that it consists chiefly of carbohydrates, but on ac- count of the small range of ultimate composition shown by these substances no indications are afforded of the specific carbohy- drates present. 170. Digestible carbohydrates. — Since both the digested crude fiber and the digested nitrogen-free extract have approxi- mately an ultimate composition corresponding to the formula C6HioO5, it has become customary in estimating the nutritive values of feeding stuffs to add together the digestible portions 122 NUTRITION OF FARM ANIMALS of these two groups and to designate the sum as the " digestible carbohydrates." The practice dates from the early experi- ments of HennebergandStohmann, but in the light of our present knowledge has little justification. In the first place, as just stated, the agreement in composition is but approximate and variable. The essential point, however, is that a digestion experiment can show simply that a certain amount of material of a certain ultimate composition has failed to reappear in the feces of the animal, and by itself affords no information as to the changes which it has undergone nor as to the nature of the products actually resorbed. As a matter of fact, a large share of the " digested " portion of these two groups, especially in the case of ruminants, has been fermented rather than digested. A considerable proportion of it has been excreted in gaseous form as carbon dioxid and methane and only a residue of organic acids has been resorbed. Such being the case, the term digestible carbohydrates is a palpable mis- nomer. 171. Digested ether extract. — No determinations of the composition of the digested ether extract similar to those on crude fiber have been made, but a few determinations of the heat of combustion of the digested extract are reported by Kellner.1 The presence in the feces of ether soluble excretory products (165) interferes with the accuracy of such a comparison and its results must be regarded as approximations. The ether extract of the feces was found to have a higher heat of combus- tion than that of the hay fed, doubtless on account of the presence in the former of the indigestible waxes, etc., while the computed heat of combustion of the digested portion was distinctly lower than that for pure fats, which average about 9.5 Cals. per gram. The heats of combustion per gram on the average of five trials were : — Ether extract of hay .... 9.194 Cals. Ether extract of feces .... 9.824 Cals. Digested ether extract . . . . 8.322 Cals. 1 Landw. Vers. Stat., 47 (1896), 301. CHAPTER IV CIRCULATION, RESPIRATION AND EXCRETION1 § i. CIRCULATION 172. Distribution of nutrients. — The digestive changes by which the ingredients of the feed are prepared for the nutrition of the organism take place outside the body proper (113). In order that the products formed shall serve their purpose they must not only be taken up into the body by the processes of resorption described in the preceding chapter but they must be distributed through it, so that each of its myriad cells may receive the substances which it requires. The chief vehicle of this distribution is the blood, into which the resorbed nutrients are transferred, directly or indirectly, and the distribution is accomplished by means of the circulation of the blood, dis- covered by Harvey in 1621. 173. The blood. — This familiar but highly complex fluid serves a variety of purposes, being not only the carrier of the resorbed feed ingredients to the tissues and cells but transmitting to them the equally necessary oxygen and carrying away the products of their activity to be used in other parts of the body or to be excreted. The blood of the higher animals is a thickish, somewhat viscid fluid, having a faint but peculiar odor, a slightly salt taste and a color varying from bright to dark red. It is somewhat heavier than water (sp. gr. 1.045-1.075), and contains about 21 per cent of total solids. Under the microscope it is seen to consist of a clear fluid, the plasma, holding in suspension a vast number of small, solid bodies, the corpuscles. The latter are of two kinds, known as the red corpuscles, or ery throcytes, and the white corpuscles, or leucocytes. 1 Only such a very general consideration of the outlines of these functions as seems essential for a proper comprehension of the phenomena of metabolism and of the processes of nutrition is attempted here. For a more complete elementary discus- sion, the reader is referred to Hough and Sedgwick's The Human Mechanism, Chapters IX, X and XI, and for further details to the larger treatises on physiology. 123 I24 NUTRITION OF FARM ANIMALS 174. Red blood corpuscles. — These are by far the more numerous of the two kinds. In man they are round like "a coin but thicker at the edges than in the center, and have a diameter of 0.0060-0.0085 millimeter. Their number is enormous, being estimated at 4 to 5^ millions per cubic millimeter of blood. To them the color and opacity of the blood are due. The corpuscles of each species of animal are peculiar to it, both as to shape and size, but their general characteristics are the same in all. Those of most animalsaresmaller than those of man. Each corpuscle is a cell, having no nucleus but containing as its characteristic ingre- dient the conjugated pro- tein haemoglobin to which the red color of the blood is due. Haemoglobin is a crystalline substance and it has recently been shown by Reichert that the haemoglobin of each spe- cies of animal has its spe- cific crystalline form and properties. 175. White blood corpuscles. — The white corpuscles are colorless, nucleated cells which are not confined to the blood but which, by means of ameboid movements, are able to pass through the walls of the blodd vessels and the lymph spaces of connective tissue as the so-called " wander- ing cells." They have important functions, especially in protecting the body from disease, but need not be further considered here. 176. Blood platelets. — In addition to the two kinds of cor- puscles, the blood contains more minute nucleated cells, rang- ing in diameter from 0.0003-0.0005 millimeter, called blood platelets, or thrombocytes. They are much more abundant than the white corpuscles and are thought to be concerned in the coagulation of the blood. FIG. 16. — Blood corpuscles. Above are shown nine red corpuscles, highly mag- nified; below, less highly magnified, the appearance of the blood soon after being drawn. (Hough and Sedgwick, The Human Mechanism.) CIRCULATION, RESPIRATION AND EXCRETION 125 177. Blood plasma. — This very complex fluid contains, be- sides about 90 per cent of water, a great variety of substances, the most prominent of which are the proteins, of which two groups are recognized, viz., two or more serum globulins and the so-called serum albumin, which is probably not a single chemical individual. Plasma contains also approximately 0.1-0.15 Per cent °f dextrose, from o.i to as much as i.o per cent of fat, usually in some soluble form (243), a great variety of so-called extractives which are in part waste products of cell action, and about i per cent of mineral ingredients. 178. Coagulation. — When blood is drawn from the body it usually coagulates or clots within a few minutes. The coagu- lating substance is a globulin called fibrinogen and its coagulation is an enzymatic reac- tion brought about by a ferment, throm- bin, believed to be derived from the blood platelets by a very complicated process. The coag- ulated protein con- stitutes the so-called blood fibrin, which entangles within it- self the corpuscles, producing the famil- iar blood clot. While the clot is very bulky the dry blood fibrin amounts to only 0.2- 0.3 per cent of the weight of the blood. 179. The heart.— The blood is distrib- uted to all parts of FIG. 17. — Diagram of mammalian heart. a, Left ventricle, b, Right ventricle, c. Left auricle. d, Right auricle. /, Aorta. gg, Pulmonary arteries. - , , , - op.. Pulmonary veins. (Smith, Physiology of the Domestic the body by means ol Animals.) a most interesting organ, the heart, which is in effect a living force pump. Figure 17 shows diagrammatically the structure of the mam- 126 NUTRITION OF FARM ANIMALS malian heart, which is substantially the same in all farm animals. It is divided by an impervious partition into a right and left half, and each of these is subdivided by a cross partition into two chambers, communicating with each other by a valve in the dividing wall. The upper and smaller of these divisions are known as the right and left auricles, and the lower and larger as the right and left ventricles. Into these cavities of the heart open several large blood vessels, whose mouths are closed with valves so arranged that the blood can only flow into the auricles and out of the ventricles. 180. Arteries. — The blood vessels which conduct the blood from the heart to the various organs of the body are called arteries and may be described as tubes with strong, elastic and contractile walls, to withstand the force with which the blood is pumped into them by the heart. Their walls consist of an outer layer of elastic and connective tissue, a middle layer of muscular tissue and an inner layer of epithelium. The ar- teries originate in the aorta (h, Fig. 18), which receives the blood from the left ventricle, and as they extend farther and farther from the heart subdivide and throw off branches to the various organs, the more minute of which are called arterioles, finally ending in the capillaries. 181. Capillaries. — The capillaries are exceedingly minute blood vessels which penetrate all the tissues of the body and form the connecting link between the arteries and veins. Their walls are thin and delicate, and through them the nutritive mat- ters of the blood pass out into the tissues while the waste prod- ucts of cell activity pass from the tissues into the blood. In Fig. 1 8, n represents the capillaries of the posterior part of the body, o those of the stomach and intestines, t those of the kid- neys, p those of the liver, and m those of the anterior part of the body. The capillaries gradually unite again into larger vessels, the veins, which convey the blood back to the heart and lungs. 182. Veins. — The veins are tubular vessels somewhat similar to the arteries but with weaker and non-elastic walls, the pres- sure of the blood on them being slight, owing to the interposi- tion of the capillaries between them and the arteries and to the fact that their total cross section is greater than that of the CIRCULATION, RESPIRATION AND EXCRETION 127 arteries. To prevent any possible flowing back of the blood, the veins are provided at intervals with valves which permit the blood to pass toward the heart but not in the opposite direc- tion. The smaller veins unite to form larger ones, and finally empty their contents through two branches into the right auricle of the heart. From the capillaries of the intestines the blood carrying the re- sorbed nutrients passes through the portal vein, s, to the liver, p, is there distributed through another system of capillaries and then rejoins the blood from the extremities through the hepatic vein, u. Into the branch, k, coming from the head and anterior parts of the body, the nutrients which are resorbed by the lacteals enter by way of the thoracic duct. 183. Course of the blood. - The blood returning through the veins from the extremities of the body to the heart enters first the right auricle (a, Fig. 1 8), through two large veins, k and /, coming from the an- terior and posterior parts of the body. The auricle then contracts and the blood, being prevented from returning into the veins by the valves at their mouths, is forced through the FlG- l8- ~ Scheme of circulation of , . ' ,, .... 11 •, blood. (Armsby, Manual of Cattle valve in the partition wall into Feeding.) the right ventricle, b. This, in turn, contracting, the blood, prevented as before by a valve from turning back in its course, is forced out of the 128 NUTRITION OF FARM ANIMALS ventricle into the pulmonary artery, c, which divides into two branches leading to the capillaries of the right and left lungs, d, d. The entrance to this blood vessel, like that of the others, is provided with a valve which prevents the return of the blood. The blood, after passing through the lung capillaries, returns to the left auricle, /, through the pulmonary veins, represented by e. The auricle then contracting, sends the blood into the left ventricle, g, which, in its turn, contracts powerfully and expels the blood into one large vessel, the aorta, h. The aorta, soon after leaving the heart, divides into two branches, i and j, and these repeatedly subdivide, forming the arteries which carry the blood to the arterioles and capillaries, whence it returns again through the veins to the right side of the heart. The passage of the blood from the left side of the heart through the body capillaries and back to the right side is called the greater or systemic circulation; that from the right side of the heart through the lung capillaries, the pulmonary cir- culation. The appearance of the blood in the veins and arteries is strikingly different. In the veins it has a dark, cherry-red color, but after it has passed through the lungs and is sent out by the heart to the arteries it has a bright scarlet color. The former is called venous, the latter, arterial blood. An exception to this rule, that the arteries carry bright red blood and the veins dark, is found in the pulmonary circulation, where, of course, the vessels leading from the heart to the lungs carry venous blood, and those leading from the lungs to the heart, arterial. Nevertheless, the general nomenclature is adhered to, and the former are called arteries and the latter veins. Ar- teries conduct the blood from the heart, veins toward it. 184. Mechanics of circulation. — While it is not uncommon to speak of the flow of the blood, or of the blood stream, sug- gesting an analogy to a brook or river, the circulation is not in reality a flow of this sort but resembles rather the movement of the water pumped into a hose by a force pump. The heart constitutes the force pump and the arteries correspond to the hose. The powerful muscular contraction of the ventricle drives the blood into the arteries by successive impulses, as the water is driven into the hose by the pump. If the end of the hose were left open the water would issue in a series of spurts CIRCULATION, RESPIRATION AND EXCRETION 129 corresponding to the strokes of the pump. By the addition of a nozzle of smaller diameter than the hose this intermittent outflow is converted into a steady stream. The resistance of the nozzle to the passage of the water gives rise to a pressure which stretches the walls of the hose, and their elastic force maintains the flow between the strokes of the pump. Substantially the same conditions exist in the body. The walls of the arteries are elastic while the capillaries in which the arteries terminate may be compared to the nozzle of the hose. The resistance caused to the flow of the blood by these minute channels tends to hold it back and produces a pressure in the arteries which, like the pressure in the hose, causes a steady movement of blood through the capillaries. In other words, the immediate cause of the motion of the blood through the capillaries is the elasticity of the arterial walls. If the latter become weakened and lose their tone or become hardened as in old age (arteriosclerosis), the driving force is lessened and the circulation slows down, since the veins can return blood to the heart only as fast as it is forced through the capillaries by the arterial pressure. The blood pressure in the arteries, therefore, is an important indicator of the activity of the circulatory sys- tem. The veins serve substantially as a return system, the blood being pushed along them by the residual pressure from the capillaries, perhaps aided somewhat by the expansion of the auricle of the heart, while valves prevent any backward flow. As compared with the arterial pressure, therefore, the blood pressure in the veins is low. 185. The lymph. — The body cells are not closely packed to- gether but are imbedded more or less loosely in connective tis- sue (83) leaving spaces between them (intercellular spaces). These spaces contain a colorless transparent fluid called the lymph which is the real nutritive medium in which the cells live. From it, by means of osmosis through their outer mem-- branes and perhaps in other ways, the cells derive the substances required for their vital activities and into it they discharge the waste products of their action. The lymph in its turn stands in relation to the blood, from which it is separated by the thin walls of the capillaries. While the minute capillaries penetrate all the tissues and convey blood to all parts of the body, it should be understood that the cir- 130 NUTRITION OF FARM ANIMALS culatory apparatus is a closed system. Even the very thin delicate walls of the capillaries are continuous and the blood does not come into direct contact with the living cells, except, of course, those lining the blood vessels. The accompanying diagram (Fig. 19) illustrates schematically the anatomical re- lations of the cells, intercellular spaces, capillaries and lym- phatics, A representing a minute artery, or arteriole, subdi- viding into capillaries which are reunited to form the small vein V. Through the capillary walls the nutritive substances contained in the blood pass, partly by osmosis and partly by FIG. 19. — Relation of cells to blood vessels and lymphatics. (Hough and Sedgwick, The Human Mechanism.) filtration, into the lymph to maintain its stock, while the waste products of cell action pass in the opposite direction into the blood and are carried off. 186. Lymphatics. — In the intercellular spaces there orig- inates another set of minute vessels, the lymphatics, which unite like the capillaries to form larger ones (L in Fig. 19) and finally form two main lymphatic trunks, the thoracic duct and the small lymphatic trunk, which empty into the great veins near the heart. The lacteals of the intestinal villi, through which the fats are chiefly resorbed, belong to the lymphatic system. CIRCULATION, RESPIRATION AND EXCRETION 131 In the lymphatics there is a continuous slow movement of the lymph from the tissues towards the main trunks, the lymphatics, like the veins, being provided at intervals with valves prevent- ing a backward flow. This lymph flow is sustained in part by a slightly greater pressure in the lymphatic spaces but largely by the rhythmic motions of breath- ing, and is aided by muscular activity. Thus, in addition to the exchange of substances between the lymph and the blood through the walls of the capillaries, there is a general movement of the lymph itself over the surface of the cells which tends to facilitate the exchanges between it and the protoplasm. 187. Adjustment of circula- tion. — The activity of the various tissues varies at different times. A muscle, for example, is some- times at rest and sometimes actively contracting. Conse- quently, a greater or less supply of food material and of oxygen will suffice according to circumstances, and the blood supply needs to be regulated accordingly. This regulation is effected in substantially two ways. First, when the cells of any particular tissue increase their activity they consume more oxygen and give „ J ? FIG. 20. — Mam lymphatic trunks off more waste products than be- (in white). (Hough and sedgwick, fore, tending to produce a relative The Human Mechanism.) deficiency of the one and an ex- cess of the other in the lymph and blood. These conditions bring about an increase in the heart action (194), probably by means of a nerve stimulus, so that the amount of blood 132 NUTRITION OF FARM ANIMALS passing through* the heart is increased and a more abundant supply of it reaches the active tissues. Second, there may be a partial shunting of the blood supply from one region of the body to another as one set of organs or another calls for a larger amount. This is accomplished through the agency of the middle or muscular coat of the arterioles, controlled by the so-called vaso-motor nerves. When a larger supply of blood is called for in the muscles, for example, these fibers relax and allow the arterioles to enlarge, thus reducing the resistance offered to the blood flow and allowing the arterial pressure to force blood into the capillaries more rapidly. To compensate for this there is a contraction of the arterioles of the internal organs, especially of the abdominal organs, resulting in a di- minished blood supply. The effect of the performance of work upon digestion, discussed in Chapter XVI (721), is possibly connected with this effect upon the blood flow. On the other hand, after a hearty meal the arterioles of the digestive tract relax, while the superficial blood vessels tend to contract and the blood supply is partially diverted from the surface tissues to the internal organs. This power of the body to regulate the supply of blood to different regions is of special importance, as will appear later (321), in connection with the regulation of the body temperature. § 2. RESPIRATION 188. The oxygen supply. — By means of the processes described in the preceding section the nutritive materials de- rived from the feed and taken up by the intestinal capillaries and lacteals are distributed to the various tissues and cells. Equally necessary with a supply of feed materials to the living cells, however, is a supply of oxygen, and this another set of or- gans, those of respiration, are engaged in furnishing to the blood through another set of capillaries for transmission to the cells. 189. The lungs. — The transfer of oxygen from the air to the blood is effected in the lungs, which, with the heart and large blood vessels, fill the cavity of the thorax, or chest. This cav- ity is enclosed on the sides by the ribs and their connections, forming the chest walls, and is separated from the abdominal cavity, containing the digestive organs, by a strong, arched, mus- CIRCULATION, RESPIRATION AND EXCRETION 133 cular partition, convex toward the chest, the diaphragm. The air enters the lungs through the trachea, or windpipe, from the mouth and nostrils. The trachea, after reaching the chest, divides into two branches, or bronchi, one leading to the right and one to the left lung. Each bronchus subdivides repeatedly into a multitude of fine tubes, the smallest of which are called bron- chioles (little bronchi), each of which finally ends in an alveolus, the inner surface of which is much , FlG- "• ~ AlveoU of luns- (wil- 111. , ckens, Form und Leben der Land- increased by being arranged in wirthschaftiichen Hausthiere.) the form of pits or air cells. In Fig. 21, c represents a bronchiolus, aa two alveoli and bb the air cells. Figure 22 shows diagrammatically on a large scale a cross section of two alveoli. FIG. 22. — Section of two alveoli. (Hough and Sedgwick, The Human Mechanism.) 134 NUTRITION OF FARM ANIMALS The walls of the trachea and bronchi consist of cartilaginous rings which prevent them from collapsing. The alveoli and bronchioles are surrounded and bound together by connective tissue consisting largely of elastic fibers so that the minute air cavities of the lungs are extensible and their walls elastic. In this connective tissue are found the larger branches of the pulmonary artery and pulmonary vein, connected by a net- work of capillaries which are spread out over the inside of the alveoli in direct contact with their lining membrane. Each lung is enclosed in a double-walled sack, the pleura, one wall of which covers the lungs and the other the chest walls and diaphragm, the narrow cavity between the two being filled with a liquid. 190. Mechanics of breathing. — In breathing, the lungs themselves play an essentially passive role, the movement of air into and out of them being effected by changes in the capac- ity of the chest brought about by the movements of the dia- phragm and ribs. Since the diaphragm is convex toward the chest its contrac- tion tends to pull the apex of the dome toward the abdomen, 'thus increasing the volume of the chest cavity and by pressure on the digestive organs distending the abdominal walls. When the diaphragm relaxes again the volume of the chest is reduced and the abdominal walls return to their former position. This type of breathing is what is called abdominal breathing. The ribs pass obliquely around the chest from the spine to the breast bone (sternum). By means of the intercostal muscles, located between .them, the ribs can be elevated, turning on their attachments to the spine and sternum, thus increasing the diameter of the chest both from side to side and from front to back and so increasing the capacity of the chest cavity. This type of breathing is called costal, or rib, breathing. The two types of breathing are ordinarily combined. By their joint action the size of the closed pleural cavity contain- ing the lungs is increased and the atmospheric pressure forces more air into the extensible alveoli of the lungs, so that the latter expand along with the chest cavity, the whole constitut- ing the act of inspiration, or breathing in. When the dia- phragm and the intercostal muscles relax, the elasticity of the CIRCULATION, RESPIRATION AND EXCRETION 135 chest walls causes them to return to their original position and this, together with the elasticity of the lung tissue itself, com- presses the air in the alveoli and forces part of it out through the trachea, this constituting the movement of expiration. Inspiration is an active process, while expiration is chiefly passive. The respiratory movements are ordinarily what are called involuntary, i.e., they go on independent of conscious- ness, being governed by automatic nerve impulses, conveyed by nerves of various origin but controlled by the so-called " res- piratory center," although the movements can be accelerated or retarded or even suspended entirely for a few moments by an effort of the will. From the foregoing, it is plain that the ventilation of the lungs does not consist in the passage of air through them but of a surging or tidal movement in and out. The alveoli are never entirely emptied of air even in forced expiration. In inspiration the new or tidal air enters the trachea and bronchi, gives up by diffusion some of its oxygen to the residual air in the alveoli and receives from the latter some of the carbon dioxid which it contains. In this way, by the ebb and flow of the tidal air and by diffusion between it and the residual air, fresh oxygen is being continually introduced into the lungs and carbon dioxid continually removed. 191. Absorption of oxygen. — The oxygen introduced into the alveoli of the lungs in the manner just described is still outside the body proper, just as is the feed in the digestive tract. In order to fulfill its functions it, like the feed, must be transmitted to the blood for distribution to the tissues. This transfer is accomplished in the lung capillaries as is that of the feed in the intestinal capillaries. In the lung capillaries the blood is separated from the air of the alveoli only by a thin mem- brane. The coloring matter of the red corpuscles, haemoglobin, has the power of entering into combination with oxygen, of which it can take up a maximum of about 1.66 c.c. per gram, forming a loose chemical compound known as oxy haemoglobin. The red corpuscles of the venous blood as it comes to the lungs contain chiefly haemoglobin. In their passage through the lung capillaries they are exposed to the oxygen of the alveolar air and, aided by the relatively large surface of the blood cor- puscles, their haemoglobin takes up more or less oxygen and is 136 NUTRITION OF FARM ANIMALS converted partly or wholly into oxy haemoglobin, the amount of the oxygen taken up ranging from eight to twelve volume per cent. The color of haemoglobin is a dark red or purplish, while that of oxyhagmoglobin is bright scarlet. To this differ- ence of color is due the marked difference in appearance be- tween venous and arterial blood. 192. Respiration of tissues. — The term respiration is very commonly applied to the mechanical processes of breathing just described or to the exchange of gases in the lungs. In reality all these are preliminary to the real respiration, which takes place in the tissues. The vital processes in the body cells consist, broadly speaking, as will appear in detail in the next chapter, of a series of oxidations. The requisite oxygen is necessarily drawn from the lymph in which the cells exist (185), while the carbon dioxid produced by oxidation is discharged into it. The lymph, therefore, tends continually to become richer in carbon dioxid and poorer in oxygen. In the manner just described the blood takes up oxygen in the lungs and acts as a carrier through the body. Through the capillary blood vessels of the body generally, therefore, there are continually passing red blood corpuscles charged with loosely combined oxygen, while on the other side of the capillary wall is a fluid (the lymph) in which the partial pressure of oxygen is relatively low. Accordingly, the combination of oxygen and haemoglo- bin is dissociated to a greater or less extent and oxygen passes into the lymph as required to supply the needs of the cells. At the same time the excess of carbon dioxid in the lymph passes in the opposite direction into the blood and is thus removed from the neighborhood of the cell.1 It is this continual con- sumption of oxygen and elimination of carbon dioxid by the cells which constitutes the real act of respiration, while the complex structure of the lungs and the elaborate mechanism of breathing and of the blood corpuscles are simply means for providing oxygen to the cells and taking away carbon dioxid. That the movements of breathing are not an essential part of respiration is strikingly shown by the fact that it is perfectly possible by suitable devices to maintain oxygenation of the blood 1 In these exchanges, as in other similar ones, while diffusion doubtless plays a large part, its effects are no doubt modified by the special properties of the living cells. CIRCULATION, RESPIRATION AND EXCRETION 137 of an animal in the absence of any respiratory movements what- ever. 193. Respiration regulated by cell activity. — It is apparent from the foregoing that the amount of oxygen taken up by the blood in the lungs depends in the first instance upon the amount of this element consumed by the body cells. When they are relatively inactive they take up correspondingly little oxygen from the lymph and the tension of oxygen in the latter is low- ered but little. As a consequence there is less dissociation of the oxyhaemoglobin in the blood and the corpuscles tend to return to the lungs still carrying more or less oxygen and there- fore capable of taking up relatively less. On the other hand, as the tissues become more active they consume more oxygen, the oxyhaemoglobin in the corpuscles is more extensively dissociated and the corpuscles tend to come back to the lungs relatively exhausted of oxygen and ready to take up the maximum amount. Any considerable degree of tissue activity, however, calls for a more rapid supply of oxygen than can be provided for in this way and this need is met by a nerve stimulus to the heart, caus- ing it to beat faster and more powerfully, thus increasing the arterial pressure and therefore the amount of blood passing through the capillaries in a given time. In these two ways the amount of oxygen absorbed in the lungs is very accurately adjusted to the needs of the organism. It is impossible to stimulate the body oxidations by a free supply of air as, for example, by deep and rapid breathing, as one might blow up a fire with a bellows, or to get more intense combustion by re- placing air with pure oxygen. In the body such additional air or oxygen never reaches the fire. Each corpuscle is a recep- tacle which can carry only a definite amount of oxygen and if it comes back to the source still partly filled it takes up so much the less on its next trip, or if it travels slowly it is less efficient than if it returns more frequently. The respiration of the tissues can no more be affected by increasing the ventilation of the lungs than the amount of water delivered by a pump is by the volume of the stream from which the water is taken. 194. Regulation of the rhythm of breathing. — The illus- tration just used is true, of course, only on the condition that the stream carries at least as much water as the pump can handle. So, too, the amount of oxygen available in the lungs 138 NUTRITION OF FARM ANIMALS must at least equal the amount required by the tissues. It is a familiar observation that the rate of ventilation of the lungs varies with the varying activity of the body cells. This is true of all these activities, but is most familiar in the case of muscular work which, as everyone knows, promptly increases the rate and depth of breathing, so that severe exercise, such as rapid running, for example, brings into play all the reserve re- sources of the breathing mechanism. As already stated (190), the muscles which are used in breathing are ordinarily controlled from the so-called " respiratory center " and it is through this center that the regulation is effected. If, for example, an animal be supplied with air largely diluted with some indifferent gas, such as nitrogen or hydrogen, the partial pressure of the oxygen in the alveoli is so reduced that the hae- moglobin of the blood is only partially saturated with oxygen. Such a deficiency of oxygen stimulates the respiratory center and produces more active breathing and a corresponding in- crease in the rapidity with which the air in the alveoli (residual air) is renewed. Under ordinary conditions, however, the stimulus to the respiratory center is not a lack of oxygen in the blood but an excess of carbon dioxid. As has already been implied, the lungs serve not only for the absorption of oxygen but for the elimination of the carbon dioxid produced by respiration, which passes by way of the lymph to the blood and thence to the air in the alveoli of the lungs. Any increase in the ac- tivity of the tissues by which more carbon dioxid is produced tends to increase the content of this substance in the blood. Even a very slight increase, however, promptly stimulates the respiratory center and so causes greater activity of the muscles concerned, resulting especially in deeper and to some extent more rapid breathing. By this means the ventilation of the lungs is augmented and so provision is made for the removal of a greater amount of carbon dioxid. It is plain, however, that a simple increase in the lung ven- tilation alone is not sufficient, except in a limited degree, to carry away more carbon dioxid from the tissues. Along with the increased ventilation there must be an increase in the rapidity of the blood current which is the medium by which the transfer of gases between the lungs and the lymph takes place. Accordingly, CIRCULATION, RESPIRATION AND EXCRETION 139 we find that substantially the same stimuli which cause more active breathing also stimulate the heart action and vice versa. Lack of oxygen or excess of carbon dioxid are the two prin- cipal factors in regulating the breathing rhythm but by no means the only ones. They are, however, the ones of most impor- tance in the present connection. 195. Gaseous exchange through the skin. — In addition to the exchange of gases between the air and the blood which goes on in the lungs, a similar process takes place, though to a much smaller extent, through the skin. The true skin, under- lying the cuticle or scarf-skin, is penetrated by capillary blood vessels, and in its passage through these capillaries the blood gives off some carbon dioxid and takes up some oxygen by dif- fusion through the skin. The amounts given off and taken up are small compared with the corresponding amounts in the lungs, but still are not inconsiderable, and must be taken into account in accurate experimental work. § 3. EXCRETION 196. Excretory products. — As already implied, the vital activities of the body cells lead to the formation of products which must be removed from the cells and some of which must ultimately be discharged from the body. The next chapter will be concerned with the nature of the more important of these products and with some of the steps by which they are formed. For the present, it suffices to say that the gradual oxidations of non-nitrogenous material taking place in the cells give rise substantially to the production of carbon dioxid and water, while the proteins and related substances yield in addition certain comparatively simple nitrogenous substances of which the most abundant is urea. In addition to these substances, more or less of the mineral ingredients also pass into the excreta. 197. Excretion of carbon dioxid. — As stated in the previous section, the carbon dioxid produced by the tissue respiration passes by way of the lymph into the blood and is excreted through the lungs and to a minor degree through the skin. In the blood the carbon dioxid is carried by both the corpuscles and the plasma, but chiefly (two-thirds or more) by the latter, in combination with proteins and haemoglobins, but especially 140 NUTRITION OF FARM ANIMALS with the alkalies. As in the case of oxygen, the amount of carbon dioxid contained in the blood depends upon the partial pressure of this gas in the surrounding medium. Since the ten- sion of the carbon dioxid in the alveolar air is less than that in the blood of the alveolar capillaries, the carbon dioxid passes from the latter to the former. If the air were stationary the process would continue until an equilibrium was reached. Since the air is being continually renewed by breathing, the tension of carbon dioxid in it is kept permanently lower than that in the blood and there is, therefore, a continual passage of carbon dioxid from the blood to the alveolar air. It is by means of this tendency to equilibrium that the mech- anism for the regulation of breathing is set in motion. In- creased tissue respiration discharges more carbon dioxid into the blood, where its tension increases. This causes a more rapid diffusion of the gas into the alveolar air and tends to raise its carbon dioxid tension also, so that with an unchanged rate of lung ventilation the carbon dioxid level of both the alveolar air and the blood would be raised. Even a very slight rise in the carbon dioxid tension in the blood, however, as already stated, acts promptly upon the respiratory center and stimu- lates the muscles of breathing, resulting in an increased lung ventilation and consequently a more rapid excretion. At the same -time the rapidity of circulation is increased and in these two ways the level of carbon dioxid tension in the blood and in the alveolar air is maintained very constant. On the other hand, if the lung ventilation be artificially increased, as by artificial respiration or by the use of oxygen, the carbon dioxid excretion may be so facilitated that the amount in the blood falls below the normal and the movements of breathing may be temporarily suspended (apncea). 198. Excretion of nitrogenous products. — The urea and other nitrogenous products of cell action, like the non-nitrog- enous products, pass ultimately into the blood. In its course through the body the blood passes through a capillary system in two bean-shaped organs, the kidneys, indicated by t in Fig. 1 8, situated in the abdominal cavity on either side of the spine near the loins. In these organs the urine is being continually secreted, passing thence through the ureters into the bladder from whence it is voided at intervals. CIRCULATION, RESPIRATION AND EXCRETION 141 The chief stimulus to the secretion of water by the kidneys is the water content of the blood, the kidneys acting as regu- lators of this important factor and eliminating more or less water as the blood contains a larger or smaller percentage of it. As regards the excretion of dissolved matter, very interesting relations exist. With one important exception (hippuric acid) the kidneys do not manufacture the excretory products. Their essential function is to maintain the composition of the blood constant. For each substance capable of being excreted at all in the urine there exists a certain minimum concentration in the blood above which it begins to pass through the kidneys into the urine. For the normal excretory products, as well as for foreign substances, this minimum approaches zero, that is, only very minute amounts of these substances can be retained in the blood. For dextrose the limit is approximately 0-2-0-3 per cent, for sodium chlorid 0-6 per cent, etc. So long as the percentage of one of these substances in the blood does not exceed its own particular limit, none of it is excreted through the kidneys. On the other hand, & slight rise above this limit causes an excretion of the substance concerned. This function of the kidneys has been likened to the working of an overflow valve on a tank. It should be added that each particular sub- stance has its own minimum, independent to a large degree of all the others. The functions of the kidneys, however, in this respect are not so simple as those of an overflow valve for the reason that the concentration of the excreted substances is greater in the urine than in the blood. In other words, the kidney does its work by transferring substances from a fluid of lower to a fluid of higher osmotic pressure and the expenditure of energy in this work is not inconsiderable. This is notably true o£ urea and the other nitrogenous waste products, of which only traces can be detected in the blood. In addition to the nitrogenous substances excreted in the urine there are present in the feces, as already noted (154), excretory products which represent a certain fraction of the organic body waste. Finally, small amounts of urea and other nitrogenous substances are excreted in the perspiration. 199. Excretion of ash ingredients. — Being non-volatile the ash ingredients are excreted chiefly through the feces or 142 NUTRITION OF FARM ANIMALS urine according as the intestines or kidneys form the normal path of excretion, although they are contained to a small ex- tent also in the perspiration. The intestines are the usual path of excretion for certain mineral substances, notably iron, calcium and to some extent magnesium. To these must be added in the case of the her- bivora phosphoric acid, which, under ordinary conditions, is excreted in the feces. The urine of herbivora, especially when they consume roughage freely, or in more general terms when the basic predominate over the acid ingredients of the ash, is alkaline and contains but minute amounts of phosphoric acid. On the other hand, during fasting or upon a ration having an acid ash, the urine may have an acid reaction and then, like the acid urine of carnivora or omnivora, may contain phosphoric acid. The urine is the normal vehicle for the excretion of sulphur, chlorin and the alkalies. 200. Excretion of water. — The motions of air in and out of the lungs are the means of removing from the body more or less incidentally large amounts of water by simple evapora- tion. The presence of water vapor in the expired air is a fa- miliar fact, shown by its condensation on a cold surface or in cold air. The skin likewise acts, by means of its sweat-glands, as a channel for the removal of water from the system, con- siderable being continually evaporating from the skin in the form of the " insensible perspiration." Under certain circum- stances the excretion of water is so rapid as to give rise to the formation of visible drops (sweating). The amounts of water excreted in these two ways are larger than are sometimes realized. For example, a thousand pound ox at ordinary temperature and on light feed may easily ex- crete through the lungs and skin eight to ten pounds of water in twenty-four hours, the amount depending to a considerable extent upon the temperature and amount of movement of the surrounding air. The feces also contain a large percentage of water and in the case of herbivorous animals the amount thus eliminated is very considerable. Finally, water is excreted in the urine, serving as a solvent for the nitrogenous products of cell activity which are removed through this channel. The amount of water thus excreted de- pends in part upon the amount consumed, in part upon the CIRCULATION, RESPIRATION AND EXCRETION 143 quantity of nitrogenous material which must be dissolved and in part upon the amount eliminated through the lungs and skin. Most of the water excreted by animals is, of course, con- sumed as such, but it includes also that formed by the oxida- tion of organic hydrogen — the so-called metabolic water. Babcock has shown that in some classes of animals, notably insects, this metabolic water suffices for all the needs of the organism, so that they are not dependent upon a supply of drinking water. CHAPTER V METABOLISM § i. GENERAL CONCEPTION 201. Assimilation and excretion. — The cell has already been defined (73) as the biological unit of life. It is the living proto- plasm of the body cells which is the seat of the multifarious activities of the organism. Every such activity requires an expenditure of energy, de- rived from the breaking down of constituents of the .proto- plasm itself or of cell enclosures and solutes and their transfor- mation into other forms. The presence of oxygen is essential to these changes and while, as will appear, they seldom are primarily direct oxidations, nevertheless, they yield products which are ultimately oxidized to carbon dioxid, water and other comparatively simple compounds. Two things, then, are necessary for the continued life of the cell : first, a supply of material from without to replace that consumed and, second, the removal of the waste products of its activities. Both conditions are fulfilled in the higher ani- mals by the circulation of the blood and lymph. In the pro- cesses of digestion, the heterogeneous nutritive materials con- tained in the feed are gradually brought into solution by a series of molecular cleavages, so that the resorptive organs transmit to the blood and lymph current a qualitatively uniform material consisting of substances of comparatively simple molecular structure (146, 147), while oxygen is supplied to the blood cor- puscles through the lung capillaries. The mechanism of cir- culation is continually distributing to each tissue and cell oxygen from the lungs and nutritive material from the digestive tract and carrying away the waste products of cell action to the various organs of excretion which remove them from the body. 202. Definition of metabolism. — It is clear from the fore- going that the body is the seat of extensive chemical trans- 144 METABOLISM 145 formations. On the one hand, molecules of resorbed digestion products are being assimilated by the body cells and built up into the structure of their protoplasm, while, on the other hand, molecules of protoplasm or of cell enclosures are being broken down and oxidized, yielding finally the relatively simple excretory products. The term metabolism is commonly used to designate the to- tality of the chemical changes which the constituents of the resorbed feed undergo in the course of their conversion into the corresponding excretory products. Similarly, one- may speak in a more restricted sense of the metabolism of single ingredients of the feed, as of the proteins, carbohydrates or fats, protein metabolism, for example, signifying the chemical changes undergone by the digestion products of the proteins of the feed during their assimilation and subsequent transfor- mation into excretory products. The adjective metabolic is also used to describe these chemical changes. 203. Anabolism and katabolism. — The term metabolism, as just defined, includes processes of two distinct kinds, viz., those by which molecules of sugars, organic acids, amino acids, etc., are built up into more complex compounds in the body and those by which these complex compounds are broken down again into simpler substances and finally into the excretory products. The building up metabolism has received the name anabolism, while the breaking down or oxidative phase is called katabolism. Any change in the direction of greater molecular complexity is spoken of as an anabolic change, while one in the direction of greater molecular simplicity is a katabolic change. It must not be inferred from what has been said that anabolism always precedes katabolism. Neither is the breaking down of cell constituents by any means a process of uninterrupted katabolism. On the contrary, many instances are known in which it is interrupted at various stages by anabolic changes of one sort or another. While the general direction of the change is towards simplification, there are eddies in the current. Moreover, it is by no means probable that all the resorbed substances are actually built up into protoplasm before being katabolized. It is true that, to trie best of our knowl- edge, the metabolic processes take place within the cells but it ap- pears unlikely that the relatively large amounts of material some- L 146 NUTRITION OF FARM ANIMALS times katabolized must first become integral parts of the protoplasm. In other words, it is probable that the cells have the power to katab- olize substances present within them but not structurally a part of them. 204. Synthetic processes in the body. — The foregoing conception of metabolism implies that the body has power to carry out extensive chemical syntheses, contrary to the idea still current that the course of chemical change in the organic world is toward the building up of complex compounds in the plant and their breaking down to simpler ones in the animal. Synthethic chemical changes were long regarded as peculiar to the vegetable kingdom, while the reactions in the ani- mal body were supposed to be exclusively analytic. The first syn- thetic action to be recognized in the animal was the formation of hippuric acid from benzoic acid, discovered by Keller and Wohler in 1824, and which attracted wide attention. More recent physio- logical investigations have shown that this is by no means an isolated case, but that syntheses in great variety are executed in the animal body. No such sharp distinction between animal and vegetable organisms exists as was formerly supposed. The fundamental laws of metabolism are the same for both and both execute synthetic as well as analytic processes. It is only the special synthetic activity of the chlorophyl in green plants which tends to obscure this funda- mental likeness. The conception, then, that the digestive cleavages supply to the body cells comparatively simple "building stones" which are synthesized to produce the complex ingredients of cells and tissues is quite in harmony with our general knowledge of the nature of metabolism. 205. Metabolism oxidative and analytic. — Metabolism re- garded as a whole may be characterized chemically as an oxi- dation. Oxygen is introduced into the system through the blood and reacts with the feed or tissue materials or with the products of their breaking down, and the final excretory products are either completely oxidized substances, like carbon dioxid and water, or substances approaching this condition, like urea, etc. From a slightly different point of view, metabolism as a whole may be characterized as an analytic as opposed to a synthetic process. The general tendency is toward the formation of simpler molecules. For example, the molecule of dextrose or levulose contains 24 atoms and those of the three most com- mon fats, respectively, 155, 167 and 173 atoms, while the molecules of carbon dioxid and water resulting from their metab- METABOLISM 147 olism contain but 3 atoms each. Even the cleavage products of protein which are resorbed from the digestive tract are, with few exceptions, much more complex than the final products which result from their metabolism. 206. Metabolism a gradual process. — While metabolism has just been characterized as an oxidative process, and is often loosely spoken of as a burning of the feed or tissue ingredients, it is in fact radically different from what is commonly under- stood by these terms. The building up and breaking down of materials in metabolism is a gradual, i.e., a step by step, process. Metabolism is the sum of the chemical reactions through which the life of the cells is manifested. These reactions, however, differ from tissue to tissue and from cell to cell, and even in the same cell from time to time, and the resulting products are correspondingly numerous and varied. Between the nutrients supplied to the cells by the blood and the final products of metabolism as excreted from the body there are innumerable intermediate products, comparatively few of which, in all proba- bility, have been recognized. We know the first and last terms of the series and thus are able to measure, as it were, the alge- braic sum of the changes, but of the single factors making up the so-called intermediary metabolism as well as of the specific tissues concerned in the changes, we are largely ignorant, al- though we know that they are numerous. Furthermore, while metabolism results in the formation of highly oxidized products, it does not consist primarily in the direct union of oxygen with feed materials, i.e., the step by step processes of which it is made up do not consist of a series of partial oxidations. The primary processes of metabolism are of the nature of cleavages and hydrations and it is only the comparatively simple molecules resulting from these which unite directly with oxygen. Correspondingly, the extent of metabolism is determined by the amount of functional activity of the various cells and not, as in the case of direct oxidation in a fire, by the supply of oxygen (193) . The somewhat com- mon notion that an increased proportion of oxygen in the air or a voluntary increase in the rate and depth of breathing may cause more material to be oxidized in the body is without foundation, except -so far as increased breathing involves increased mus- cular exertion. 148 NUTRITION OF FARM ANIMALS 207. Purpose of metabolism. — As implied at the opening of this chapter, the vital activities of the body are essentially transformations of energy. The living body is continually doing work upon its surroundings and continually loosing heat to them and the energy for the production of work and the main- tenance of the body temperature is derived, as already stated, from the transformation of the chemical energy contained in the substances broken down, tiiis transformation being indeed the essence of the whole process. This fact is familiarly, if not altogether accurately, expressed in the statement that the feed is the fuel of the body. There will be occasion later to consider this aspect of the matter in detail, but it is important at the outset to grasp the conception that the final end and aim of metabolism is to sup- ply energy for the vital activities and that the demand for en- ergy is the controlling factor in all its processes. It is these transformations of energy which, if not synonymous with life, are at least its objective manifestation. But while it is essential to hold fast to this broad general con- ception of metabolism, it is also important to understand clearly that the processes by which this end is reached are exceedingly complex. A volume would be required for any adequate dis- cussion even of existing knowledge regarding the details of the metabolic processes. Such a discussion lies outside the scope of the present work. All that is attempted in this chapter is to outline the metabolism of the principal groups of feed sub- stances and, as preliminary to a subsequent consideration of their values as sources of matter and energy to the body, to indicate the functions which they perform in the building up and maintenance of the organism and the support of its activ- ities. § 2. ENZYMS AS AGENTS IN METABOLISM Enzym action has come to play so large a part, even if a more or less hypothetical one, in the current conceptions of the pro- cesses of metabolism that a brief outline of the prevailing views seems called for. 208. Extracellular enzyms. — The enzyms of the digestive tract are those which are most familiar in physiology. As has been seen (114), the digestion of all three of the chief classes METABOLISM 149 of feed ingredients is brought about largely or wholly by their agency and is often effected by different enzyms in successive stages. Thus the ptyalin of the saliva converts starch into maltose while the further conversion of the latter into dextrose is effected by the maltase of the intestine. Quite similar are the successive actions of pepsin, trypsin and erepsin on the pro- teins. In all these cases, as well as in the even more familiar case of the diastase of germinating seeds, the enzyms act at a dis- tance from the cells which produce them and have, therefore, been called extracellular enzyms. 209. Intracellular enzyms. — From the fact that the most obvious cases of enzym action were those in which the ferment acted at a distance from the cells producing it, enzyms came to be regarded as substances whose action belonged in a different category from that of living cells. A sharp distinction was drawn between unorganized substances, acting substantially as chemical reagents, and organisms producing chemical changes by virtue of their life. The action of the yeast plant upon sugar afforded a typical example of this distinction. It was shown that yeast secreted an enzym (invertase) which was capable of inverting sucrose independently of the action of the yeast cell, while, on the other hand, the alcoholic fermentation of mono- saccharids was held to be a vital function of the living yeast cells. Buchner, however, in 1897, showed that by suitable means there could be extracted from yeast a substance (zymase) which fermented the simple sugars exactly like yeast in the absence of any living organism whatever; i.e., it acted as an enzym. It became evident, then, that the yeast cell ferments monosaccharids not because it is alive but because it contains zymase. The only essential difference between the yeast fer- mentation and that, for example, produced by diastase or by the invertase of yeast is that the enzym normally acts within the cell which produces it. Later it was shown that what is true of the yeast fermentation is true also of the fermentation caused by the lactic acid bacillus. It, too, is due to an intra- cellular enzym which can be separated from the cell and act independently. Investigators are inclined, therefore, to re- gard all fermentation as the work of enzyms, some of which, like the digestive enzyms, are excreted by the cells and may 150 NUTRITION OF FARM ANIMALS act at a considerable distance from their point of origin, while others normally produce their effect within the secret- ing cell. 210. Intracellular enzyms in the body. — Still more recently the presence of intracellular enzyms in all parts of the animal body has been recognized. It has been shown that a very considerable variety of reactions which are known to take place in the body may also be brought about outside the body by the action of extracts of various tissues and organs under conditions apparently excluding the action of any living organisms. Con- sequently, they have been ascribed to the action of enzyms originally present in the cells, and the reactions in the body have been regarded as due to these same enzyms. The idea of intracellular enzyms has thus been extended to account for the metabolic activities of the organism, and this explana- tion has been very generally accepted by physiologists. Accord- ing to this view, the body cells bring about metabolic changes substantially in the same way as do the cells of yeast or of the lactic acid bacillus, viz., by the formation of appropriate enzyms which act upon the substances to be metabolized. This phase of the subject is a comparatively new one and unanimity as to individual cases has by no means been reached, but of the value of the general conception as a working hypothesis there can be little question. The word explanation is used above, of course, in a limited sense. It is not known how the cell produces enzyms, nor with any degree of certainty how an enzym acts. Nevertheless, this hypothesis, if confirmed, is a real explanation as far as it goes, in that it enables related phenomena to be grouped to- gether from a broader standpoint, as will be apparent from the following paragraphs. 211. Enzym reactions reversible. — A chemical reaction is said to be reversible when it may progress in either direction according to the conditions. For example, if a mixture of hydro- gen and iodin in molecular proportions be heated to 448° C. hydrogen iodid is produced. If, however, hydrogen iodid be heated to the same temperature it yields hydrogen and iodin. The reaction between these two elements, then, is represented by the equation H2 + I2 ^± (HI)2 METABOLISM 151 At the temperature of 448° C., 79 per cent of 'the matter exists as HI and the remainder as free H2 and I2 ; the HI is dissoci- ated at the same rate at which the H2 and I2 unite, and a con- dition of chemical equilibrium exists. At a different temper- ature, the point of equilibrium is different, but otherwise the result is the same. In theory, all chemical reactions are regarded as reversible, but in many cases the reverse action is so slight as to be incapable of detection under attainable experimental conditions, and such reactions are often spoken of as irreversible. In addition to the temperature, the position of the point of equilibrium in a reversible reaction is affected by the relative mass of the ingredients. Thus if, in the example just given, the iodin be removed from the field of chemical action (as, for example, by condensing it to the solid form in a cold portion of the apparatus) the dissociation of the hydrogen iodid will proceed until it is practically complete. On the other hand, if the hydrogen iodid be removed (as by allowing it to react with calcium carbonate) the reaction may be pushed to completion in the reverse direction. Similarly, an increase in the concentration of one of the reacting substances tends to dis- place the equilibrium in the opposite direction. It is a matter of much interest that at least some enzym reactions have been shown to be reversible. One of the best authenticated cases appears to be that of the action of lipase on fats. It has been shown by Kastle and Loevenhart 1 that this enzym acts on ethyl butyrate according to the equation C2H5 • C4H7O2 + H2O ^± C2H5OH + C4H8O2 A similar reaction has also been shown to take place with monobutyrin, the glycerol ester of butyric acid, which may be regarded as a simple fat, while it is at least very probable that the higher fats are acted on in the same way. Another example of reversible enzym reaction is claimed to be that of the con- version of maltose into dextrose by the action of the ferment maltase, it appearing, according to the researches of Croft Hill,2 that the same ferment may also convert dextrose into maltose. Similar, although less decisive, results have also been reported regarding the action of the proteases. 1 Amer. Qiem. Jour., 24 (1900), 491. 2 Jour. Chem. Soc., Trans., 73 (1898), 634. 152 NUTRITION OF FARM ANIMALS 212. Reversibility of metabolic reactions. — It would appear, then, that the action of the intracellular enzyms which are be- lieved to play such an important part in metabolism may be synthetic as well as analytic, and that the metabolic processes may be conceived of as a complex of reversible chemical reac- tions, now accelerated and now retarded by appropriate en- zyms.1 The idea that each cell of the body thus exists in a state of constantly shifting chemical equilibrium, according as the concentration of one or another substance in its domain changes, is an attractive one in its breadth and comparative simplicity, and there seems to be little doubt that it contains elements of truth and will prove an important aid to research. As yet, however, it is to be regarded as a probable hypothesis rather than as a fully established fact. § 3. THE METABOLISM OF THE CARBOHYDRATES The hexose carbohydrates 213. Glycogenic function of the liver. — The monosac- charids (principally dextrose) produced in the digestive cleavage of the carbohydrates are resorbed chiefly or wholly by the blood capillaries of the intestines. These capillaries unite into the portal vein leading to the liver, where it subdivides into a capillary system in which the blood is brought into intimate contact with the cells of that organ and from whence it passes by way of the hepatic vein into the posterior vena cava, thus entering the general circulation (182). The proportion of dextrose found in the blood of the general circulation is remarkably constant, and if any considerable excess be introduced it is promptly excreted through the kid- neys (198). On the other hand, the supply of carbohydrates from the digestive tract may be more" or less intermittent or fluctuating, so that there is -evidently need for some regula- tory mechanism to prevent a waste of sugar by excretion in the urine. This regulation is effected chiefly in two localities, viz., in the muscles and in the liver. The function of the liver 1 That syntheses can be effected by the agency of enzyms seems established, but that enzym reactions in general are reversible is questioned by good authorities. For example, the ferment maltase, acting on dextrose, is stated to produce not mal- tose but isomaltose, and it is claimed that a different enzym is required to reconvert isomaltose into dextrose. METABOLISM 153 in this respect was the earliest to be discovered and may be appropriately considered first. When dextrose is being freely resorbed from the digestive tract, it undergoes dehydration and polymerization in the liver, yielding the polysaccharid glycogen (25), which is stored up in the liver cells. If, on the other hand, the resorption of dex- trose from the intestines is insufficient to maintain the supply in the blood, glycogen previously formed may undergo the reverse process of hydration and cleavage, giving rise to a pro- duction of dextrose. This regulatory activity, discovered by Claude Bernard in 1853, by which carbohydrates are held back or released according to the demands of the body, is called the glycogenic function of the liver. While this function has other aspects, as will appear later, as respects the digested carbohy- drates the liver may be likened to a storage reservoir by which the flow of a stream is controlled. 214. Mechanism of regulation. — It is of interest to note that this phase of carbohydrate metabolism illustrates two of the general conceptions formulated on preceding pages. First, the formation of glycogen is a synthetic reaction. The comparatively simple molecules of dextrose are built up tem- porarily into the more complex molecules of the polysaccharid. In other words, almost the first step in carbohydrate metabolism is an anabolic change (203). Second, the process of the formation and destruction of gly- cogen is susceptible of explanation as a reversible enzym re- action (212). It is known that the conversion of glycogen into dextrose is effected by an enzym or enzyms which may be ex- tracted from the liver and which, it would seem, must be similar to those of the digestive tract. The action of one of the latter, maltase, however, is claimed to be reversible (211), and one is naturally tempted to infer that the synthesis of the liver gly- cogen is effected by the same enzym which brings about its cleav- age, although experimental proof that such is the case is lacking. According to this hypothesis, the changes taking place in the liver would be represented by the equation »(C6H1206) $ C6nH10w05n + n(H20) An excess of dextrose in the blood would have the effect of displacing the point of equilibrium in the direction of the for- 154 NUTRITION OF FARM ANIMALS mation of glycogen, while a deficiency of dextrose would have the contrary effect. If it be supposed further that the glycogen as soon as formed combines with the protoplasm of the liver cells, forming compounds which withdraw a considerable por- tion of it from the sphere of action of the enzym, after the anal- ogy of the precipitation of an insoluble compound, we have a plausible, even if chiefly hypothetical, scheme of the chemical mechanism of the process. Whether or not it adequately rep- resents the actual facts, it may at least serve as a concrete il- lustration of the manner in which the conception of enzym ac- tion may be applied to metabolic processes. 215. Muscle glycogen. — While the glycogenic function of the liver has been the subject of very extensive investigation, the presence of glycogen is by no means confined to this organ. Indeed, glycogen seems to be a normal constituent of animal protoplasm. It is found in greater or less amounts in practi- cally all tissues, being particularly abundant where rapid cell multiplication is taking place, as in embryonic tissues or in rapidly growing tumors. It is estimated that in an animal in normal condition roughly one-half of the glycogen of the body is contained in the liver. Of the other half by far the larger proportion is found in the muscles (96). The glycogen of the muscles (and other organs) is not simply glycogen which has been formed in the liver and transported to the muscles, but is produced independently from the dextrose of the blood, apparently in much the same manner as in the liver. That this is true is shown by the fact that glycogen is still formed in the muscles when, by surgical interference (Eck fistula), the blood is prevented from passing through the liver. In fact, the formation of glycogen in the muscles, etc., appears to be the primary process, while the liver serves rather as a secondary reservoir which may be eliminated without seriously affecting the general carbohydrate metabolism. With the liver excluded from the circulation, the dextrose resorbed from the digestive tract is still converted into glycogen and the animal is still able to digest considerable quantities of carbohy- drates without the appearance of sugar in the urine. Even in the normal animal, however, the power to dispose of sur- plus sugar is not unlimited. If large quantities of sugar are consumed, the conversion into glycogen, together with the normal katabolism, METABOLISM 155 may not keep pace with the resorption and there occurs an excretion of sugar in the urine — the so-called "alimentary glycosuria." The amount of sugar which can be resorbed without producing alimentary glycosuria, — i.e., the limit of tolerance for sugar — varies with the kind of sugar, being highest with dextrose (220). 216. Carbohydrates formed in the body. - — In their relation to the carbohydrates of the feed, the muscles and liver act, as has been seen, as a sort of storage reservoir or regulator of the sugar supply to the blood. The total withdrawal of carbohy- drates from the feed, however, by no means results in the disappearance of these substances from the body. The car- bohydrates appear to be essential to the normal course of metab- olism and if they are absent from the feed, they are manufac- tured in the body from other materials. A carnivorous animal, e.g., fed exclusively on meat or fat, shows a normal percentage of dextrose in its blood, while its liver and muscles contain a normal amount of glycogen. It is true that in such an experi- ment small quantities of glycogen are contained in the meat consumed, but their amount is entirely insignificant as com- pared with the quantities of dextrose which there is reason to believe are produced and katabolized in the organism. This dextrose must obviously have its origin either in the proteins or the fats. Which of the two is the source or whether both can be thus utilized will be considered later in connection with the metabolism of those substances (235, 253). 217. Formation of fat. — The mutual transformations of sugar and glycogen tend to keep the dextrose content of the blood approximately constant, while holding a supply of readily available carbohydrate material at hand to meet promptly any sudden demand. The amount of carbohydrates which can be disposed of in this way is, however, limited. For man it is estimated at about 300 grams and for cattle at about 2 kilo- grams (96) . It is evident, then, that if the feed contains a per- manent excess of carbohydrates over the needs of the body the capacity to store them up as glycogen will soon be exhausted. A surplus of carbohydrates over the amount which can be dis- posed of in this way is applied by the organism to the pro- duction of fat, which may be stored up in very large amounts in the cells of connective tissue through the body, but especially in those immediately beneath the skin and about the abdominal 156 NUTRITION OF FARM ANIMALS organs, constituting the adipose tissue (94). This tissue con- stitutes a reserve of non-nitrogenous material which may be mobilized later if need arises. Of the chemistry of the conversion of carbohydrates into fats, as well as of the organ or organs where it is effected, our knowledge is still meager, but the fact of such a change is undisputed and it is perhaps the most notable example of a synthetic and anabolic process in the animal body. The physiological evidence for this fact and the quantitative relations of the process may be taken up more conveniently later (249). 218. Katabolism of carbohydrates. — The physiological sig- nificance of the dextrose of the blood and the glycogen of the muscles and liver appears most clearly when they are regarded, not in the light of a more or less temporary storage of matter in the body, but rather as carriers of energy for the physiological processes. Of these processes, the most obvious one, which vastly predominates over all others, is the performance of work by the muscles, external and internal, but what is true of mus- cular work is in the main true also of the subordinate forms of glandular and cellular activity. The former, therefore, may be taken as typical. In the performance of muscular work, as will appear later, there is a rapid katabolism of non-nitrogenous material and especially of carbohydrates, largely, it would appear, in the form of dextrose. The resulting impoverishment of the blood in dextrose causes a conversion of stored up glycogen into dextrose to supply the lack. If the view of the formation of glycogen which regards it as a reversible reaction may be accepted, we may say that the chemical equilibrium between the dextrose and the glycogen is disturbed by the removal of the former during muscular work. As long as the work is continued, the process of conversion of glycogen into dextrose also continues, and by prolonged work it is possible to reduce the glycogen con- tent of an animal to a very low limit. It should be clearly understood that the foregoing is only a highly schematic view of the chemistry of muscular contraction as related to the katabolism of the carbohydrates. Some further consideration is given in Chapter XIV (630) to the very com- plicated chemical mechanism of the process. METABOLISM 157 219. Intermediary katabolism. — Regarding the intermediary katabolism of the carbohydrates, not very much is known. It appears probable, however, that dextrose undergoes pre- liminary cleavage with the formation of glyceric aldehyde, pyruvic aldehyde (methyl glyoxal) and either lactic or pyruvic acid, which is then further oxidized to acetic acid, carbon dioxid and water. Many facts, including especially those derived from a study of the fermentation of sugar, seem to point to the possibility of such reactions. Lactic acid is also widely dis- tributed in the body, although its presence is also susceptible of explanation as arising in the katabolism of protein (233), and, moreover, it has been shown that lactic acid may give rise to glycogen or dextrose in the animal body. Accordingly, these changes, like the mutual transformations of glycogen and dextrose, may be conceived of as constituting a series of re- versible reactions. (Dextrose) Glycogen ;£ CH2OH(CHOH)4CHO (Glyceric aldehyde) CH2OH(CHOH)4CHO ^ 2CH2OH - CHOH • CHO (Pyruvic aldehyde) CH2OH - CHOH - CHO-H2O ^ CH3 • CO • CHO (Lactic acid) CH3 • CO • CHO + H2O :£ CH3CHOH • COOH or (Pyruvic acid) CH - CO • CHO • + O ;£ CH3CO - COOH The conversion of dextrose into lactic acid is a nearly iso- thermic process, the resulting lactic acid containing almost the same amount of chemical energy as the dextrose. If, then, these cleavages occur in the katabolism of carbohydrates they are obviously preparatory to the actual oxidation in which the principal portion of the energy is liberated. The pentose carbohydrates The foregoing paragraphs have treated of the metabolism of the hexoses, which constitute the chief carbohydrate supply of man, and of the carnivora so far as the latter consume carbo- hydrates. The feed of herbivora, however, contains also consid- erable amounts of various pentose carbohydrates which are in part 158 NUTRITION OF FARM ANIMALS digestible, or at least disappear from the feed during its passage through the alimentary canal. 220. Pentose sugars. — In general, it may be stated that the pentose sugars (in particular arabinose and xylose), whether administered by the stomach or injected into the blood, are at least partially oxidized in the body. The pentoses differ from the hexoses chiefly in the fact that the limit of tolerance in the blood (215) is lower. Excessive amounts of hexose carbohy- drates cause an excretion of sugar in the urine. The same effect is produced by the pentoses, but much smaller quantities, relatively, are required to bring it about. Most, although not all, investigators have found an increase in the glycogen of the liver consequent upon the ingestion of pentoses, but in every case it has been the ordinary Ce glycogen, indicating that the effect is an indirect one. 221. Pentosans. — The investigations upon the soluble pentose sugars or their derivatives just referred to have shown that they are to a greater or less extent assimilable. The pen- tose carbohydrates in the feed of herbivora, however, exist to a very limited extent, if at all, in this form. They are chiefly polysaccharids, being either pure pentosans or combinations of pentosans and hexosans. In discussing the nutritive value of these pentosans, it seems to have been frequently assumed that they are converted into pentoses during digestion. As a matter of fact, however, there is no direct evidence that such is the case, while Kellner's results (129) afford reason to believe that they are largely fermented along with cellulose, yielding, besides gaseous products, chiefly organic acids. If this is the case, farm animals do not acquire from their feed any considerable amounts of pentoses and conclusions drawn from experiments with the pentose sugars regarding the nutritive value of these substances are inapplicable to ordinary stock feeds. Their true value in the latter would be simply that of the products of their fermentation. The organic acids 222. Formation in digestion. — As was shown in Chapter IV (128-130, 132), a considerable proportion of both the hexose and pentose carbohydrates contained in the feed of herbivora undergoes fermentation in the digestive tract, giving rise, -in METABOLISM 159 addition to gaseous products, to the formation of various or- ganic acids. In particular, the constituents of the cell walls of plants appear to owe their apparent digestibility chiefly to this action of the organized ferments of the alimentary canal. While, therefore, the organic acids are chemically distinct from the carbohydrates, and while some of these acids are contained as such in the feed, the amounts produced from carbohydrates are so considerable that this would appear an appropriate point at which to consider their metabolism. Unfortunately, however, little is known of the metabolism of the simpler organic acids, beyond the fact that such of them as have been subjected to experiment are katabolized to carbon dioxid and water, not more than traces of them at most ap- pearing in the excreta. A portion of the carbon dioxid pro- duced unites with alkalies and appears in the urine as carbonates. 223. Analogy with carbohydrates. — It is interesting to re- call in this connection that the carbohydrates themselves undergo cleavage, producing lactic or even acetic and formic acids, before their final oxidation (219). If it be true that these latter comparatively simple substances are those whose oxida- tion yields most of the energy supplied by the carbohydrates, there would seem to be no reason why the same acids resorbed directly from the digestive tract should not follow the same general course of metabolism and have substantially the same nutritive value. If this view be correct, there is after all a con- siderable similarity between the metabolism of the carbohy- drates and that of their fermentation products. The non-nitrogenous matter of the urine 224. Products of incomplete katabolism. — It has been im- plied in the foregoing pages that the digested carbohydrates of the feed, whatever the intermediate stages through which they may pass, are ultimately oxidized to carbon dioxid and water. Of the ordinary hexose carbohydrates this is doubtless true, but with some of the large variety of substances ordinarily grouped together in the conventional scheme of feeding stuffs analysis as " carbohydrates and related bodies," or as " crude fiber " and " nitrogen-free extract," the case appears to be otherwise. It has been shown that the urine, in addition to the nitrogenous 160 NUTRITION OF FARM ANIMALS products of protein katabolism which will be considered in the following section, contains also non-nitrogenous materials, presumably arising from the incomplete katabolism of ingredi- ents of the feed. In the urine of man and of the carnivora these non-nitrogenous substances are chiefly or wholly such as might be derived from the katabolism of proteins (phenols and other compounds of the aromatic series), and their amount is com- paratively small. In the urine of herbivora, particularly of ruminants, however, their quantity is relatively very consid- erable, and it seems impossible to regard any large portion of them as products of protein katabolism. 225. Origin. — Apparently these non-nitrogenous organic substances originate in some way from the roughages. Their proportion in the urine is relatively large when the ration con- sists exclusively of roughage, and the addition of such feeding stuffs to a basal ration causes a marked increase in their amount, while, on the other hand, such concentrates as have been in- vestigated do not produce this effect to any very considerable extent. Furthermore, their amount seems to bear no fixed relation to the protein of the feed. When the amount of the latter ingredient is small, the total organic matter of the urine has in some cases exceeded the digested protein of the feed, thus demonstrating that a portion at least of the non-nitrogenous urinary constituents must have had some other source. As the proportion of protein in the feed increases, the amount of nitrogenous products in the urine likewise increases, while that of the non-nitrogenous products appears to be more constant, so that the ratio of urinary nitrogen to carbon increases. The most plausible explanation of these facts seems to be that the substances in question are derived from some of the non-nitroge- nous ingredients of the roughages, but from what ones, or what is the nature of the products, we are still ignorant. § 4. THE METABOLISM or THE SIMPLE PROTEINS Anabolism 226. Synthesis of proteins from digestive products. — The simple proteins are resorbed (139, 152) in the form of com- paratively simple cleavage products; largely as amino acids but in part perhaps as more or less complex polypeptids. Out METABOLISM l6l of these substances the body builds up the great variety of specific proteins which are peculiar to itself and which differ in properties and chemical structure from the proteins of the feed, especially from those of the vegetable kingdom (147). This process of building animal proteins from the fragments of vegetable proteins is the most conspicuous example at once of the synthetic powers of the animal organism and of the object of the digestive cleavage. 227. Seat of protein synthesis. — As regards the place where this synthesis of proteins occurs, opinions are divided. Until recently, most experimenters have not been able to detect the products of digestive cleavage with certainty in the blood, either in the general circulation or in the portal vein, and the current view has been, therefore, that of Abderhalden, viz., that the " building stones " of the proteins are synthesized in the epithelial cells of the intestine and that the resulting proteins — in particular serum albumin — are passed on to the blood to serve as nourishment to the protein tissues of the body. Various investigators, however, have reported the presence in the blood of greater or less amounts of non-protein nitrogen and with the aid of more refined chemical methods Folin and Denis1 and Van Slyke and Meyer2 seem to have shown beyond question that amino acids may pass through the resorbing epithelium unchanged and be found in the blood and tissues in amounts sufficient to account for practically all that was administered (152). The latter experimenters have likewise shown that after meat feeding the proportion of amino acids in the blood may be doubled and that the increase affects the blood of the entire circulatory system and not that of the portal vein only, while Abel,3 by a diffusion method, has been able to secure considerable amounts of amino acids from the circulat- ing blood of living animals. The evidence at the present time, therefore, seems decisively in favor of the view that the frag- ments into which the protein molecule is split during digestion pass without material change into the blood current and serve as a common source from which the proteins, both of the blood and the various tissues, are built up and that every living cell, each in its own measure, has this anabolic power. 1 Jour. Biol. Chem., 11 (1912), 87. 2 Jour. Biol. Chem., 12 (1912), 399. 3 Jour. Pharmacol. and Expt'l Therap., 5 (1914), 275. M 1 62 NUTRITION OF FARM ANIMALS Katabolism 228. Nitrogenous end products. — The total katabolism of the proteins results in the elimination of all their nitrogen through the kidneys in the form of the various relatively simple crystalline products found in the urine. Of the nitrogenous excretory products of man and the carnivora, urea is the most prominent, while others, such as uric acid, creatin, creatinin, ammonia, etc., are of subordinate importance quantitatively. Traces of hippuric acid are also found in the urine of man and carnivora, while it is present in relatively large amounts in that of herbivora along with considerable quantities of ammonia and apparently but little urea. The nitrogenous ingredients of the urine of mammals other than those just mentioned are either derived chiefly from the nucleoproteins, whose metabolism will be considered later, or are present in such small amounts as to call for no special consideration from the present very general point of view. Finally, it should be noted for completeness that a small amount of nitrogenous products is eliminated in the perspiration and also that from one point of view the incompletely katabolized nitrogenous excretory products of the feces (154) may also be regarded as products of protein katabolism. Urea, or dicarbamid, CO (NH2)2, is the chief nitrogenous product of the katabolism of the simple proteins in carnivora and omnivora. In human urine from 80 to 90 per cent of the nitrogen is ordinarily present in this form, although the proportion may be considerably diminished under special conditions, notably on a low protein diet. Urea, however, is not simply split off as such from the proteins as some earlier schematic statements have sometimes been taken to imply. The immediate antecedent of urea is ammonium carbonate, which undergoes a dehydration in the liver or elsewhere, while there is evidence in favor of the view that the ammonia is brought to the liver in the form of ammonium lactate. At any rate it is an accepted fact that most, if not all, of the nitrogen of the simple proteins passes through the ammonia stage on its way to excretion as urea.1 That the formation of urea from ammonia is not exclusively a function of the liver is shown by the fact that it still continues when this organ is excluded from the circulation by means of an Eck fistula. 1 It has been shown that the liver, kidneys and other organs contain an enzym which splits off the guanidin group from arginin (47) producing urea and ornithin. METABOLISM Hippuric acid in small amounts is a normal constituent of the urine of mammals but is especially abundant in that of herbivora. Its formation is the result of a synthesis (204). When benzoic acid or other compounds containing the benzoyl radicle are introduced into the circulation they are paired with glycin, one of the cleavage products of the proteins, in the kidneys and excreted as hippuric acid, which, chemically, is benzoyl-glycin, or benzamidoacetic acid, (C6H5 • CO)NHCH2 • COOH. The normal presence of small quan- tities of hippuric acid in the urine arises from the fact that the putre- faction of the proteins in the intestines yields compounds containing the benzoyl radicle which are resorbed and combine with glycin to form hippuric acid. But a small proportion of the hippuric acid pro- duced by herbivora can be thus accounted for, however. Most of it appears to owe its origin to the roughages consumed by these animals, especially those derived from plants of the gramineae, while, on the other hand, concentrates do not seem to increase its amount. Ap- parently its formation bears some relation to some of the ingredients of the cell walls, but to what ones in particular is not clear. 229. The non-nitrogenous residue. — In addition to the nitrogenous products eliminated in the urine, the complete oxidation of the protein molecule gives rise to the production of considerable amounts of carbon dioxid and water, which are excreted through the same channels as those derived from the katabolism of carbohydrates or fats. To put the matter in the reverse way, while the urinary products account for all the nitrogen of the protein, they contain but a relatively small part of its carbon, hydrogen and oxygen. This is clearly shown by comparing the average amounts of these elements in 100 parts of protein with the quantities contained in the urea corresponding to the total nitrogen of the protein. Disregard- ing the sulphur of the proteins, the results of such a computation are as follows : — PROTEINS UREA RESIDUE Carbon C5 o 6 86 46.16 Hydrogen . . ... 7 o 2. 2O 4.71 Oxygen Nitrogen 24.0 26.0 Q.I4 16.00 14.86 IOO.O 34-29 65-71 1 64 NUTRITION OF FARM ANIMALS After abstracting the elements of urea, there remains con- siderably over half the hydrogen and oxygen of the protein and the larger part of its carbon. A substantially similar result is reached in case of the other nitrogenous metabolic products. The splitting off of these products from the proteins leaves a non-nitrogenous residue. 230. Two stages of protein katabolism. — Two general stages in the katabolism of the proteins may be distinguished. The first is a hydrolysis by which the proteins are split up into their constituent amino acids. The second is a deaminization of the amino acids in which the nitrogen of these acids is split off as ammonia. 231. Protein hydrolysis. — The first stage in the katabolism of the body proteins is a hydrolytic cleavage, more or less similar to that effected in digestion and like the latter brought about by enzyms, which in this case are contained in the body cells — the intracellular enzyms (209). The truth of this view is attested by the facts that the pres- ence of proteases in almost all of the tissues and organs of the body has been demonstrated and that under proper conditions they effect a rapid solution of the tissue proteins — the so-called autolysis. Further confirmation is afforded by the known facts regarding the transformation of one protein into another in the body, while finally the production in the organism of some of the cleavage products of the proteins, presumably as products of katabolism, may be indirectly shown. 232. Is protein hydrolysis a reversible process ? — If the katabolism of body proteins is initiated by an enzymatic cleav- age in the body cells, this is precisely the reverse of the syn- thetic action by which it is believed that body proteins are built up out of the products of digestive cleavage (226), and the question at once arises whether we have to do here with a reversible enzym reaction, analogous to that which has been suggested as occurring in the case of the carbohydrates (214), the general nature of which may be represented by the formula Protein ^ Amino acids It must be freely admitted that proof of the reversibility of the action of proteolytic enzyms is as yet lacking, such phenom- ena as the formation of the plasteins discovered by Okunew METABOLISM 165 and the alleged formation of paraneuclein by the action of pep- sin as reported by Robertson being apparently due to adsorp- tion phenomena.1 On the other hand, however, many authori- ties2 are inclined to regard reversibility as a general charac- teristic of enzym action and mere negative evidence cannot, of course, disprove this belief. At any rate the conception of a reversible reaction between the amino acids of the blood and lymph and the proteins of the cells affords a comparatively simple and unforced explanation of the facts outlined in the foregoing paragraphs, as well as of others relating to the in- fluence of the supply of feed protein on metabolism which will be considered later (402). In particular, it may be observed that, according to this view, by no means all the amino acids resorbed into the blood stream would undergo synthesis to proteins but that, especially if the amino acid supply were liberal, a large part of them might pass directly to the 'second stage of protein katabolism, viz., deaminization. Finally, since the proportions of the single amino acids sup- plied from the digestive tract vary, one must conceive, not of a single reaction between protein and amino acid, but, speak- ing broadly, of as many independent reversible reactions as there are amino acids concerned. 233. Deaminization. — The second general stage of protein katabolism seems to be the splitting off of the NH2 group from the amino acids, the products being the corresponding or closely related non-nitrogenous organic acids on the one hand and ammonia on the other. This also, it would appear, is a case of enzym action, although the discovery of deaminizing enzyms in various tissues is comparatively recent and its biological im- portance is still to some extent speculative. The ammonia resulting from the deaminization of the amino acids is believed to be the immediate antecedent of urea, into which it is rapidly converted, chiefly although not exclusively in the liver (228). In this way the nitrogen of any amino acids resorbed in excess of the immediate demands of the body cells for protein building material is promptly converted into ex- cretory products and so disposed of, while the larger part of their carbon and hydrogen remains in a series of substances 1 Compare Rohonyi ; Biochem. Ztschr., 53 (1013), 179- 2 Compare Bayliss; The Nature of Enzym Action (1908), Chapter V. 1 66 NUTRITION OF FARM ANIMALS bearing a more or less close relation to the fatty acids and to- gether constituting the " non-nitrogenous residue " of the proteins (229). It is important to note that these non-nitrogenous products contain the larger share of the chemical energy of the original proteins, the ammonia carrying off but little and both the di- gestive cleavage and the deaminization being nearly isothermic processes. The cleavage and deaminization of proteins, there- fore, do not necessarily involve a destruction of their nutritive value and the excretion of a given amount of nitrogen in the urine is not to be regarded as indicating the total destruction of a corresponding amount of protein. It has ceased to exist as protein, but its non-nitrogenous residue is made up of substances which are closely related chemically to both the carbohydrates and fats, and which, like these, may be katabolized to supply energy. 234. Deaminization reversible. — Since deaminization in the body appears to be an enzymatic reaction, it is natural to in- quire whether in this case, as in the other enzymatic reactions already considered, there is any evidence that the reaction is a reversible one. So far as direct experiments with deaminizing enzyms are concerned, no such evidence has been produced, but Knoop l andEmbden and Schmitz2 have demonstrated a fact of funda- mental significance in metabolism, viz., that amino acids may be formed in the body from ketonic or hydroxy acids and am- monium salts. In other words, the animal body can manufac- ture some at least of the " building stones " of the proteins, and from the latter presumably the proteins themselves (226), out of the ammonium salts of the corresponding ketonic or hydroxy acids. The full significance of this comparatively recent discovery is not yet fully apparent. The question of the utilization of ammonium salts will be considered later. In this connection the important fact is that these results indicate that the reaction, or series of reactions, by which deaminization takes place is reversible, so that the whole process of protein metabolism may be represented schematically as follows : — -04-- ^. A • -j -> [ Organic acids Proteins X Ammo acids 2. S • [ Ammonia 1 Ztschr. Physiol. Chem., 67 (1910), 489. 2 Biochem. Ztschr., 29 (1910), 423. METABOLISM 167 235. Formation of carbohydrates from proteins. — It has already been stated (216) that carbohydrates may be manu- factured, in the bodies of carnivorous animals at least and probably in those of other species, but the question whether the proteins or the fats or both serve as the source was left open. Without entering into experimental details, it may be stated, as the general result of many trials in which the possibility of a production from fats was excluded as completely as pos- sible, that carbohydrates have been produced in such large amounts, and in quantities so closely paralleling the quantities of protein katabolized, as to amount to a proof of their formation from the latter. The acceptance, however, of the view that carbohydrates may be a product of protein katabolism by no means excludes the possibility of their formation also from fats. Indeed, in view of the importance of carbohydrates in metab- olism it seems altogether likely that the body has the power to manufacture them from both fats and proteins, while, as already stated (217), the reverse process of the formation of fats from carbohydrates has been demonstrated. With the increasing knowledge of the details of protein katabolism afforded by recent investigations, the question under consideration has assumed a somewhat different aspect, the discussion shifting from the fate of the proteins as a whole to that of the single amino acids and of the non-nitrogenous products of their katabolism. It has been shown, especially by the work of Lusk and his associates, that some at least of the amino acids (glycin, alanin, aspartic acid, glutamic acid, histi- din), after deaminization may yield dextrose. In the case of glycin and alanin all the carbon of the amino acid could be re- covered in the form of dextrose. In the case of aspartic and glutamic acids, on the other hand, only three out of the four or five carbon atoms respectively were found in the dextrose pro- duced. Still other amino acids, notably leucin and tyrosin, apparently do not yield dextrose, but instead compounds like /3 hydroxybutyric acid and aceto-acetic acid which are the distinctive products of the katabolism of the higher fatty acids (252). In the case of some, then, but apparently not all, of the prod- ucts of protein katabolism, the relations between protein and 1 68 NUTRITION OF FARM ANIMALS carbohydrate metabolism may be schematically expressed thus : - Glycogen $ Dextrose ^\ X* (" Lower fatty acids -> CO2 and H2O Proteins ^ Amino acids $ 1 INHs- — >Urea 236. Formation of fat from proteins. — Since the non-nitrog- enous products of protein katabolism appear to consist largely of comparatively simple substances closely related to the lower members of the fatty acid series, and since some at least of these may in all probability be synthesized to carbohydrates, while the latter can undoubtedly give rise to fats, it is natural to conclude that the non-nitrogenous products of protein katabolism may serve as a source of fat, either by direct syn- thesis of the simpler fatty acid chains or possibly by way of the carbohydrates. The conclusion is one which has been hotly debated and much of the earlier evidence in its favor has been shown to be inconclusive. The experimental evi- dence may be more conveniently considered in connection with a discussion of the sources of animal fat (247-249). For the present, it may suffice to say that the formation of fat from protein seems altogether probable, but that on the other hand the amount of fat thus formed under normal conditions is usually unimportant. § 5. THE METABOLISM or THE NUCLEOPROTEINS The metabolism of the conjugated proteins, with the excep- tion of the nucleoproteins, offers few features of special in- terest. In general it may be said that they are split up into their constituents during digestion and that the cleavage products undergo substantially the same metabolic changes as if con- sumed by the animal in the uncombined form. In the case of the nucleoproteins, however, the metabolism of the nucleic acid portion of the molecule calls for more specific consider- ation. Anabolism 237. Fate of digestive products. — The nucleic acids undergo extensive enzymatic cleavages in digestion (139), the products METABOLISM 169 passing into the circulation being essentially phosphoric acid, pentoses, and purin and pyrimidin bases. By analogy, with the simple proteins, one might expect, therefore, to find that these fragments of the nucleic acid molecule are rebuilt into nucleoproteins in the body cells, of which they constitute such an indispensable ingredient (75). The occurrence of such a synthesis, however, has been seriously questioned. One argument against it is the fact that the in- gestion of nucleoproteins, or more specifically purin bases, re- sults in a prompt excretion in the urine of end products of their katabolism which, although it has not been proved to be quan- titative, is certainly large, while the amounts excreted on a purin free diet are small and notably uniform. It has been argued, therefore, that the so-called " exogenous " purins, i.e., the nucleic acid constituents derived from the feed, are simply katabolized and excreted without serving to rebuild nucleic acids in the cells. Precisely the same argument might be made, however, against the synthesis of the simple proteins from their cleavage products, since in this case also an increase in the supply causes a prompt and almost quantitative increase in the ex- cretion of the end product, urea (402). 238. Autogenesis. — It is true that the formation of nucleo- proteins differs from that of the simple proteins in that the latter is a reconstruction of the molecule from its fragments 1 rather than a synthesis in the stricter sense, while it has been demonstrated that the body can build up nucleic acids out of a feed supply containing neither purins, pyrimidins nor pentoses. One of the most striking instances of this is seen in the development of the embryo of birds and insects. The eggs contain practically none of the substances just men- tioned, yet the bodies of the young animals contain normal amounts of nucleic acid. Equally significant is the case of the 'suckling mammal, which receives in the milk a food very poor in purins, pyrimidins and pentoses, yet which maintains a rapid growth and cell multiplication with its accompanying formation of nucleoproteins. So, too, in Osborne and Mendel's extensive investigations 2 upon the nutritive values of the proteins, normal 1 The possibility of the formation of proteins from ammonia (234) is of little sig- nificance under ordinary conditions of nutrition. 2 Carnegie Institution of Washington, Publication No. 156, p. 85. 1 70 NUTRITION OF FARM ANIMALS growth of rats through two generations was secured on purin- and pyrimidin-free feed. Another fact pointing in the same direction is that the body does not appear to require a supply of phosphorus in organic combination but can build up its organic compounds from phosphates (258). 239. Regeneration from, cleavage products. — In view of the capacity of the body to produce nucleoproteins in the entire absence of their constituent " building stones " may it be supposed that when the latter are supplied in the feed they may be recombined in the cells somewhat as are the ' amino acids of the simple proteins? No positive answer can be given to this question. It would seem, however, that the first steps in the autogenesis of the nucleoproteins must be the formation of pentoses and of the purin and pyrimidin bases, i.e., of precisely those substances which result from the digestive cleavages. Even though it be assumed that, in the former case, they are produced within the cells where they are further synthesized to nucleic acid, it is not altogether clear why the same substances brought to the cell by the blood current should not be available for the same purpose. Provisionally, at least, it seems perfectly possible to regard the entire stock of these " building stones " contained in the body, whether derived from the feed or produced by the body cells, as potentially available for the regeneration of nucleic acids. From this point of view, the increased excretion of purins which results from their ingestion would be con- sidered as a consequence of their increased concentration in the blood and as analogous to the increased excretion of urea which follows the ingestion of simple proteins or of amino acids (402). Katabolism 240. Cleavages. — The katabolism of the nucleic acids bears a close general resemblance to that of the simple pro- teins. As in the case of the latter, the first general stage of the process consists of a series of enzymatic cleavages. These cleavages are quite analogous to those of the simple proteins and yield as final products the comparatively simple " building stones " of the nucleic acids. Since it is to be supposed that the autogenesis (238) of these compounds is via these same METABOLISM 171 " building stones " it would appear that we have here, as in the case of the simple proteins, a complex of reversible enzym reactions. Phosphoric acid Nucleic acid Pentose Purin or pyrimidin bases 241. Deaminization. — The phosphoric acid which is split off from the nucleic acids is, of course, added to the general stock of this substance in the body. The pentose may be pre- sumed to be katabolized or possibly built up into a hexose. The bases, on the other hand, like the amino acids derived from the proteins, undergo, as the second general stage of their katabolism, an enzymatic deaminization and oxidation. The NH2 groups are split off as ammonia and converted into urea, while the ring formations are largely unbroken, the principal end products of purin katabolism being uric acid in man and allantoin in most other mammals. Of the katabolism of the pyrimidin bases little is known. The deaminization is never complete, however, purin and pyrimidin bases appearing in the urine along with the end products of katabolism. 242. Synthesis of uric acid. — In birds and reptiles, uric acid is the principal nitrogenous constituent of the semi-solid urine. Since no considerable portion of its nitrogen can have existed as preformed purins in the feed, it is evident that these animals must synthesize uric acid. This synthesis appears to take place in the liver, the antecedents probably being lactic acid and urea. § 6. THE METABOLISM OF 'THE FATS Anabolism 243. Re synthesis of feed fat. — In considering the resorp- tion of the fats (152) it was shown that, while the products of their digestion are glycerol and fatty acids (or their salts), after resorption only neutral fats have been recognized in the epithelial cells and in the lymph of the intestinal lacteals. The cleavage of the fats in digestion is reversed in the epithelial cells. It seems altogether plausible to ascribe this resyn thesis to the action of an intracellular lipase, the more since the action 172 NUTRITION OF FARM ANIMALS of lipase has been shown to be reversible in some cases (211). Whether this resynthesis be regarded as part of the process of resorption or be classed as one of the metabolic processes is a matter of indifference. In either case the material transmitted to the blood current consists substantially of fats. The digested fats are contained in the lymph in the emulsified form and in this state pass from the thoracic duct into the blood of the subclavian vein. The blood itself, however, although sometimes containing as much as i per cent of fat, does not normally carry emulsified fats, and the fat globules entering it from the thoracic duct do not long persist. The nature of the change is still uncertain ; by some, it has been regarded as a cleavage into fatty acids and glycerol and by others as a union with proteins. But whatever the nature of the change it seems to be well established that the fat of the blood exists in some sort of combination which is soluble in water and diffusible and which may be called for convenience " soluble fat." 244. Storage of fat. — A liberal supply of fat to the blood from the digestive tract may give rise to a storage of reserve fat in the adipose tissues (94) of the body. It is to be presumed that this deposition of reserve fat is substantially a reversal of the process, whatever it is, by which it was brought into solu- tion in the blood, the " soluble fat " of the latter passing into the cells and being there reconverted into the emulsified form and so giving rise to the globules characteristic of fat cells. 245. Formation of cell lipoids. — The fats deposited in the adipose tissues, as already implied, are a store of reserve ma- terial, laid aside temporarily from the body metabolism when the feed supply is more than adequate for immediate needs. The various more complex lipoids (37-39, 75), however (cholesterins, . lecithins and other phosphatids, cerebrosids, etc.), appear to be essential ingredients of protoplasm and to perform specific functions in the cell. All these substances have as their basis fatty acid molecules coupled with other groups and it is a reasonable assumption that the former are derived from the " soluble fat " of the blood and synthesized in the cells into the specific lipoids as required. 246. Manufacture of fats. — But while the feed fats may serve as a source of body fats, the organism is by no means dependent upon the former for its supply of these substances, METABOLISM 173 but may, as has already been indicated (217, 236), manu- facture fats from other ingredients of its feed. This view, first propounded by Liebig in 1843, was contrary to the opinion then prevailing and led to a lively controversy which, however, was definitely resolved in favor of the newer be- lief. Indeed, the feed fats, especially in case of herbivorous ani- mals, are usually of subordinate importance as sources of body fat, a large share of the latter being produced de now in the body. This fact explains in part the general uniformity of composition of the body fat of each species. The steer produces beef fat and the sheep mutton fat on substantially identical rations largely because the fat deposited in the body is derived only in small part from the feed fat, most of it being produced by the specific metabolic activities of the body cells. The seat of this synthetic production of fat, however, as well as the manner in which it is deposited in the reserve tissues, are still unknown. The sources of animal fat 247. Experimental evidence. — The sources of animal fat have been already indicated. Aside from whatever feed fat may be stored up in the adipose tissues, the body can produce fat from the carbohydrates of the feed (217) and in all prob- ability from the non-nitrogenous residue of the proteins (236). In view of the historic interest attaching to the long controversy over this question, however, as well as of its intrinsic importance, an outline of the experimental evidence seems appropriate. That the feed fat is a source of body fat was never seriously questioned. When the correctness of Liebig's contention that the animal body can also manufacture fat had been demon- strated, it was assumed that the source of this new-formed fat was to be found in the carbohydrates of the feed and this was for years the accepted view. Following Liebig's termi- nology, the proteins were designated as. the " plastic materials," serving to build up tissue, while the carbohydrates and fats were " respiratory materials," serving as sources of heat and of fat. 248. Fat from protein. — Several earlier investigators ob- served facts pointing to the formation of fat from protein in the animal body, but Carl Voit l was the first to distinctly ad- 1 Ztschr. Biol., 5 (1869), 79-169. 174 NUTRITION OF FARM ANIMALS vocate the belief that protein constitutes an important source of animal fat, this conclusion being based largely on the famous respiration experiments of Pettenkofer and Voit at Munich in which a dog was fed lean meat freed from visible fat as carefully as possible (this being the nearest practicable approach to a pure protein diet) and the balance of nitrogen and carbon (287, 292) determined. The results showed in many cases a retention of carbon by the animal greater than corresponded to the quantity of protein gained, and this difference was inter- preted, according to the methods described in Chapter VI (293), as showing a production of fat. Pettenkofer and Volt's experiments were long accepted as conclusive until Pflliger l subjected them to destructive criti- cism, showing the possibility of material errors in the estimates of the carbon of both feed and visible excreta. It scarcely need be said that this result does not prove that fat is not formed from protein, but simply that Pettenkofer and Voit's experiments fail to demonstrate it. Of later experi- ments on the subject, a number seem to show clearly the for- mation of a small amount of fat from protein, even after every allowance has been made for the objections raised by Pfliiger in his criticisms of the experiments. A number of negative results have, it is true, also been reported, but naturally nega- tive results are of much less value than positive ones. Moreover, the indirect evidence in favor of the possibility of the formation of fat from protein seems practically conclu- sive. As already stated (235), it has been established beyond reasonable doubt that carbohydrates may be produced from protein in the body. If this is true, however, it almost neces- sarily involves the possibility of the formation of fats from pro- tein, since carbohydrates are undoubtedly a source of fat. 249. Fat from carbohydrates. — Pettenkofer and Voit,2 how- ever, went further than to demonstrate, as they believed, the formation of fat from protein. Their experiments included a number in which carbohydrates were added to a ration of protein (lean meat). Assuming with Henneberg 3 that 100 grams of protein might yield 51.4 grams of fat, they computed that all the fat produced by the animal in these experiments 1 Arch. Physiol. (Pfluger), 51 (1892), 229. 2Ztschr. Biol., 9 (1873), 435. 3 Lanclw. Vers. Slat.., 10 (1868), 455. METABOLISM 175 could, with only one or two exceptions, be accounted for by the fat and protein of the feed. They, therefore, characterized the formation of fats from carbohydrates as improbable. The some- what general impression that Voit absolutely denied the pro- duction of fat from carbohydrates is incorrect, although he re- garded it as improbable and unproved. Indeed, he came later to admit the truth of the opposite view and even furnished from his own laboratory experimental evidence in its support. Never- theless, his earlier opinion as to its improbability obtained wide currency and in the hands of his followers became almost a dogma, so that protein was given a vital and preponderant im- portance the effect of which has been unfortunate both for the development of the science of nutrition in general and upon the theory of stock feeding in particular. Henneberg's estimate of the maximum fat production from protein was soon virtually accepted as an established fact and with the use of this high figure it was easy to compute from most of the experiments on fat production then on record that the fat and protein of the feed were sufficient to account for the fat produced. Similar computations upon a large number of later feeding experiments l yielded similar results, so that belief in the non-formation of fat from carbohydrates was further strengthened. One notable exception to the rule, however, were the experi- ments made by Lawes and Gilbert in 1850 upon the fattening of swine. These were comparative slaughter tests (284) in which the gain of fat was determined by comparing the weight and composition of similar animals, one before and the other after fattening. They were, accordingly, subject to a somewhat considerable range of error, but even on the most extreme as- sumptions it was impossible in^six out of the nine experiments to account for the fat actually produced by the supply of fat and protein in the feed. These investigators, therefore, con- tinued to maintain, in spite of much adverse criticism, the formation of fat from carbohydrates, although their experi- ments hardly secured the recognition which they deserved. As time went on, however, results began to accumulate which, like Lawes and Gilbert's showed a much larger production of fat than could possibly be ascribed to the fat and protein of See the author's Manual of Cattle Feeding, p. 177. 176 NUTRITION OF FARM ANIMALS the feed. This was particularly the case as it came to be more clearly recognized that Henneberg's estimate of a production of 51.4 grams of fat from 100 grams of protein was in all prob- ability too high, and especially after it was shown that what had been regarded as digested protein in many of these experi- ments (i.e., digestible N X 6.25) consisted in part of much simpler nitrogenous compounds. The ready formation of fat by the hog rendered this animal a very suitable subject for experiment, and the great majority of investigations on this animal have supported the view that fat is produced from carbohydrates, but similar results upon other species have not been lacking, while respiration experiments upon swine, geese, dogs, and especially the extensive investigations by G. Klihn l upon cattle have completed the demonstration.2 In the light of all these results, the formation of fat from carbohydrates in the animal body is now universally admitted, while its production from protein is still questioned by a few and in any case is of little economic significance, so that we have come back by a curious reversal of views almost to Lie- big's classification of the nutrients into plastic and respiratory. This conclusion applies specifically to the pure hexose carbo- hydrates, particularly starch. In many of the experiments cited, however, the non-nitrogenous material digested by the animal consisted to a not inconsiderable extent of those sub- stances of uncertain chemical nature included in the terms crude fiber and nitrogen-free extract. Postponing for the present any discussion of the nutritive value of these groups, it may suffice to say here that Kellner's investigations3 in particular show that both of them, including the pentosans, may serve as sources of fat. Katabolism 250. Body fat a reserve. — The stored fat of the adipose tis- sues, aside from its mechanical functions, constitutes the great reserve of energy-yielding material in the body. In the lack of an adequate feed supply, common observation shows that this 1 Kellner : Landw. Vers. Stat. ; 44 (1894), 257. 2 Compare the author's Principles of Animal Nutrition, pp. 165-184. 3 Landw. Vers. Stat.; 51 (1900). METABOLISM 177 reserve is drawn upon for the support of the internal activities of the body and as a source of energy for the performance of external work. 251. Mobilization of reserve fat. — In order that the stored fat may be used for the general metabolism of the body it must first be transferred from the adipose tissue cells to the localities where it is needed. Presumably this is accomplished by its reconversion into " soluble fat " and its passage through the walls of the cells into the blood, that is, by a reversal of the process by which it was laid down. Since the transfer of fat through the epithelial cells in resorption is effected by a hydrolytic cleavage (152), one is tempted to imagine a similar reversible enzymatic process in this case. Direct evidence of this is lacking, but apparently such a cleavage takes place some- where at an early stage in the katabolism of the fats, the re- sulting glycerol perhaps serving as a source of dextrose. From that point on the katabolism is , that of the fatty acids. 252. Oxidation at the p carbon atom. — The oxidation of the fatty acids, either saturated or unsaturated, to carbon dioxid and water, like the other katabolic processes already considered, is a step by step process. The researches of Knoop, Embden, Dakin and others 1 have rendered it highly probable, if not cer- tain, that the oxidation, at least in the case of the normal satu- rated acids, begins at the /3 carbon atom (i.e., at the second carbon atom from the COOH group) and results in the splitting off of two carbon atoms at a time. The products are carbon dioxid, water and a fatty acid containing two less carbon atoms than the original one and with which the same process of erosion is repeated. If it be true that the fatty acids thus undergo katabolism in the body by stages of two carbon atoms each, and particularly if it may be regarded as probable that they may be built up again in a similar manner from simpler atomic chains, there is afforded a plausible explanation of the rather striking fact that nearly all of these compounds found in the animal body contain an even number of carbon atoms. This scheme does not provide for the oxidation of the three lower acids of the series, propionic, acetic and formic, and in 1 Compare, Dakin, Oxidations and Reductions in the Animal Body, 1912, pp. 17-47- N 178 NUTRITION OF FARM ANIMALS fact, while these acids are known to be freely oxidized in the body, the chemical mechanism of the process is little under- stood. 253. Formation of carbohydrates from fats. — In discussing the probability of the formation of carbohydrates from pro- teins (235), it was pointed out that their origin might often be ascribed to either proteins or fats or both. It was there shown that in many cases the probabilities strongly favored a forma- tion from proteins. In other instances, however, the proba- bilities seem equally strong that fats give rise to carbohydrates. In particular, experiments upon pjiloridzin diabetes of the dog have shown the production of more sugar than could be formed from the quantity of protein katabolized during the same time, while the stock of glycogen in the animals experi- mented on had been so exhausted by fasting and muscular work that it seems scarcely possible to interpret the results other- wise than as showing the formation of sugar from fat. It should be added, however, that it has been seriously questioned whether the conditions of the experiments were sufficiently controlled to warrant the conclusions drawn. The very low values for the respiratory quotient (296) which have been reported in some cases for hibernating animals have also been interpreted as indicating a production of carbohydrates from fat. In the conversion of fat into sugar, there must ob- viously be an absorption of oxygen with no corresponding evolu- tion of carbon dioxid, the tendency of which would be to lower the respiratory quotient. The value of the latter for the direct oxidation of fat is 0.7. In hibernating animals, however, figures as low as 0.3 have been reported, while the weight of the fasting animal increased. While these facts, of course, do not demonstrate the formation of sugar from fat, they are quite compatible with that interpretation and seem to indicate a storage of oxygen. The more recent experiments on hibernat- ing animals, however, have failed to give such low quotients as were obtained by earlier observers. § 7. METABOLISM OF ASH INGREDIENTS 254. Certain chemical elements of the body and of the feed are found wholly or in part in their ash when these materials METABOLISM 179 are burned and are therefore spoken of as ash ingredients, al- though, as already pointed out (3, 5), this does not necessarily imply that they existed in the original material in " inorganic " combination. Most of these elements are as essential to the vital processes as the more abundant elements carbon, nitro- gen, hydrogen and oxygen of the so-called " organic compounds," although unfortunately the laws regulating their metabolism have been much less extensively studied. Among these ele- ments sulphur and phosphorus are of special importance in this connection. Sulphur 255. Sources. — While feeding stuffs may contain small amounts of sulphur in the form of sulphates, by far the greater part of this element in the feed of animals exists in organic compounds. Such, for instance, are the allyl sulphid (CsHs^S, contained in garlic and other members of the genus allium, and the allyl sulphocyanat, CsHs • CNS, found in mustard and other genera of the cruciferae. Ordinarily, however, the chief car- riers of organic sulphur, both in feeding stuffs and animals, are the proteins, which contain the element in the form of the di-amino acid cystin (47). 256. Katabolism. — The question whether the animal body can build up its sulphur compounds from inorganic sulphur does not appear to have been investigated. The katabolism of the cystin component of proteins pre- sumably follows the same general course as that of the other amino acids, i.e., it is split off from the proteins by hy- drolytic cleavage and subsequently deaminized. One of the products of. the katabolism of cystin appears to be taurin, CH2 • NH2 • CH2 • SOsH, contained in the taurocholic acid of the bile. To the extent, therefore, to which the latter com- pound escapes resorption in the lower intestine, it carries small amounts of sulphur into the feces. Both cystin and taurin, however, are readily oxidized in the body, the larger part of their sulphur taking ultimately the form of sulphuric acid and being excreted in the urine. The sulphuric acid of the urine exists in combination in part with aromatic radicles derived from the putrefaction of the proteins in the lower intestine and in part with bases. In human urine about one-fifth of I &0 NUTRITION OF FARM ANIMALS the total sulphur exists in a less completely oxidized form known as neutral sulphur, the nature and origin of which is obscure. Phosphorus 257. Forms ingested. — The phosphorus supply of the body is received substantially in the four forms indicated in Chapter I (5), viz., as phosphates, as phosphatids, as phospho- and nucleo-proteins and as phytin. Of these the various " organic " forms usually predominate. It appears probable, however, that all these various forms of phosphorus are resorbed into the blood stream in the form of phosphoric acid. Of the phosphates ingested as such this is certainly true. There seems good reason for believing that the phosphoric acid radicle contained in the nucleic acid of the nucleoproteins is quite completely split off by the digestive enzyms and reaches the blood as phosphoric acid (139), and the same thing is presumably true of the phosphoproteins. The phosphatids are probably acted on by the Upases of the digestive tract, but whether the glycerophosphoric acid resulting from their cleavage is further split up is unknown. The ready cleavage of phytin in seeds would suggest that probably its phosphorus also is resorbed as phosphoric acid. 258. Anabolism and katabolism. — The animal body contains a large store of phosphorus in the " inorganic " form, especially in the skeleton. For the maintenance or increase of this store the resorbed phosphoric acid is naturally available. The body also contains, however, organic phosphorus com- pounds, which, although less in amount than the inorganic, are of the highest significance for the vital functions. The be- lief that the phosphorus supply of the body is resorbed chiefly in the form of phosphoric acid necessarily implies, therefore, that the organism is able to utilize inorganic phosphorus for the synthesis of nucleic acids, phosphoproteins, phosphatids, etc., and the experimental evidence is strongly in favor of this belief (497). In this respect, as in many others, the synthetic power of the organism appears to be greater than was long supposed. Little is known regarding the course followed by the phos- phorus in the katabolism of the nucleoproteins, phosphatids, METABOLISM 181 etc. Ultimately it takes the form of phosphoric acid and is excreted in the feces or urine (199), but whether any inter- mediate compounds are formed is not known. Other elements While the other so-called ash ingredients are no less impor- tant than the two just considered, little is known regarding their katabolism in the ordinary sense, i.e., of the chemical changes which they undergo in the body. That they may exist in feeding stuffs in organic as well as in inorganic forms is probable. That they enter into organic combination in the animal body is likewise to be assumed but is positively known in only a few instances like that of the iron of the haemoglobin and the iodin of the thyroid glands. 259. Sodium and potassium. — Both sodium and potassium are contained in the ordinary foods and feeding stuffs and in addition man and farm animals consume not inconsiderable amounts of common salt, although it appears probable that this serves to a considerable extent as a condiment and that the amount actually necessary is less than is often supposed. Bab- cock, e.g., was able to keep cows for over a year without access to salt, except that contained in their feed, without any obvious ill consequences. Both potassium and sodium, as well as the chlorin com- bined with the latter in the form of salt, are excreted in the urine. 260. Calcium and magnesium. — These elements, calcium in particular, are especially important in their relations to the growth and maintenance of the skeleton, but they are not lacking in the soft tissues also, where they perform important functions. Of their intermediary metabolism little is known. As noted in Chapter IV (199), the normal path of excretion of calcium, and to some extent of magnesium, is through the lower in- testine, so that the apparent digestibility of these elements is no measure of the amount actually resorbed and utilized in the body processes. 261. Iron. — A long controversy has been carried on over the question whether inorganic iron may be resorbed and if so whether it can be utilized for the synthesis of the haemoglobins 182 NUTRITION OF FARM ANIMALS and for other purposes in the body. Both questions, however, may be regarded as settled in the affirmative. The matter is of importance in its relations to the use of iron in medicine but is of no special significance in stock feeding. The excretion of iron takes place almost exclusively through the intestines and this fact led to the earlier conclusion that inorganic iron cannot be resorbed. § 8. FUNCTIONS or THE NUTRIENTS 262. General scheme of metabolism. — In considering the metabolism of the several classes of nutrients in the foregoing paragraphs, it was found that the main features of the process FIG. 23. — Diagrammatic scheme of metabolism. in each case might be conceived of in accordance with the ideas suggested in § 2 (210-212) as consisting of a complex of rever- sible reactions accelerated or retarded by intracellular enzyms. By combining the equations used to represent those reactions, it appears possible to take a further step and formulate the fol- lowing highly generalized scheme for the total metabolism which may serve to show the interrelations between the metabolism METABOLISM 183 of the chief classes of organic nutrients. For the sake of sim- plicity some of the intermediate steps mentioned on previous pages have been omitted. The central portion of the diagram includes the feed substances taken up into the blood. At the extreme left are shown the main groups of tissue ingredients, and at the extreme right the excretory products. It cannot be too strongly emphasized that any such diagram as the foregoing is of necessity in the highest degree schematic. For one thing, neither the enzymatic nature nor the revers- ibility of the changes indicated in the diagram has been estab- lished except in a few cases. As already pointed out (212), this conception of the nature of metabolism is still to a large extent hypothetical, although the hypothesis harmonizes well with the present state of our knowledge. Moreover, aside from the mere omission from the diagram of certain recognized products of the intermediary metabolism, the chemical processes in the body are doubtless infinitely more complex than can be indicated in any such way. A vast num- ber of different substances have been identified in the animal body, many of which are known to have important functions in keeping the organism in running order but which are not even hinted at in this scheme. In brief, the scheme is concerned with the results of the metabolic processes so far as they are related to nutrition rather than with the mechanism by which these results are brought about. It seeks to show in outline how the principal groups of nutrients are related, on the one hand, to the building up of body tissues, and, on the other hand, to the formation of excretory products, and to indicate the mutual relations of the several groups. For this purpose it may perhaps serve a use- ful end as an aid to memory, provided its limitations are clearly understood. 263. Dual function of feed. — As pointed out in § i of this chapter (207), the animal body may be regarded in the light of a transformer of energy. By the agency of the protoplasm of its cells, in ways largely hidden from us, it converts the chemical energy supplied in its feed into the various forms characteristic of living matter. From this point of view the feed has a two- fold function. First, the feed ingredients are carriers of energy. The higher 1 84 NUTRITION OF FARM ANIMALS plants transform the radiant energy of the sun into the chemical energy of their various constituents, to be yielded up by the latter to the animal organism through the processes of metab- olism. This conception has become a familiar one and much emphasis has been laid upon it in recent years. Second, the feed supplies the specific materials required for building and maintaining all the complex structures of the body and for their harmonious functioning, i.e., it is the source of structural and repair material. That the proteins, fats and mineral ingredients which make up by far the larger part of the dry matter of the body (99, 280) are derived ultimately from the feed needs no special demon- stration, but the importance of many substances present in the body in only minute amounts tends to be overlooked. For example, the enzyms of the body, both extra- and intra-cellular, form no considerable portion of its mass, yet they are essential to its vital activities. So, too, the various hormones and se- cretions of the ductless glands, while ignored in the broad scheme of metabolism just presented, are essential to the vital processes. Clearly, the feed must supply material for the production of these and other similar substances. In other words, no amount of energy-yielding material will suffice to support life in the absence of those specific substances which are necessary in order that the machinery of conversion shall operate properly, much as *LO amount of coal under the boiler will enable an electric plant to furnish a normal amount of current if the insulation of the generator is defective. For example, if tryptophan is necessary for the formation of some essential internal secretion, a diet lacking that substance, however much energy it might furnish, would fail to support the organism permanently unless the body can manufacture trypto- phan from other substances. The latter qualification is a very important one. The animal is very far from being dependent upon the presence in its feed of all the varied chemical compounds required for its operation. Indeed, quite the reverse is the case. As has appeared in pre- vious sections, the actual substances resorbed are comparatively simple and uniform and upon them the animal body executes a great variety of chemical changes, both analytic and synthetic. What is necessary is that the resorbed feed shall include sub- METABOLISM 185 stances out of which the body can manufacture the compounds which it requires. 264. Functions of the proteins. — The proteins furnish at once the most familiar and the most striking example of this dual function of the feed. Since the proteins may be katabolized in the body with the formation of products (carbon dioxid, water, urea, etc.) con- taining either no available energy or but a small fraction of that found in the original proteins, it is clear that the latter serve as carriers of energy. In fact, it has been shown to be possible to maintain a carnivorous animal in normal activity for an indefinite time on a diet containing substantially nothing but protein as a source of energy. But proteins serve also as building material. Aside from water, the working machinery of the body is composed largely of proteins, while very many at least of the special substances already mentioned are nitrogenous and probably derived from the proteins. These protein tissues and other substances must be built up in the growing animal and maintained in the mature one, and for this purpose only proteins or their cleavage prod- ucts can be utilized, and their presence in the feed is indis- pensable. A point which sometimes causes perplexity is that the same portion of protein may not only serve as structural material but also yield energy for the vital processes, so that in esti- mating the energy supplied by a feeding stuff that of its protein as well as that of its other ingredients is included. The difficulty disappears, however, when it is remembered that any given portion of protein does not perform both these functions at the same time. If a gram of protein in the feed of a mature animal is used for structural purposes it practically takes the place of an equal amount of tissue protein, while the latter is katabolized and yields substantially the same amount of energy as would have been available from the gram of feed protein had that been katabolized instead. The latter, with its store of energy, has been temporarily set aside from the katabolic process but at some later time may itself be replaced by another gram of feed protein and katabolized in its turn, liberating the corresponding amount of energy. The repairing of a wooden building may serve as an illustration. The old wood taken out to make way 1 86 NUTRITION OF FARM ANIMALS for new material, as well as any surplus of new wood over that immediately required, may be used indifferently as fuel for warming the building. The case of the young animal, in which protein is permanently set aside for growth, is a trifle more com- plex but substantially the same considerations hold good. 265. Functions of fats. — In the case of the fats the energy- bearing function is the predominant and obvious one. Fats are a concentrated form of fuel, containing much more energy per unit than any of the other nutrients. They supply much energy in a small bulk and are, therefore, well adapted for the storage of reserve energy in the body. The fats and closely related bodies (the lipoids), however, are also important and apparently essential constituents of protoplasm (75). The lipoids, therefore, have important structural functions and an adequate supply of them in the body is indispensable. From this point of view, some interest attaches to the results obtained by a number of investigators who claim to have shown that a certain minimum supply of lipoids in the feed is essential, especially for growing animals. The evidence, however, is negative evidence, i. e., experimental animals failed to grow normally on a lipoid-free diet. In view of the positive results obtained by Osborne and Mendel,1 as well as of the fact that both the simple fats and the phosphatids, at least, can be synthesized freely in the organism, and taking into consideration the extensive synthetic power of the body in general, it is difficult to believe that the presence of lipoids in the feed is indispensable, and more recent investigations have afforded a different explanation of the observed facts (498). On the other hand, it has been shown that the lecithins stimulate growth and also that the fats appear, within certain limits, to favor the production of milk fat. 266, Functions of carbohydrates. — The carbohydrates even more distinctly than the fats serve chiefly as carriers of energy. While containing less energy per unit than fats, they can, on the other hand, be consumed in larger quantities and they practically supply the greater part of the energy in the diet of man and of farm animals. While the presence of carbohydrates (dextrose) in the blood and lymph is essential, this appears to be chiefly on account of their ready availability as fuel material. 1 Jour. Biol. Chem., 12 (1912), 81. METABOLISM 187 The carbohydrates seem, however, to have a specific, function in relation to the katabolism of fats. When the body is com- pelled to draw its energy supply chiefly from the fats, as in fasting or in diabetes (in which the power of katabolizing car- bohydrates is lost), or when carbohydrates are absent from the diet, the katabolism of the fats fails to be complete and con- siderable amounts of beta-oxybutyric acid as well as the ab- normal katabolic product aceton are excreted unoxidized in the urine. 267. Non-nitrogenous nutrients in general. — While it thus appears that both the fats (or lipoids) and carbohydrates may serve special purposes in the body, it is, nevertheless, clear that their chief function is to supply energy. Their amounts in ordinary rations are so abundant that as compared with their functions as carriers of energy any specific purposes which they serve in the body are amply provided for. As related to the nutrition of farm animals in particular, it is of special interest to note that not only the fats and carbohydrates di- gested as such but also the products of the bacterial fermenta- tion of the insoluble carbohydrates are available as sources of energy. 268. Functions of ash ingredients. — While the non-nitroge- nous organic nutrients serve chiefly as carriers of energy and only in a minor degree to provide the compounds necessary for the performance of specific bodily functions, the so-called ash ingredients represent the other extreme in this respect. They introduce practically no available energy into the organism but, on the other hand, they are not only essential structural com- ponents of the body tissues but likewise supply and maintain certain conditions indispensable to the performance of the bodily functions. The structural importance of the ash ingredients is most manifest in the case of the skeleton, which, in the higher ani- mals, contains relatively large amounts of calcium and phos- phoric acid and small quantities of magnesium, sodium and carbonic acid (81) which impart to it certain necessary me- chanical qualities of strength and rigidity. The necessity for a supply of these substances in the feed, especially in that of growing animals, is too obvious to require discussion. The ash ingredients, however, have other equally important functions 1 88 NUTRITION OF FARM ANIMALS in providing the necessary conditions for the chemical and physi- cal activities of the various tissues. 269. Osmotic pressure. — The cells of the various tissues draw their nourishment from the lymph which constitutes their immediate nutritive environment (185) and from which they are separated by cell walls which partake of the nature of semi- permeable membranes. In order to maintain normal condi- tions in the protoplasm of the cells the osmotic pressure of the lymph, and therefore that of the blood from which it is derived, must be maintained approximately constant. The osmotic pressure of the blood is stated to be approximately about 8 atmospheres, due largely to the ash ingredients contained in solution. With an adequate supply in the feed the concen- tration of mineral matter in the blood is regulated chiefly by the excretory activity of the kidneys. Thus, in the case of sodium chlorid, for example, it is estimated that the blood of an average man contains approximately 30 grams of this sub- stance, of which hardly half a gram is excreted daily when none is consumed. If, however, salt is added to the diet, the excess is promptly excreted in the course of the next twenty- four hours. What is true of salt in this respect is true also of other diffusible ingredients of the blood. 270. Ionic concentration. — The various salts are contained in the body largely in dilute aqueous solution. In such solu- tions, however, it is believed that salts are largely dissociated into their constituent ions, a dilute solution of common salt, for example, containing in addition to some unchanged NaCl the ions Na and Cl, one of calcium sulphate the ions Ca and SO4, etc. Acids are similarly dissociated, yielding hydrogen ions (H2SO4^:H + SO4), while alkalies yield OH ions (KOH ^ K + OH) . Some of these ions have been shown to have specific effects on certain cellular activities. For example, a frog muscle kept in 0.7 per cent NaCl solution retains its irrita- bility for one or two days. In a solution of a non-electrolyte, like sugar, asparagin, etc., having the same osmotic pressure, the muscle soon loses its irritability, but if NaCl be added to the solution it regains it. Since a number of other sodium salts produce the same effect, while chlorids of other metals do not, it is apparent that the effect is due to the Na ions. On the other hand, Na ions alone cause long continued rhythmic con- METABOLISM 189 traction of muscles, which, however, is suspended by the pres- ence in the solution of certain (not all) dyad ions like Ca or Mg. Numerous other examples of such antagonistic actions of ions are known, such as those observed by Loeb, for example, in the development of the egg. In general it may be said that cell activities are dependent among other things upon a suitable ionic concentration of various elements in their surroundings, and it is a striking and interesting fact that the so-called physi- ological salt solutions in which living organs may be kept func- tionally active for a longer or shorter time contain the various salts in approximately the same proportions as are found in sea water. Another example of the influence of ionic concentration is afforded in the case of the digestive enzyms. Ptyalin, for example, is sensitive to a very slight excess of hydrogen ions. Pepsin, on the other hand, is most active in the presence of hydrogen ions, while trypsin acts best in the presence of an excess of OH ions. 271. Maintenance of neutrality. — Closely connected with the foregoing topic and constituting indeed a special case of it, is that of the maintenance of neutrality in the body fluids. A fluid is neutral in the chemical sense when it contains no excess of H nor of OH ions, an excess of the former being equivalent to acidity and an excess of the latter to alkalinity. It has been shown that the blood serum, as a representative of the body fluids, is very nearly neutral, its content of H and OH ions being approximately 0.4 X io~7 and 7.2 X io~7, i.e., it has an alkalinity equivalent to about 0.000012 gram NaOH per liter.1 The body katabolism is continually producing acids, espe- cially carbonic, phosphoric and sulphuric acids (256, 259), which tend to increase the acidity of the blood. These acids are in part neutralized by the ammonia produced in the katabolism of protein (233), but it has been shown by the investigations of L. J. Henderson that the salts of the blood serum, especially the sodium phosphates and bicarbonates, play an important part in maintaining its neutrality. They are present in such 1 Blood is commonly said to be alkaline because it gives an alkaline reaction to ordinary indicators, such as litmus. Such a reaction, however, gives no definite measure of the true alkalinity or acidity. 1 90 NUTRITION OF FARM ANIMALS proportions that their solution possesses nearly the maximum capacity for the preservation ol neutrality, while they also, particularly the phosphates, serve as a means of elimination of an excess of acid through the kidneys in the form of the acid phosphates of the urine. 272. Other functions of ash. — The three general functions just enumerated by no means exhaust the list of offices performed by the ash ingredients. Iron, for example, is an essential in- gredient of haemoglobin, the coloring matter of the red blood corpuscles, which is the vehicle by which oxygen is distributed throughout the body (191). Although contained in the body in relatively minute amounts, this element is, therefore, one of prime necessity. lodin appears to be an essential ingredient of the thyroid glands, and although we are ignorant of its exact functions it is known that the absence of these glands, or their failure to function, gives rise to serious disturbances (goitre, myxcedema). Recent investigations seem to indi- cate that manganese and boron, and perhaps other elements not heretofore regarded as essential, may have important functions as catalysts in plants and perhaps, therefore, in animals also, although this is at present a conjecture. It is likewise possible that other elements present in small amounts may later be shown to have physiological functions. 273. Functions of water. — Its very familiarity tends to make us overlook the striking nature of the fact that life as we know it is impossible in the absence of water. If protoplasm may be regarded as a collodial solution, one may almost say that life is possible only in aqueous solutions. Some reasons for this are fairly obvious. The phenomena of osmotic pressure and ionization, for example, whose impor- tance has just been indicated, are substantially solution phe- nomena. It is possible also that there are more fundamental reasons for this striking fact. Certainly the larger share of our present chemical knowledge relates to the chemistry of either aqueous solutions or gases, two states resembling each other in many respects and in which chemical action seems to occur most readily, if indeed it ever takes place in the solid state. Moreover, it has been shown that some reactions, at least, in which water is not commonly regarded as concerned, are dependent upon the presence of minute amounts of this sub- METABOLISM 191 stance, or at least proceed with extreme slowness in its absence. Such, for example, is the action of chlorin on metallic copper or iron, or of oxygen upon many of the elements even at high temperatures. Aside from these considerations, however, the importance of the part played by water in the animal economy is sufficiently obvious, while it constitutes in most cases more than half of the weight of the body (97) and therefore may be regarded as having structural importance. CHAPTER VI THE BALANCE OF NUTRITION § i. GENERAL CONCEPTION 274. The animal as a prime motor. — The living animal constitutes what is known as a prime motor ; that is, it gener- ates power for its own operation and is able to produce a surplus which may be applied to do external work. In particular, a fairly close analogy may be drawn between the animal body and what are known as internal combustion motors. In such motors, a fuel (gas, gasoline, alcohol, etc.) is burned in the cylinder of the engine itself and its available chemical energy is transformed in part into motion and in part into heat. In a somewhat similar manner the compounds supplied to the cells of the body by the processes of digestion, resorption and circulation are katabolized, combine with the oxygen introduced through the lungs, and yield energy for the various activities of the organism. It should be noted that these activities include not merely ex- ternal work done by the animal but likewise a variety of internal work, such as that of circulation, respiration, digestion, resorp- tion, secretion, etc. In other words, the animal machine is always in operation, even when performing no external work. 275. Expenditure by the body. — When in operation, a me- chanical prime motor (a gasoline engine, for example) consumes two things. First, the material of which the working parts are composed is gradually worn away so that ultimately repairs are necessary, and second, fuel is consumed in amount depend- ing upon the work done. Substantially the same thing is true of the animal body. The working machinery of the body may be regarded as composed essentially of water, ash and protein. This ma- chinery, like that of the engine, is continually wearing out ; that is, the protein in particular is being continually katabolized and 192 THE BALANCE OF NUTRITION 1 93 the products of its oxidation excreted. In addition, the activi- ties of the body, like those of the engine, require a supply of fuel material containing available chemical energy equivalent to the work to be done. For this purpose the body utilizes in the first instance the substances contained in its own cells and tissues. As shown in Chapter V, all the organic ingredients of the body — protein, fat and carbohydrates — undergo katabolism, giving rise to carbon dioxid, water and compara- tively simple nitrogenous products, accompanied by a trans- formation of their chemical energy into other forms. In other words, the body is a storehouse of chemical energy as well as a mechanism. This stored-up energy of the body is contained particularly in its fat, and to a minor degree in its glycogen, while the body protein, although it likewise yields energy when katabolized, especially through the oxidation of its non-nitrog- enous residue (229), usually plays a small part quantitatively. The fat of the body constitutes its great reserve of energy. The store of reserve material in the body may be compared, for the sake of illustration, to the gasoline in the tank of an automobile, with the difference, however, that the body derives more or less energy from the combustion of the material (protein) of the engine itself. 276. The feed. — Neither the automobile nor the animal can long depend entirely upon its own stock of material without disaster. Sooner or later it must obtain supplies from the out- side. The supplies required in both cases are obviously of two classes, corresponding to the two classes of materials consumed in the operation of the machine, and may be briefly designated as repair material and fuel. In the automobile, parts of the machinery, the tires, etc., as they wear out must be replaced by new ones of the same kind, while the gasoline tank must be filled at intervals and the work- ing parts must be suitably lubricated. The case of the animal is precisely similar. In the first place, it must be supplied in its feed with materials from which, by the processes of digestion and resorption, it can secure the particular atomic groupings (amino acids, peptids, ash ingredients, etc.) which will exactly fit into its protoplasm and replace those eliminated by the vital activities. In the second place it must also derive from its feed molecules which it may, according to circumstances, break o 194 NUTRITION OF FARM ANIMALS down (katabolize) at once for the sake of their energy or store up as a reserve of energy (fat, glycogen) for future use. Finally, to carry the analogy a step further, it must obtain from its feed such amounts and proportions of the several ash ingredients as will maintain the necessary working conditions of osmotic pressure, ionic concentration and the like, somewhat as the engine must be lubricated. 277. Balance of income and expenditure. — It is evident from the foregoing considerations that the body exhibits two sets of activities, those concerned in its actions as a prime motor, tending to destroy it, and those of nutrition, tending to build up and increase it. Whether the body gains, is main- tained or falls away depends upon the balance between these two sets of activities. In a broad general way, of course, this fact is perfectly obvious. We do not need a physiologist to teach us that the horse or cow cannot long continue to do work or to yield milk unless supplied with sufficient feed to make good the resulting loss of body material. Similarly, we are familiar with the fact that those operations of the body which go on in a state of so-called rest likewise require material for their support, so that the mere maintenance of an animal calls for an expenditure of feed. What is needed in a scientific study of nutrition is something more than the mere general knowledge of these familiar facts ; namely, a quantitative measure of the extent to which the various feeding stuffs or their single ingredients contribute to the nutritive functions of the body under varying conditions. § 2. METHODS OF INVESTIGATION 278. Investigation of details of metabolism. — One method of attacking the problem just stated is by investigating the details of the metabolic processes. In the study of metabolism (including the chemical changes in digestion and resorption) the attempt is made to follow the various ingredients of the feed through the body and to trace in detail how, where and to what extent they contribute to the maintenance or growth of tissue or supply energy for the use of the organism. Such studies are of fundamental importance. They reveal to us how the animal mechanism operates. When carried to their ultimate con- THE BALANCE OF NUTRITION 195 elusion, and when accompanied by a complete knowledge of the chemical ingredients found in feeding stuffs, they will make it possible to give an exhaustive account of nutrition as a physico- chemical process. It is hardly necessary to say that the reali- zation of this ideal lies in the distant future. 279. Total nutritive effect. — Meanwhile, students of stock feeding are interested primarily in a somewhat different aspect of the subject, viz., in the aggregate effect of the varied and com- plex metabolic processes in reducing, maintaining or increasing the stock of matter and of chemical energy in the body. Is the body under any given regimen maintaining itself and making due growth, or is the animal doing work or yielding milk or other products at the expense of its own tissues? This is evi- dently a question of balance. Is the income of the body equal to its outgo ? 280. The schematic body. — The idea of the organism as dependent upon a balance between constructive and destructive activities may be made more specific by means of the conception of the schematic body, which regards the body of the animal, aside from water, as consisting essentially of ash, protein and fat, together with an amount of glycogen so small that it may for many purposes be neglected. The justification for this conception is found in the data con- tained in Chapter II, § 3, regarding the composition of the animal as a whole. It will be recalled that in the investigations there recorded the water, ash and fat were determined directly, the difference between the sum of these and the total weight of the animal, of course, showing the amount of fat- and ash-free dry matter. In those cases in which the total nitrogen contained in the body was also determined, it appeared (99) that, with one exception, the percentage of nitrogen in this fat- and ash-free dry matter closely approximated that in the animal proteins. In other words, the amount of glycogen and other substances included in the fat- and ash-free dry matter is so small as to be negligible and the latter may be considered to consist essen- tially of protein. From this point of view, it is evident that the effect of any feeding stuff or ration in causing a gain or preventing a loss of ash, protein and fat (and glycogen) shows its aggregate nutri- tive effect. Or, since the organic matter of the body may be 196 NUTRITION OF FARM ANIMALS looked upon in the light of stored energy, a still simpler ex- pression of the nutritive effect may be obtained by determining the effect of the feed upon the store of protein and of chemical energy in the body.1 Experiments directed to the determination of the gain or loss of matter and of energy by the body have been of two general kinds, viz., comparative slaughter tests and what are called balance experiments. Both have played an important role in the study of nutrition. 281. Live weight as a measure of nutritive effect. — At the very outset, however, the question arises whether the simple and obvious method of weighing an experimental animal is not sufficient to determine the aggregate effect of a ration, without the necessity for any elaborate experimental devices. The answer to this question depends largely upon the object of the experiment. If it be one undertaken to answer a com- mercial question, the increase in live weight during a considerable period, when determined with the necessary precautions, may be entirely adequate as a measure of the results obtained. If, for example, the question under investigation is the relative profits of two methods of fattening cattle, the gains made by a considerable number of animals, together with the judgment of the market regarding the quality of the finished animals, will substantially determine which method is to be preferred. The use of more elaborate experimental methods would not only be a needless refinement but might actually interfere with the settlement of the economic question involved. So, too, in the handling of young stock or in milk production, the general appearance and condition of the animals, together with the gain in live weight or the yield of milk, furnishes a sufficiently ac- curate indication of the practical results obtained, provided a sufficient number of individuals be employed. If, however, the purpose of the investigation is to study some question relating to the fundamental principles of nutrition, 1 To make the demonstration absolutely complete, of course, it would be neces- sary to show that the stock of each different kind of protein in the body had been maintained and that all the energy containing material derived from the feed was actually capable of yielding up its energy to the organism. Usually, however, especially on a mixed diet, it may be assumed that if the body maintains its stock of protein, each particular kind is practically maintained, while no considerable storage of unavailable energy in the body has been recognized. THE BALANCE OF NUTRITION 197 such, for example, as the relative values of the carbohydrates and fats, the changes of live weight are of little value as indica- tors. For this there are two principal reasons. 282. Fluctuations in live weight. — In the first place the live weight of an animal fluctuates considerably from day to day, even when taken under what seem to be identical conditions, chiefly on account of variations in the weight of the contents of the digestive tract. This is true of all animals, but especially of herbivora on account of their comparatively bulky feed, and reaches the extreme in ruminants. For example, a steer which had been receiving daily for two months a uniform ration of 6.35 Kgs. of timothy hay and which was kept under as uniform conditions as possible was weighed daily 24 hours after watering. On February 19 he weighed 419.0 Kgs. and on March 6 practically the same amount, 418.6 Kgs. The inter- mediate weights, however, were as follows: — February 19 419.0 Kgs. February 20 43 1 .6 Kgs. February 21 43i-o Kgs. February 22 440.6 Kgs. February 23 431.2 Kgs. February 24 444-8 Kgs. February 25 427.6 Kgs. February 26 427.9 Kgs. February 27 437.8 Kgs. February 28 436.0 Kgs. March i 437-2 Kgs. March 2 443-Q Kgs. March 3 428.4 Kgs. March 4 433.4 Kgs. March 5 436.8 Kgs. March 6 418.6 Kgs. It is evident that conclusions based upon a comparison of single weighings would have been entirely untrustworthy. Thus a com- parison of the live weight of February 19 with that of March 6 would have led to the conclusion that the animal was being practically main- tained. If, however, the initial weight had chanced to be taken on February 20, a comparison with that of March 6 would have shown a loss of 13 Kgs., while on the other hand, a comparison of February 19 with March 5 would have shown a gain of 17.8 Kgs. Even aver- 198 NUTRITION OF FARM ANIMALS aging two or three successive daily weighings, as is often done, while it reduces the error does not eliminate it. For example, a comparison of the average of February 19-21 with that of March 3-5 shows a gain of 8.7 Kgs., while if each average be taken a day later, viz., February 20-22 and March 4-6, the comparison shows a loss of 4.8 Kgs. By increasing the number of single weighings averaged, the uncertainty may, of course, be further reduced but not entirely elimi- nated, even ten-day averages varying materially, as is illustrated by the following figures. February 24-March 5, inclusive, 435.3 Kgs. February 25-March 6, inclusive, 432.7 Kgs. February 26-March 7, inclusive, 432.6 Kgs. February 27-March 8, inclusive, 434.2 Kgs. A similar reduction of the error may be obtained by the use of a number of animals combined into a group which is treated as an in- dividual, the fluctuations in the single animals tending to balance each other. These fluctuations are such as to preclude the use of the gain in live weight as a measure of the nutritive effect in exact scientific investigations, while it is evident that they must also be considered in the planning and interpreting of commercial experiments, as well as in judging the effects of rations in prac- tice. Such experiments should extend over a considerable length of time and include a considerable number of animals, while the weights on which comparisons are based should be the average of as many single weighings as possible. 283. Composition of increase. — In the second place, even were it possible to ascertain the gain or loss in weight by the body tissues proper, exclusive of the contents of the digestive tract, i.e., the empty weight, the composition of the material gained would still be unknown. An increase of a kilogram in tissue weight, for example, might consist chiefly of adipose tissue containing 10 or 12 per cent of water (95), or it might be largely muscular tissue with 75 or 80 per cent of water (87). Moreover, aside from the difference in water content, the dry matter of adipose tissue carries more chemical energy than that of muscular tissue, so that a gain of a kilogram in the former case would be equivalent to the storage of seven or eight times as much energy as in the latter. Finally, a knowledge of the THE BALANCE OF NUTRITION IQQ kind of material gained or lost is necessary. In the study of growth, for example, it is important to know how much of the increase in weight is due to a storage of protein, ash, etc., i.e., to real growth, and how much to a mere storage of fat or water, or both. For all these reasons the increase or decrease in live weight, while not valueless, is by itself an entirely inadequate measure of nutritive effect in investigations into the principles of nu- trition. In such experiments it is essential to determine at least the gain or loss of the great groups of substances of which the body is composed, viz., water, ash, protein, fat and if pos- sible carbohydrates, by one of the two general methods already mentioned as available for this purpose, viz., the comparative slaughter test or the balance experiment. 284. The comparative slaughter test. — This method seeks to determine by analysis the actual weights of water, protein, fat, etc., or the quantities of chemical energy, contained in the body of the experimental animal at the beginning and at the end of the experiment. Since, however, it is obviously im- possible to analyze the same animal twice, its original stock of protein, etc., is ascertained by the use of a check animal as exactly like the other in age, weight, condition, conformation, etc., as it is possible to select, which is slaughtered and analyzed at the beginning of the experiment. Assuming initial identity of percentage composition for the two animals, the results of this analysis are used to compute the weights of the several ingredi- ents contained in the body of the experimental animal at the outset of the experiment, while the animal itself is analyzed at its close. The method of comparative slaughter tests has the advantage of being a direct determination of the amounts of each ingredient gained and of requiring comparatively simple appliances. Furthermore, it may be applied not only to the conventional groups of protein, fat, etc., but to any substance capable of accurate analytical determina- tion. Finally, in addition to the total amount of any substance, its distribution between different parts of the body may be ascertained. On the other hand, the method has certain drawbacks. In the first place, it requires relatively long experimental periods. Assuming the work of weighing, sampling and analysis to be correctly performed, the accuracy of the results evidently depends upon the 200 NUTRITION OF FARM ANIMALS care and skill exercised in the choice of the check animal. The as- sumed identity of composition of the two animals cannot in the nature of things be proved and is very unlikely to be absolute. In a short experiment, therefore, the error thus possibly introduced may be rela- tively large. Its importance diminishes the greater the increase made over the original weight, i.e., the longer the period covered by the experiment. Furthermore, an experiment by this method can be divided into periods only by the use of additional check animals, involving additional assumptions as to identity of compo- sition at different times, while even these subdivisions, for the reason just stated, must be fairly long. Finally, the method is labori- ous, especially with the larger animals. The different parts of the carcass must be separated, the weight of each part accurately deter- mined, avoiding mechanical losses and making due allowance for evaporation of water. A correct sample of each part must be taken promptly and at once so treated as to preclude any changes previous to its analysis. The task of analyzing the carcass of a hog or sheep, and still more that of a steer, with the degree of accuracy required in -a scientific investigation is not one to be un- dertaken lightly. 285. The balance experiment. — The comparative slaughter test attempts to determine the weights of the several ingredients contained in the body at two different times. The balance experiment, on the contrary, consists of a comparison of in- come and outgo and does not attempt to determine the original stock in the body. If I know that I have a balance of $50 in bank at the beginning of the month and $150 at the end, it is clear that I have gained $100 in the meantime. This is the principle of the comparative slaughter test. On the other hand, if I know that my deposits during the month were $500 and my drafts $400, I am equally sure that I have gained $100, even if I do not know whether my balance at the beginning was $50 or $500. This is the principle of the balance experiment. If, for example, a steer digests 750 grams of protein out of his daily ration and if the amount of nitrogenous products excreted in 24 hours shows that he has katabolized 500 grams of protein, it is evident that his original stock of protein, whatever its amount may have been, has been increased by 250 grams. By compari- sons based on the same general principle, although more com- plicated as to details, the increase or decrease of the body's stock of fat, glycogen, ash and water or of chemical energy may THE BALANCE OF NUTRITION 2OI be determined. The specific methods used for such comparisons are described in the two following sections. A great advantage of the balance experiment is the comparatively short time which it requires. A period sufficiently long for the deter- mination of the digestibility of a ration (159) is in general suffi- cient also for a balance experiment, while for the requisite determina- tion of the respiratory products or of the heat produced twenty-four to forty-eight hours suffice, and even this short period may be divided into a number of subperiods of a few hours each. For this reason, and also because the animal is not injured in the process, repeated experiments may be made on the same subject, so that the effect of various rations or conditions may be compared on the same individual, while the method of comparative slaughter tests necessarily involves comparisons between two different animals. On the other hand, the complete balance experiment requires elab- orate and expensive apparatus, while opinions as to the relative amount of labbr involved in the two classes of experiments would perhaps depend largely upon the previous experience of the experi- menter. Furthermore, the balance experiment shows only the amounts of the constituent groups — protein, fat, etc. — gained or lost. It affords no opportunity to subdivide these and determine the fate of single chemical compounds nor does it give any clue to the particular region of the body where the gains have been deposited. 286. The balance of nutrition. — The phrase " balance of nu- trition " used as the title of this chapter refers in a general way to the balance between income and outgo of matter and energy in the body as determined by the methods of the balance experiment. Logically, of course, the comparative slaughter test, if com- bined with determinations of the feed consumed, may also be regarded as a balance experiment. In it the income of the body and the resulting gain are determined, leaving the outgo to be inferred, while in a balance experiment in the technical sense, the income and outgo are determined and the gain is inferred. Nevertheless, the latter type of experiment has played so large a part in the study of the balance of nutrition, both for physio- logical and for agricultural purposes, that a clear conception of its methods and postulates is essential for a comprehension of many of the results to be considered in subsequent chapters. The subject may be conveniently considered under the two heads of the balance of matter and the balance of energy. 202 NUTRITION OF FARM ANIMALS § 3. THE BALANCE or MATTER The gain or loss of protein 287. The nitrogen balance. — Feed protein which fails to be stored up in the body is not excreted as protein but in the form of the various products of its katabolism. The gain or loss of protein, therefore, cannot be determined by a direct comparison of its income and outgo because there is no outgo of protein as such. Since, however, the protein of the schematic body (280) is equivalent to total nitrogenous matter, the gain or loss of protein may be inferred from that of its characteristic element, nitrogen, and this is readily ascertained by a com- parison of the total nitrogen of the feed with the total nitrogen of the excreta, i.e., by a determination of the nitrogen balance. 288. Free nitrogen not excreted. — In Chapter V (228) it was stated that all the nitrogen of the protein katabolized is found in the urea and other organic compounds which are ex- creted in the urine. Obviously this is a point of fundamental importance. If nitrogen leaves the body only as combined nitrogen in the urine and in the feed residues and nitrogenous excretory products found in the feces, it is a comparatively simple matter to compare the income and outgo. If, however, the metabolic processes or the fermentations of the feed in the digestive tract yield also gaseous nitrogen, then the nitrogen of the respiratory products must also be determined, a task of no small difficulty. The question of the excretion of gaseous nitrogen has been the subject of a vast amount of investigation and controversy. Substantially two general methods of experimentation have been followed, viz., a comparison of the income and outgo of com- bined nitrogen and direct investigation of the respiratory products, and the results of both have been in substantial accord. The cumulative force of the great number of experi- ments in which substantial equality between income and outgo of combined nitrogen has been observed under condi- tions which precluded the possibility of any considerable gain or loss of body protein, together with the fact that the very careful and accurate investigations of recent years upon the respiratory excretion of free nitrogen have given negative results, amount to THE BALANCE OF NUTRITION 203 a demonstration that nitrogen leaves the body only in the combined form in the visible excreta. 289. Determination of nitrogen balance. — There being no excretion of gaseous nitrogen, a determination of the nitrogen balance requires simply a determination of the amounts of this element contained in the feed and in the visible excreta. Evi- dently this end is already partially attained in a digestion ex- periment (158). It is only necessary in addition to provide for the quantitative collection and analysis of the urine and, in very accurate experiments, of the perspiration and of the epi- dermal excreta, in order to obtain data for a comparison of the income and outgo of nitrogen, and the same precautions as to length of period, uniformity of feeding, etc., which are necessary in a digestion experiment, suffice also to render the results of a balance experiment representative. 290. Example of a nitrogen balance experiment. — The digestion experiment with clover hay used as an example in Chapter III (160) may serve also to illustrate the nature of a nitrogen balance experi- ment. In that experiment the hay consumed daily contained 3.144 Kgs. of dry matter and the daily feces 1.267 Kgs., while the average daily weight of the urine for g days was 5.449 Kgs. Analysis showed the following percentages of nitrogen : — In dry matter of hay 2.271 % In dry matter of feces 2.240% In fresh urine 1.074% The brushings of the steer (hair, dandruff, etc.) were found to con- tain 1.87 grams of nitrogen per day. The daily nitrogen balance may accordingly be computed as follows, showing a loss from the body which, of course, must be placed in the income column to complete the balance. TABLE 22. — EXAMPLE OF A NITROGEN BALANCE INCOME Grms. ODTGO Grms. Nitrogen in hay 71 4.O Nitrogen in feces Nitrogen in urine Nitrogen in brushings 28.40 5*8.50 1.87 Nitrogen lost from body 17-37 88.77 88.77 204 NUTRITION OF FARM ANIMALS 291. Computation of protein. — The conception of the sche- matic body (280) upon which balance experiments are based regards the total nitrogenous matter of the animal as consisting substantially of protein. All the vast number of other substances containing this element which have been identified as constituents of the body are insignificant in amount as compared with the great mass of protein which it contains. Accordingly, a gain or loss of nitrogen is interpreted as signifying a gain or loss of protein and the amount of the latter may be computed from the former just as the protein of a feeding stuff is computed from its protein nitrogen, it being only necessary to fix upon a suitable factor or factors, i. e.} to know the average percentage of nitrogen in body protein. From the results of analyses of entire bodies of animals cited in Chapter II, the average nitrogen content of the fat- and ash-free dry matter was computed (99) to be : — In Lawes and Gilbert's experiments . . . . 16.11% In Chaniewski's experiments 16.06% It is probable that in both cases the supposedly fat-free matter still contained some fat, it having been subsequently shown that extraction with ether does not remove the last trace's of it from ani- mal tissues. Kohler's analyses (88) of the fat- and ash-free lean meat of vari- ous species, after correction for the glycogen content of the horse flesh, show an average nitrogen content of 16.64 per cent. Since the material of Lawes and Gilbert's and of Chaniewski's experiments doubtless in- cluded some residual fat and other non-nitrogenous substance, and since the larger share of the protein of the body is contained in the muscular tissues, it appears justifiable to regard Kohler's figures as representing with substantial accuracy the average elementary com- position of body protein as a whole, especially since they are the results of direct analysis while the others are derived from slaughter experi- ments in which the limits of error are somewhat wide. Assuming, on the basis of Kohler's results, that average body protein contains 16.64 Per cent °f nitrogen, the corresponding protein factor is 6.0, and the gain or loss of nitrogen observed in a nitrogen balance experiment multiplied by this factor gives the gain or loss of protein. This is, of course, an approximation, since protein is not the only nitrogenous substance contained THE BALANCE OF NUTRITION 2O$ in the body and since not all the animal proteins contain exactly 16.67 Per cent °f nitrogen, but the error involved is insignificant in most cases so far as it relates to the question of the balance between income and outgo. On this basis, the steer in the foregoing example was losing daily 17.37 X 6.0 = 104.22 grams of body protein. Evidently the results of an experiment in which a gain of nitrogen occurs can be computed in precisely the same way. The gain or loss of fat and glycogen 292. The carbon balance. — By a method quite similar in principle to that just described for protein, it is possible to com- pute approximately the gain or loss of body fat from the com- bined income and outgo of nitrogen and carbon, while if the balance of hydrogen and of oxygen can also be determined the computation may be made considerably more exact and may include glycogen also. The experimental methods, however, are necessarily much more elaborate than those required for a simple determination of the nitrogen balance, since it is evident that, in addition to the carbon of the feed and of the visible excreta, it is necessary to determine the amount of this element contained in the gaseous excreta, viz., carbon dioxid and methane, while if the balance of hydrogen and oxygen is to be included, the hydrogen of the feed, the water excreted and the amount of oxygen taken up from the air must also be ascertained. An outline of the experimental methods employed for these purposes is given in a succeeding paragraph (297), but at the outset it seems desirable to confine attention to the principles involved. 293. Computation of gain or loss of fat. — According to the conception of the schematic body (280) on which the whole scheme of the balance experiment is based, substantially all the carbon of the body is regarded as existing in the two forms of protein and fat. Evidently if a comparison of the income and outgo of carbon shows a gain of that element it can, according to the fundamental assumption, have been only in one or the other or both of these two forms. The nitrogen balance, how- ever, shows the amount of protein gained and the carbon con- tent of protein is known. If the carbon of the protein gained 206 NUTRITION OF FARM ANIMALS be subtracted from the total gain of carbon, the remainder can have been gained only in the form of fat and the corre-. spending amount of this substance can be readily computed. 294. Example of a carbon balance. — In a respiration experi- ment on a steer a complete statement of the nitrogen and carbon balances is as follows : — TABLE 23. — NITROGEN AND CARBON BALANCES OF A STEER NITROGEN CARBON Income Grms. Outgo Grms. Income Grms. Outgo Grms. 6088 grms timothy huv 56.4 2I.Q 33-5 32.4 i-3 ii. i 2831.7 172.6 1428.7 124.2 8.0 1290.2 106.6 46.6 400 grms. linseed meal 16610 grms feces 4357 grms. urine 37 grms. brushings 4730 grms. carbon dioxid .... 142 grms. methan Gain by body 78.3 78.3 3004.3 3004.3 The nitrogen balance shows that the animal gained n.i X 6.0 = 66.6 grams of protein. According to Kohler's results (88), the average protein of cattle contains 52.54 per cent of carbon. Conse- quently, the protein gained in this experiment contained 66.6 X -5254 = 35.0 grams of carbon. The total gain of carbon, however, as shown by the carbon balance, was more than this, viz., 46.6 grams, and we accordingly have the following : — Total gain of carbon 46.6 grams Carbon in protein gained . . . 35.0 grams Carbon gained as fat .... 1 1.6 grams The elementary composition of animal fat was shown in Chapter I (34) to be very uniform, averaging 76.5 per cent of carbon. A gain of 0.765 gram of carbon in the form of fat, therefore, is equiva- lent to a gain of one gram of fat or a gain of one gram of carbon to 1.31 grams of fat, and accordingly the gain of n.6 grams of carbon in the form of fat shows a gain by the animal of n.6 -f- 0.765, or 11.6 X 1.31 = 15.2 grams of fat. Substantially the same method of THE BALANCE OF NUTRITION 207 computation can, of course, be applied when there is a loss of nitro- gen or carbon or both.1 295. Gain or loss of glycogen. — The only non-nitrogenous organic substance other than fat present in the body in sufficient amounts to affect the foregoing computations is glycogen. It is generally assumed that under reasonably normal conditions of feeding the glycogen con- tent of the body does not fluctuate materially, so that any consider- able or long continued gain of carbon, other than that contained in protein, is in the form of fat. Probably this is not equally true in the case of a loss of carbon, and in any case the results of computations like that of the preceding paragraph are evidently subject to a degree of uncertainty as regards a possible gain or loss of glycogen by the body. While this is probably not serious in reasonably long periods it may be relatively important in short experiments. If, however, there can be added to the determination of the nitrogen and carbon balance that of the balances of hydrogen and oxygen the means are afforded for a more accurate calculation, since it is evident that the amounts of the latter two elements, especially of oxygen, retained in the body would be greater in the case of a storage of glycogen than in that of a storage of fat containing the same amount of carbon. The method of computation is, however, somewhat complicated and need not be gone into here.2 296. The respiratory quotient. — The respiratory quotient is the ratio of the volume of carbon dioxid excreted by an animal to the volume of oxygen taken up during the same time, i.e., it is ' • The respiratory quotient will obviously Vol. G£ vary according to the kind of material which is being katabo- lized in the body. Thus in the oxidation of carbohydrates each liter of oxygen consumed gives rise to the production of one liter of carbon dioxid and the respiratory quotient therefore is i.o. On the other hand, when fat is oxidized, a portion of the oxygen unites with the hydrogen of the fat and only the re- mainder is available for the production of carbon dioxid. It is easy to compute, therefore, that each liter of oxygen con- sumed in the oxidation of fat will give rise to the production of 1 To avoid errors in computation it is convenient to regard losses in such compu- tations as negative gains and to carry through the computation exactly as in the above experiment, using the algebraic sum or difference in every instance. 2 See Atwater and Benedict, A Respiration Calorimeter, etc., Carnegie Institu- tion of Washington, Publication No. 42 (1905). 208 NUTRITION OF FARM ANIMALS 0.7 liter of carbon dioxid. Similarly, it may be computed that if protein of average composition be oxidized to urea, carbon dioxid and water, the respiratory quotient will be approxi- mately 0.8, although in reality the quotient for protein varies according to the nature of the nitrogenous products formed and the amount of carbon thereby withdrawn from oxidation to carbon dioxid. Ordinarily, however, the proportion of the gaseous exchange of the body due to the katabolism of protein is comparatively small, so that if, for example, the respiratory quotient closely approaches i.o, it is clear that the katabolism must be chiefly that of carbohydrates, while if, on the other hand, its value approaches 0.7, it is equally evident that the katabolism must be chiefly that of fat. Values for the respira- tory quotient intermediate between these extremes imply that the katabolism is in part that of fats (or proteins) and in part that of carbohydrates. The respiratory quotient of course affords no information regarding the balance between income and outgo but its deter- mination gives valuable information as to the nature of the material which is being katabolized in the body, particularly in short periods. 297. The respiration apparatus. — A determination of the gaseous exchange of an animal, such as is necessary in order to formulate the complete balance of matter, requires the use of some form of special apparatus known as a respiration apparatus. In its simplest and earliest form the respiration apparatus consisted of a closed chamber of known capacity, such as was used by Crawford, Mayow, Black, Priestly, Lavoisier and others in their early experiments. The animal was placed in the hermetically sealed apparatus and the changes in the com- position of the enclosed air which were brought about by its respiration were determined. Evidently, however, the method, while charmingly simple, is open to objections. The oxygen of the air is gradually consumed, while the carbon dioxid and other products of respiration accumulate. Even if the experi- ment be broken off before fatal results to the animal ensue, it is made under varying and increasingly abnormal conditions, while no very long trials are possible. Two obvious methods of avoiding this difficulty at once suggest themselves; either to absorb the products of respira- THE BALANCE OF NUTRITION 2OQ tion and replace the oxygen consumed or to conduct a current of air through the apparatus. Correspondingly, two different types of respiration apparatus have been evolved, known re- spectively as the closed circuit and open circuit apparatus, or, from the names of the investigators who first developed them into practicable appliances, as the Regnault-Reiset and the Pettenkofer apparatus. Each of these two types may be sub- divided into those intended to determine the total gaseous ex- change of an animal and those which take account only of the pulmonary exchange. 298. The Regnault-Reiset apparatus. — In the closed cir- cuit, or Regnault-Reiset apparatus, respiration takes place in RESPIRATION CHAMBER 0 used FIG. 24.- Scheme of closed circuit respiration apparatus. (Atwater and Benedict, Carnegie Institution of Washington, Publication No. 42.) a confined volume of air, the possibility of any exchange be- tween it and the outside atmosphere being carefully guarded against. By suitable mechanical means (a blower, for instance) the confined air is kept in circulation over suitable absorbents which take up the water and carbon dioxid given off, while the oxygen consumed is replaced from a gasometer or a cylinder of the compressed gas. The general scheme for such an ap- paratus is shown in Fig. 24. The increase in weight of the' ab- sorbents plus any increase in the amount of carbon dioxid and 2IO NUTRITION OF FARM ANIMALS water contained in the air of the apparatus shows the amounts of these substances produced, while the amount of fresh oxygen admitted minus any increase of the oxygen contained in the air of the apparatus shows the quantity of this element absorbed. u Any methane or hydrogen excreted accumulates in the ap- paratus and may be determined by an analysis of the contained air at the close of the experiment. The amount of nitrogen contained in the apparatus should, of course, remain unchanged if the apparatus is working properly. THE BALANCE OF NUTRITION 211 212 NUTRITION OF FARM ANIMALS If the entire respiratory exchange is to be determined, the subject is placed in the respiration chamber represented in the diagram. If only the pulmonary exchange is under investi- gation, the respiration chamber is replaced by a mask or mouth- piece or even by a suitable cannula inserted in the trachea. The original form of the Regnault-Reiset apparatus l is shown in Fig. 25. The same investigators subsequently devised a larger one in which they made a number of experiments upon animals of various species including sheep, calves, swine and fowls. In theory this is the most perfect form of respiration apparatus, but numerous tech- nical difficulties arise in its use. Various later forms have been de- vised but At water and Benedict 2 were the first to construct one of a size suitable for man which was capable of a high degree of accuracy. Quite recently Zuntz 3 has constructed a respiration apparatus of this type for experiments on domestic animals, a section of which is shown in Fig. 26, while for the determination of the pulmonary exchange, Benedict 4 has devised a so-called " Universal " respiration apparatus. 299. The Pettenkofer apparatus. — In the Pettenkofer, or open circuit, respiration apparatus, the subject breathes in a continuous measured current of atmospheric air whose content of water, carbon dioxid and methane is determined before and after passing the animal, the difference, of course, showing how much of each gas the subject has added. In an apparatus suit- able for small animals the entire amount of carbon dioxid and water in the incoming and outgoing air current may be deter- mined, but in the larger forms it is necessary to measure the air current and make analyses upon relatively small samples, so that the analytical errors are multiplied by a large factor, while a de- termination of the oxygen balance has not as yet been found practicable. The general scheme of such an apparatus is shown in the diagram, Fig. 27. As in the case of the Regnault-Reiset apparatus, the respiration chamber may be replaced by a mask, mouthpiece or cannula for the investigation of the pulmqnary exchange. 1 Ann. de Chem. et de Physique, 3^me Series, 26, 299. 2 Carnegie Institution of Washington, Publication No. 42 (1905). 8Landw. Jahrb., 44 (1913), 776. 4Deut. Arch. Klin. Med., 107 (1912), 156. THE BALANCE OF NUTRITION 213 The first practicable form of open circuit apparatus was devised by Pettenkofer **for experiments on man. Its general appearance is shown in Fig. 28. The comparative simplicity of its operation and CH+ OXIDIZED ABSORBED FIG. 27. — Scheme of Pettenkofer respiration apparatus. the fact that it could be readily built of any desired size led to its extensive use in investigations upon agricultural animals, notably by Henneberg and Stohmann at Gottingen, Stohmann at Leipzig and G. Kiihn and later Kellner at Mockern. FIG. 28. — Pettenkofer respiration apparatus. Explanatory sketch. {Atwater, U. S. Department of Agriculture, Office of Experiment Stations, Bulletin No. 21.) 1 Ann. Chem. Pharm., Suppl. Bd. II, p. i. 214 NUTRITION OF FARM ANIMALS The principle of the Pettenkofer apparatus has also been very extensively used for the investigation of the pulmonary exchange, especially by Zuntz and his associates, 'to whom the development of this form is largely due. Figure 30 shows a horse equipped with a tracheal cannula for experiments with this type of apparatus. Ow- ing to the fact that the excretory gases are not diluted with many times their volume of air, as is the case when a respiration chamber is used, the results are much sharper and it is possible to determine the amount of oxygen consumed as well as that of carbon dioxid given off. FIG. 29. — The Mockern respiration apparatus. (Bailey's Cyclopedia of Ameri- can Agriculture.) 300. Investigation of pulmonary exchange. — For many pur- poses a determination of the gaseous exchange in the lungs, either with the Regnault-Reiset or the Pettenkofer type of apparatus, is preferable to determinations of the total exchange in a respiration chamber. The former method is especially adapted for short experiments. By its use, it is possible to trace sharply changes in the amount of the metabolism, the respiratory quotient, etc., produced by the administration of feed substances, drugs, etc., by experimental lesions, and es- pecially by work, — changes whose amounts would often be relatively very small as compared with the total excretion for 24 hours as measured in the respiration chamber and which, therefore, if they did not escape detection altogether, could not be as accurately determined either quantitatively or chrono- logically. On the other hand, it is impracticable to continue its use through long periods, — a day, e.g., — and since it takes THE BALANCE OF NUTRITION 215 2l6 NUTRITION OF FARM ANIMALS no account of the excretion through the skin and the alimentary canal, it is only by indirect methods that it is possible to com- pute the total balance of carbon, hydrogen and oxygen by its use. 301. Balance of water and of ash ingredients. — The res- piration apparatus of either type serves to determine the ex- cretion of water vapor by the subject as well as that of carbon dioxid and other gases and thus, in connection with the neces- sary analyses of the feed and visible excreta, to establish the gain or loss of hydrogen. Unfortunately, more or less difficulty is experienced in determining accurately the hydrogen balance owing in part to the liability to condensation of water in the apparatus and in part to the fact that the amount of organic hydrogen actually entering into the metabolism of the animal is small as compared with the amounts of water consumed and simply evaporated again. The ash ingredients, of course, with the possible exception of minute amounts of sulphur, all leave the animal in the visible excreta and the balance of these elements may therefore be determined according to the same principles as the balance of nitrogen. § 4. THE BALANCE OF ENERGY 302. Balance of nutrition includes energy. — Since the animal body is essentially a transformer of energy (207), the balance of nutrition is not only concerned with the income and outgo of matter but also, corresponding to the dual function of feed (263), with the gain or loss of energy by the body. The study of the balance of energy is a method of investigating some of the important problems of nutrition which has been especially developed in recent years and which has proved fruitful of results. Before entering upon its consideration, however, a brief review of some of the elementary concepts of energetics as related to physiological processes may not prove superfluous. Elementary principles 303. Energy. — Up to this point the word energy has been used without any precise definition. In a specific study of the THE BALANCE OF NUTRITION 217 balance of energy, however, it is important to have as definite a conception as possible of what is meant by the term. It is not altogether easy to give a simple general definition of energy, but for the present purpose that given by Noyes l may be adopted, viz., " That which gives rise to changes in the prop- erties of bodies and to the power to produce such changes." For the present purpose, however, the conception of energy may be more readily apprehended from illustrations than from defi- nitions. The subject may be conveniently approached from the side of mechanics. A moving body is capable of producing certain effects by virtue of its motion. The falling weight of a pile driver, for example, forces the pile downward against the re- sistance of the ground and at the same time produces heat at the point of impact. The projectile fired from a sixteen inch gun striking the side of the armored ship overcomes the cohesive force of the armor plate and deforms or penetrates it, while the blow also gives rise to an evolution of heat. The blows of the blacksmith, if rapid and heavy enough, may raise the iron on his anvil to a red heat. Accordingly, it is said that a moving body possesses energy in the form called kinetic energy, or energy of motion. If a body suspended above the earth is set free it falls to the ground, and at the moment of contact with the earth possesses a certain amount of kinetic energy which was generated during its fall from something which was not energy of motion. This other form of energy, which the body possessed previous to its fall by virtue of its position, may be called gravitation energy. The same relation is illustrated by a swinging pendulum. Dur- ing the downward swing, the gravitation energy which it pos- sessed when at its highest point is converted into kinetic energy, while when it rises the kinetic energy which it possesses is re- converted into gravitation energy. When we lift a weight we are conscious of expending work, which is stored up as gravitation energy, to be liberated again as kinetic energy when the weight falls. 304. Forms of energy. — In general, whenever the rate of motion of a body is increased (or, to use a more familiar if less accurate expression, whenever motion is produced) it is to be 1 General Principles of Physical Science, 1902. 2l8 NUTRITION OF FARM ANIMALS inferred, as in the case of the falling body or the pendulum, that the kinetic energy produced has been derived from some other form of energy. In the examples thus far given this other form of energy was gravitation energy. In many familiar instances, however, this is not the case. The expanding steam in the cylinder of a steam engine parts with some of its heat to produce the motion of the piston. The electric current in the wire sets the armature of the motor in revolution. The combustion of gasoline in the cylinder of an engine produces motion of the engine as well as heat. Heat, electricity and chemical action may all be sources of kinetic energy and therefore the existence of heat energy, electrical energy and chemical energy is inferred. The manifestations of energy are of the most varied charac- ter but its forms may be conveniently grouped under the fol- lowing general heads : — 1. Kinetic energy 6. Magnetic energy 2. Gravitation energy 7. Chemical energy 3. Cohesion energy 8. Heat energy 4. Volume energy 9. Radiant energy 5. Electrical energy Of these, kinetic energy, chemical energy and heat energy are those of most importance in considering the balance of en- ergy in the animal body. 305. Transformations of energy. — As is illustrated by the examples given in the previous paragraphs, and as has been assumed in speaking of energy changes in the animal body, the various forms of energy are capable of mutual transformations. Heat may be converted into motion in the heat engine. Motion in turn is converted into heat when a moving body is retarded by friction or stopped by contact with another body. When gasoline is burned freely, its chemical energy is converted into heat, but when it is exploded in the cylinder of an engine it yields also motion. This motion in turn may be stored in the form of gravitation energy in a lifted weight, or as cohesion energy in a coiled spring, or it may be made a source of elec- trical-energy which in its turn gives rise to the radiant energy of light in the filament of a lamp. In brief, all the physical phenomena of the universe of which we can take cognizance can be described in terms of changes of THE BALANCE OF NUTRITION 2IQ energy either as to form or intensity, and this fact has led some physicists to identify the concepts of matter and energy and to maintain that the former can be fully interpreted in terms of the latter. Without entering here into this debated question, it will be convenient to follow for our present purpose the more familiar course of regarding matter and energy as two distinct although indissolubly connected entities. 306. The conservation of energy. — When a unit of kinetic energy is converted into heat energy it is found that the amount of heat obtained is always the same no matter what the process employed in effecting the conversion. Similarly, if a unit of heat be converted into kinetic energy the amount of the latter obtained is always the same and moreover is always equal to the quantity of kinetic energy which disappears when one unit of heat is produced. What is true of heat energy and kinetic energy in this respect has been shown to be true of all the forms of energy. Not only are they convertible into each other but there is no loss or gain of energy in the conversion. When a quantity of energy of one form disappears an equivalent quantity simultaneously appears somewhere in some other form or forms. This great generali- zation, perhaps the most important in the history of physical science, is known as the law of the conservation of energy, or the first law of energetics. It was first clearly and distinctly formulated by Mayer in 1842 and since that time has been verified by a great number of the most exact experiments and forms the basis of modern conceptions of physical processes. In substance, it asserts that the total energy of the universe as far as man knows it is a constant quantity, subject to con- tinual changes of form but neither created nor destroyed. That the law of the conservation of energy applies to the processes taking place in the body of the animal was exceedingly probable, a priori, and has been demonstrated experimentally by the researches of Rubner upon dogs, of Laulanie on various animals, of Atwater, Benedict, Lusk and their associates upon men and of Armsby and Fries upon cattle.1 The impor- tance of this fact in relation to the study of energy changes in the body is obvious. 1 Compare the writer's Principles of Animal Nutrition, pp. 263-268 and Penna. Expt. Sta., Bui. 126. 220 NUTRITION OF FARM ANIMALS 307. Heat energy unique. — In one respect heat energy occupies a peculiar position. Other forms of energy are in general readily and completely transformed into heat but there is no known method by which heat can be completely transformed into other forms, such as kinetic energy. Whatever portion of the heat is thus transformed obeys the law of the conservation of energy but part of it always remains in the form of heat.1 308. Units of energy. — Quantities of energy are measured by converting them into the same form and comparing them with some quantity of the same form of energy arbitrarily as- sumed as a unit. Since quantities of kinetic energy can be expressed in terms of mass, space and time, a unit based on these concepts is taken as the fundamental unit of energy. The so-called C. G. S. (centimeter-gram-second) unit is the erg. An erg is a quantity of energy equal to twice the kinetic energy possessed by a mass of one gram moving with a velocity of one centimeter per second. Since this is a very small quantity, a unit called the joule, equal to ten million ergs, is often employed in practical measurements of energy, that is, i joule = io7 ergs. For purposes where a still larger unit is desired the kilo-joule equal to one thousand joules is also employed. In practice, however, heat is the form of energy which gen- erally lends itself most readily to exact determination and, since other forms of energy are easily converted into heat, units of heat are extensively employed in the study of energy. The most common unit for this purpose is the calorie, which is the quantity of heat required to raise the temperature of one gram of water one degree centigrade.2 The foregoing is known as the small, or gram calorie (cal.). Where larger quantities of heat are to be measured the large, 1 This is, of course, one aspect of the second law of energetics. Theoretically, a perfect heat engine with a lower temperature limit of absolute zero would convert heat completely into kinetic energy. Since, however, we can neither obtain the temperature of absolute zero nor construct a perfect heat engine, this theoretical conception is simply a limit which may be more or less remotely approached in practice but never attained. 2 Since the specific heat of water varies at different temperatures, an exact defini- tion of the calorie must specify the temperature at which it is measured. Practice differs in this respect but the preferable unit is the mean calorie, which is one one- hundredth of the amount of heat required to raise the temperature of one gram of water from o° to 100° C. THE BALANCE OF NUTRITION 221 or kilogram calorie (Cal.), equal to one thousand small calories, is employed, while for still larger quantities the Therm, equal to one thousand large calories, may be used. In the following pages the term calorie signifies the large, or kilogram, calorie, unless the contrary is expressly stated. Certain units of gravitation energy are also frequently used, especially in mechanics, the more important ones being the gram meter, the kilogram meter and the foot pound. The gram meter is the energy required to raise a weight of one gram vertically through one meter in opposition to gravity, the kilo- gram meter is the energy required to raise a weight of one kilo- gram through one meter, and the foot pound is the energy re- quired to raise a weight of one pound through one foot. Since the force of gravity varies at different points on the earth's surface these units as thus defined are not invariable. Taking the average force of gravity at sea level, however, as equal to 980.5 dynes, the relations between these various units are as follows : EQUIVALENCE OF UNITS OF ENERGY ERGS GRAM METERS KILOGRAM METERS FOOT POUNDS GRAM CALO- RIES KILOGRAM CALORIES gram meter . . 980.5 X 10* O.OOI 0.007236 0.002344 0.2344X io-6 kilogram meter 980.5 X io5 IOOO 7.236 2-344 0.002344 foot pound . . 135-5 Xio5 138.2 0.1382 0.3239 0.000324 calorie . . . 4.184X107 426.6 0.4266 3.087 O.OOI Calorie . . . 4.184X10!° 426600 426.6 3087 IOOO 309. Measurement of heat energy. — Quantities of heat are measured chiefly in two ways, viz., by their effects in raising the temperature of some substance or in changing its state of aggregation. Instruments for measuring quantities of heat are called calorimeters, i.e., heat measurers. In the first method, as already implied in the definition of the calorie (308), water is ordinarily used as the calorimetric sub- stance.1 For example, if the quantity of heat to be measured can be transferred without loss to a kilogram of water, and if the temperature of the water is thereby raised 2° C., it is evi- dent that the quantity of heat imparted to it is two large calo- 1 Other substances than water may, of course, be employed, but water is usually the most convenient. 222 NUTRITION OF FARM ANIMALS ries. A calorimeter constructed after this principle is a water calorimeter. Such calorimeters have been devised in a great variety of forms according to the special purpose in view. The two essential requirements are that any escape of heat by con- duction or radiation shall be either preventable or measurable and that the temperature increase be accurately determined. In the second method the heat is caused to expend itself in changing the physical state of some substance as, for example, in melting ice or in evaporating some volatile liquid. The ice FIG. 31. — Lavoisier's ice calorimeter. (Schaefer, Text Book of Physiology.) calorimeter is one of the oldest forms of calorimeter and has been extensively used for certain classes of work. Figure 31 shows a simple form of ice calorimeter used by Lavoisier. The source of heat is placed in the central vessel and imparts its heat to the surrounding ice, while the access of any extraneous heat is prevented by the outside jacket of ice. In Lavoisier's calorimeter the amount of ice melted was determined by collecting and weighing the resulting water, but a much more accurate method of measurement is based upon the contraction which ice undergoes when converted into water. THE BALANCE OF NUTRITION 223 By means of the water calorimeter, it has been. determined that the conversion of one gram of ice at o° C. into liquid water at the same temperature requires 79.24 gram calories of heat. By the use of this factor the indications of the ice calorimeter can be converted into calories. 310. Heats of Combustion. — As related to nutrition in- vestigations, the chemical energy of an organic substance is most commonly measured by converting it into heat by complete combustion with oxygen and measuring the heat by one of the methods just indicated, the result being called the heat of com- bustion of the substance. In the method most commonly used in nutrition investigations, the substance is burned in highly compressed oxygen (about 25 atmospheres) in a lined steel bomb, the heat being taken up by water. The method was first devised by Berthelot and subsequently modified by Mahler, Hempel and Atwater. One form of this calorimeter is shown in section in Fig. 32. 311. Heats of combustion do not measure total energy. — It should be clearly understood that the heat of combustion of an organic compound does not, as is sometimes erroneously stated, measure its total energy but simply the amount mani- fested in a particular chemical change. Thus, in the complete oxidation of one gram of starch to gaseous carbon dioxid and liquid water 4183 gram calories of energy are transformed into heat. How much additional energy is still contained in the resulting carbon dioxid and water we do not know, nor is it necessary that we should. In using the heat of combustion as a measure of the chemical energy of starch the possible energy content of the carbon dioxid and water is simply assumed as an arbitrary zero, much as the engineer may assume a datum plane for his levels without regard to its height above sea level. In other words, the heat of combustion of starch or of any other substance shows how much chemical energy can be secured from it for conversion into other forms by processes of oxidation such as occur in the body. 312. Law of initial and final states. — In view of the very complicated nature of the metabolic processes, the question naturally arises whether the amount of chemical energy which a feed ingredient such as starch really puts at the disposal of the organism is the same as the amount of chemical energy which 224 NUTRITION OF FARM ANIMALS ' ' i. ,• ":. . ' .. . '.:; J/IG. 32. — Section of bomb calorimeter. (Atwater, U. S. Department of Agri- culture, Office of Experiment Stations, Bulletin No. 21.) THE BALANCE OF NUTRITION 225 is transformed into heat by its almost instantaneous burning in oxygen. The answer to this question is found in what is called the law of initial and final states. This law is that in any independent system the amount of energy transformed during a change in the system depends solely upon the initial and final states of the system and not at all upon the rapidity of the transformation nor upon the kind or number of the intermediate stages through which it passes. Although this law is true in the general form here stated, it was originally propounded as related to chemical reactions. If we start with starch and oxygen and end with the corresponding quantities of carbon dioxid and water, the amount of chemical energy converted into heat or other forms is the same, no matter whether the starch be burned almost instantaneously in pure oxygen or whether it be subjected to slow oxidation in the tissues of a plant buried in the soil ; whether carbon dioxid and water are the immediate products of the action or whether the starch passes through intermediate stages like maltose, glycogen, dextrose, lactic acid, etc., etc., as in the body of the animal. It is simply necessary to determine the difference in chemical energy between the system in its initial and in its final state to obtain the amount of energy transformed during the change. 313. Measurement of kinetic energy. — The most common method of measuring the energy liberated by a machine or an animal as motion energy is its conversion, actually or virtually, into gravitation energy, which is measured by the units given on a previous page (308). In case of small amounts of energy a weight may be actually lifted, the product of weight into distance giving the number of gram centimeters or foot pounds of energy expended. In other cases, the subject may pull against a resistance produced, for example, by the friction of a brake, the traction being measured by some form of spring balance. In this case the kinetic energy is, as a matter of fact, converted into heat, but the tractive pull multiplied by the distance gives the equivalent number of gravitation units. In still another form the subject virtually lifts his own weight by climbing the inclined plane of a tread power, the body weight multiplied by the distance multiplied by the sine of the angle of ascent equaling the units of gravitation energy to be measured. Q 226 NUTRITION OF FARM ANIMALS Figure 33 shows a form of this apparatus used by Zuntz for work experiments upon horses. Another method for measuring kinetic energy consists in converting it into electrical energy by causing the subject to work against the resistance of a magnetic field. The amount of current thus gen- erated can be measured in electrical units, or, as has been done by Atwater, Bene- Idict and others, the electrical energy may be converted into heat and measured in calories. The body's income of energy. — Gross energy 314. Only chemical energy can be utilized. — As was stated in the introductory paragraphs of this chapter, the animal body resembles an internal combustion motor in being a mechanism for the conversion of the chemical energy of certain compounds contained in the feed into kinetic energy. In consider- ing the balance between in- come and outgo of energy, it is essential to recognize a further point of resemblance, viz., that neither the animal nor the motor can utilize other than chemical energy. There is no evidence that the animal body can use in any way any of the other forms of energy, such as heat, elec- tricity or solar radiation THE BALANCE OF NUTRITION 227 which reach it from its environment, any more than the gasoline engine can use the energy of falling water or of an electric cur- rent. Chemical energy is not merely a source but the only source from which the animal body can derive its supply. 315. Gross energy. — The income of energy may be ascer- tained, therefore, by determining the chemical energy con- tained in the various compounds present in the feed in the manner already indicated (309), viz., by converting it into heat and measuring the amount of the latter by means of a suitable calorimeter. In other words, the income of chemical energy is measured by the heat of combustion of the feed. In order to avoid the implication that this is the total amount of energy associated with the feed (311), it will be convenient to use the term gross energy as equivalent to the amount of energy mani- fested as heat when the feed is completely oxidized. Since the chemical energy of a feeding stuff is converted into heat for purposes of measurement, its amount is usually ex- pressed in heat units. It should be clearly understood, however, that this is simply a matter of convenience and that it is the chemical energy of feeding stuffs and not the heat produced by their combustion which is of use to the animal. It is scarcely necessary to point out that the gross energy of the feed does not measure its nutritive value. Otherwise, anthracite coal, with a heat of combustion of some 7.9 Cals. per gram, would outrank most feeding stuffs, while hydrogen gas, with a heat of combustion of more than 34 Cals. per gram would stand still higher in the list. Obviously, the feed value of a substance depends not only upon its content of gross energy but upon the proportion of the latter which the body can utilize. 316. Heats of combustion. — The heats of combustion of a great variety of organic substances have been determined. Atwater l in 1895 published a compilation of results upon a large number of compounds of importance in nutrition, Fries 2 has prepared a rather more extensive list, and Benedict and Osborne 3 have determined the heats of combustion of nineteen vegetable proteins. 1 U. S. Dept. Agr., Office Expt. Stas., Bull. 21 (1895). 2 U. S. Dept. Agr., Bur. Anim. Indus., Bui. 94 (1907). 3 Jour. Biol. Chem., 3 (1907), 119. 228 NUTRITION OF FARM ANIMALS The following tabulation may serve to give a general idea of the gross energy of some of the more important substances concerned in nutrition. It should be specially noted that the figures given are in most instances simply approximate averages. TABLE 24. — APPROXIMATE GROSS ENERGY PER KILO- GRAM PER 100 POUNDS Animal protein Cals. C7OO Therms 258 6 Vegetable protein <;6^6 2ce 7 Carbohydrates 4.18=; 189 8 Sucrose 2QC C I7Q 4. Animal fats 0 3 o d t/3 1 O X !=> UHQ W PH <*>'.£>>. Kgs. Cals. Kgs. Kgs. Kgs. Cals. Cals. Cals. a 428.1 12,541 6 7 8403 4138 80.7 Winter b 434-1 11,674 6 -6 7704 3970 76.7 Summer e 450-4 12,364 6 6 7704 4660 87.9 Winter f 449.1 11,783 6 4-75 6830 4953 93-6 Summer i 440.1 11,893 6 6 7704 4189 80.2 Winter n 448.2 11,407 4.8 o 5.1 5672 5735 108.5 Summer c 442.2 12,450 0 0 10.5 7340 5110 97.6 Summer No. n8c 434-6 11,021 4.8 0.8 1.88 4122 6899 133-3 Winter In the experiments with a standard ration of 6 kgs. of oats, one of straw, arid six (or seven) of hay, the average computed fasting katabolism per day in three winter periods was 4.33 Therms, while in a single summer period it reaches the minimum of 3.97 Therms per head, or 4.08 Therms per 1000 pounds live weight. Zuntz and Hagemann consider that the latter amount represents approximately the minimum requirement for the in- 1 For a more complete account of the method, compare the writer's Principles of Animal Nutrition, pp. 386-387. MAINTENANCE — THE ENERGY REQUIREMENTS 297 ternal work and regard the higher figures obtained in the winter experiments as indicating a stimulation of the heat production by the low temperature to which the animal was exposed ; i.e., they consider that the experiments were made below the critical temperature. The notably higher results obtained with lighter rations they ascribe to a similar cause, viz., that the heat arising from the work of digestion, together with that due to the neces- sary internal work (fasting katabolism), was insufficient to maintain the body temperature. It must be confessed that, in view of the more active, tem- perament of the horse as compared with cattle this relatively low figure for the fasting katabolism is rather surprising, and the fact should not be overlooked that it is derived from short periods in which it is probable that the animal was unusually quiet. It perhaps represents more nearly the physiological than the economic minimum of net energy required for main- tenance, and it would be of much interest to compare it with the results of 24-hour experiments. 386. Metabolizable energy in maintenance rations. — A considerable number of experiments are on record in which the amount of total digestible matter required for the main- tenance of the horse has been determined. The maintenance rations of cattle and sheep may be deter- mined with a good degree of accuracy by varying the quantity of feed given until equality between income and outgo or con- stancy of live weight is attained, but this method is not fully applicable to the horse. Owing to his more active tempera- ment, feed seems to exert a greater stimulus upon his muscular activity than is the case with the more phlegmatic ruminants, so that a considerable excess over an actual maintenance ration may be consumed by a horse and expended in the various minor activities noted in Chapter VII (348), while the balance of income and outgo may show neither gain nor loss, i.e., may appear to show that the ration is a main- tenance ration. a. Wolff's determinations. — One method of avoiding this difficulty and determining the true maintenance ration is that employed by Wolff in his extensive investigations 1 upon work production by the horse (670, 779). In these experiments the 1 Compare the writer's Principles of Animal Nutrition, pp. 531-535- 298 NUTRITION OF FARM ANIMALS horse performed a measured amount of work 1 which was so adjusted in different periods as to be as nearly as possible in equilibrium with the feed consumed. This was considered to be the case when the live weight of the animal remained sub- stantially unchanged for a considerable period and when the urinary nitrogen did not show an increase as a consequence of the additional work done (637). By comparing the work performed on a basal ration with that which could be done with a heavier one, the ratio of the work done to the additional feed consumed was established within the limits of error of the method, this being the prime object of the experiments. This being determined, however, it was a simple matter to com- pute the amount of feed corresponding to the total work per- formed, while the difference between the latter and the total ration evidently was the maintenance ration. From the total digestible nutrients (inclusive of crude fiber) required for main- tenance, as thus computed by Wolff, the equivalent amounts of metabolizable energy required for maintenance may also be computed approximately by the use of Zuntz and Hagemann's factor of 3.96 Cals. per gram (776). In Wolff's earlier experiments and in those later ones in which approximately equal proportions of hay and grain were consumed, the maintenance ration was found to be approxi- mately 4200 grams total nutrients per 500 kgs. live weight, equivalent to 16.63 Therms. In those later experiments (in- cluding the results of similar investigations by Grandeau as re- computed by Wolff) in which a larger proportion of grain was fed, the total nutrients required for maintenance ranged from 3600 to 3800 grams, equivalent to from 14.26 to 15.05 Therms. In other words, the amount of metabolizable energy required for maintenance varied with the proportion of roughage pres- ent, as would be anticipated from the results with cattle re- corded on previous pages. b. Zuntz and Hagemann's results. — From a respiration ex- periment at the Gottingen Experiment Station, Zuntz and Hage- mann compute the metabolizable energy of the maintenance 1 Wolff's experiments were made with a sweep-power arranged to serve also as a dynamometer. The actual measurements of the work performed, except in the later experiments, proved to be too low, but Wolff believes them to be relatively correct, so that the ratio between the work as measured and the additional feed required to produce it may still serve as the basis of computation. MAINTENANCE — THE ENERGY REQUIREMENTS 299 ration of the horse by subtracting from the total digested nutrients the carbohydrate equivalent of the protein and fat gained by the animal, disregarding the possible stimulating effect of the feed. In this way, they find for the maintenance ration 2955.4 grams total digested nutrients per head, equiva- lent to 11.70 Therms or 12.1 Therms per 1000 pounds live weight, a result notably lower than Wolff's. This difference is ascribed by Zuntz and Hagemann to the larger content of crude fiber in Wolff's rations, the work of digestion of this ingredient as es- timated by them (777) very nearly accounting for the differ- ence. c. Muntz's experiments. — Muntz 1 in 1878-1879 attempted to determine the maintenance ration of the horse by starting with an insufficient ration and gradually increasing it until an equilib- rium between feed and live weight was secured, seeking in this manner to eliminate the stimulating effect of excess feed (392). The trials were made on the horses of the Paris Omnibus Com- pany, their work ration being known from previous experiments. He found that a ration equal to -f% of the work ration and which may be estimated to contain 12.1 Therms of metaboliz- able energy per 1000 pounds live weight was slightly more than sufficient for maintenance. d. Grandeau and LeClerc's results. — Grandeau and LeClerc,2 in addition to the experiments mentioned in connection with Wolff's results, fed five cab horses a ration of 8 kgs. of hay during a total of 14 periods of a month each (one to five periods for each animal) during each of which the digestibility of the ration was determined. On the average of all the periods, the results per day and head were as follows : - Total digestible nutrients (fat X 2.4) 2783.7 grams Equivalent metabolizable energy at 3.96 Cals. per gram .' 11.03 Therms Daily gain in weight 0.19 kg. Average live weight 393.6 kgs. The foregoing ration, which was evidently somewhat more than a maintenance ration, is equivalent to 13.1 Therms of metabolizable energy per 1000 pounds live weight. This is 1 Annales de PInstitut National Agronomique, Tome 3, 1876-1879. 2 L'alimentation du Cheval de Trait, 1883, in. 3oo NUTRITION OF FARM ANIMALS materially less than was obtained in Wolff's earlier experiments with hay and about the same as that found by him and by Zuntz and Hagemann for rations containing much grain. The following summary of the data regarding the metaboliz- able energy required for maintenance by the horse shows a considerable range of variation which is only partially expli- cable by the varying proportions of grain and roughage con- tained in the rations. TABLE 52. — MAINTENANCE RATIONS OF THE HORSE EXPERIMENTER GRAIN TO ONE OF ROUGHAGE METABOLIZABLE ENERGY PER 1000 LB. LIVE WEIGHT Wolff (Approximate) I O Therms IS.6 Wolff and Grandeau and LeClerc . . . Zuntz and Hagemann 2.4 I 8 13-6 12. 1 Miintz 0.7 12. 1 Grandeau and LeClerc Wolff hay only hay only I3-I I7.I 387. Metabolizable compared with net energy requirement. — The net energy required for maintenance, as with other ani- mals, equals of course the fasting katabolism. This Zuntz and Hagemann compute, in the manner already described (385) to be 4.1 Therms per thousand pounds live weight. As was there pointed out, however, those of their experiments in which the external temperature was lower or the amount of feed less gave higher results. The latter was also notably the case in earlier experiments in which still lighter rations were fed. On the average of the eight most satisfactory experiments out of twelve * on Horse II the total katabolism per day and head was 11.027 Therms upon a ration consisting of 3.5 Kgs. of oats, 0.5 of straw and 2.5 of hay. Computed in the same manner as in Table 51, the expenditure of energy in the digestion of this ration is equal to 3782 Cals., which leaves a remainder of 7244 Cals., equivalent to 140.3 Cals., per square centimeter of surface. This is a higher figure than any of those contained in Table 51, although the total katabolism was not notably different. 1 Landw. Jahrb., 18, i ; 27, Ergzbd. Ill, 356-357- MAINTENANCE — THE ENERGY REQUIREMENTS 301 The authors conclude, therefore, that when the amount of heat liberated by the digestive work is small the lack is com- pensated for by an increased katabolism of body tissue. Their final result is that their animal required per head at least n.oo Therms of heat to maintain his body temperature. In other words, this is the minimum of metabolizable energy which must be contained in a maintenance ration, since if less be present, even although the ration supply the requisite amount of net energy, body tissue would still be katabolized for the production of the necessary heat. Computed per thousand pounds live weight, Zuntz and Hagemann's estimated main- tenance requirement is : — Net energy for internal work 4.1 Therms Additional required for heat production 7.8 Therms Total metabolizable energy required 11.9 Therms In computing a ration for the actual maintenance of the horse at rest, it is necessary, according to these figures, to con- sider not only whether it supplies net energy equal to the fast- ing katabolism but also whether it contains sufficient metab- olizable energy to support the necessary heat production. On the other hand no such allowance need ordinarily be made in computing work rations. The horse when at work is producing an excess of heat (compare Chapter XIV) , and during the work- ing hours no expenditure of feed energy for the sake of heat production would be called for, while any ordinary working ration would probably contain a considerable surplus of me- tabolizable energy over the maintenance demand during the hours of rest. The maintenance requirement of fowls 388. Net energy requirements. — Gerhartz l has measured the net energy requirement of fowls by means of a num- ber of respiration experiments with the Regnault-Reiset type of apparatus (298) upon two fasting hens. He has also com- puted the fasting katabolism from a number of respiration ex- periments in which the animals were fed by subtracting from the total metabolism that computed to have been due to the 1 Landw. Jahrb., 46 (1914), 797. 302 NUTRITION OF FARM ANIMALS consumption of the feed — i.e., by substantially the same gen- eral methods used by Zuntz and Hagemann for the horse (385). His results, computed per thousand square centimeters of body surface and also per 5 pounds live weight in proportion to the f power of the latter, were as follows : — TABLE 53. — NET ENERGY FOR MAINTENANCE OF HENS LIVE WEIGHT PER 1000 SQ. CM. BODY SURFACE PER 5 POUNDS . LIVE WEIGHT In fasting experiments Minimum when not laying Average when not laying Average when laying . Grins. 2350 2273 2T.ZO Cals. 58.37 76.77 Q-l. 6? Cals. 57-Qi 76.75 01.45; Computed from experiments with feed Minimum when not laying 2137 . °f protein. Obviously, however, the growth of epidermal and adipose tissue can but partially ac- count for the observed gain of protein in many of these instances and apparently a distinct increase of the nitrogenous tissue in fattening must be admitted, averaging, in these experiments, about 0.2 pound per day and 1000 pounds live weight or about 5.5 per cent of the total increase in live weight. TABLE 68. — GAIN OF PROTEIN BY MATURE ANIMALS CHARACTER OF EXPERIMENT AVER- AGE LIVE WEIGHT DAILY GAIN OF PROTEIN Per Head Per 1000 Live Wt Cattle Kiihn and Kellner Sheep 1 Henneberg, Fleischer and Miil- ler Metabolism Metabolism Metabolism Slaughter Metabolism Metabolism | Metabolism * [ Slaughter J Metabolism [ Slaughter Slaughter Metabolism Slaughter Kgs. 667 34-2 54-8 48.3 43-5 38.5 35-2 35-2 36.7 30.7 "7-5 70.0 104.0 125.0 140.0 3-9 Grams 82.0 8.50 9.24 4-05 10.36 6-55 21.49 8.42 7.67 4-55 51.96 46.92 43-32 32.76 36-48 36.45 0.123 0.248 0.169 0.084 0.238 0.170 0.611 0.239 0.209 0.124 0.442 0.670 0.416 0.262 0.261 . 9-346 Weiske .... Henneberg, Kern and Watten- berff . Henneberg and Pfeiffer . Pfeiffer and Kalb Friske Pfeiffer and Friske Swine Soxhlet Meissl Geese Schulze 1 The nitrogen of the wool is not included in the gain. 2 The same animals were used also in the slaughter tests. 356 NUTRITION OF FARM ANIMALS 445. Influence of fattening on the composition of the lean meat. — While fattening consists largely in an increase of adi- pose tissue in the ordinary sense, it has an important effect both upon the composition of lean meat in the commercial sense and upon that of the muscle tissue proper (fat-free lean meat). Percentage 0} fat. — What is commonly spoken of as lean meat is by no means free from fat, since the term includes not only muscular fibers themselves with the relatively little fat FIG. 37. — The marbling of meat. Porterhouse steak from a prime steer. (Illinois Experiment Station.) which they contain, but the masses of connective tissue of all degrees of magnitude found between the muscle bundles and between the separate muscles (86). Fattening, especially in- tensive fattening, may cause a marked increase in the storage of intramuscular fat in the lean meat, as is evident to the eye in the so-called " marbling." Such analyses of lean meat as are recorded confirm the evi- dence of the eye in this respect. A summary of the results on this point has been given by the writer 1 elsewhere. The fol- lowing example taken from that publication may serve to illustrate the point in question. 1 U. S. Dept. Agr., Bur. Anim. Indus., Bui. 108 (1908), p. 33. THE FATTENING OF MATURE ANIMALS 357 TABLE 69. — FAT IN FRESH LEAN MEAT — LEYDER AND PYRO LEAN Cow % FAT Ox • % VERY FAT Cow % Neck 1.3 I.O 2.8 Leg O.Q 4 ° 1 8 Flank 0.8 42 8.8 Tenderloin . . 2 6 8 o I 2 O Similarly, Braman l found the following percentages of fat in lean meat from a medium fat (common) and a well-fattened (prime) steer. TABLE 70. — FAT IN FRESH LEAN MEAT — BRAMAN COMMON STEER PRIME STEER Porterhouse 6 72 1 2 71 Round ? '7A 6 66 A practical difficulty in making such comparisons arises in the preparation of the sample. Obviously, the subcutaneous fat sur- rounding the meat should be discarded and the same is true of the large masses of fat found between the muscles, but just what part of the adipose tissue scattered through the meat of a fat animal should be regarded as mechanically separable and what part should be re- garded as belonging to the meat proper is difficult to decide. Dif- ferences in the trimming of the pieces may account for some of the irregular results found by recent experimenters. Extractives. — It appears to be established that fattening increases the nitrogenous extractives of the muscles as well as causes a deposition of fat in and about them. For example, in Henneberg, Kern and Wattenberg's experiments on sheep, included in Table 65, the composition of the meat from the lean and from the very fat animal (partially freed from connective tissue) computed to the fat-free state, was : — llbid., Bui. 128 (1908), p. 86. 358 NUTRITION OF FARM ANIMALS TABLE 71. — COMPOSITION OF FAT-FREE MEAT OF SHEEP THIN SHEEP % VERY FAT SHEEP C7 70 Water . . . 70 4-1 7Q O2 Insoluble protein IS 8^ T c 72 Extractives Soluble protein Non-protein I.2Q 2 l8 1-93 2 17 Ash I 27 I 1^ Total extractives A 74. ET 2C IOO.OO IOO.OO It is computed that the actual gains during the fattening of the fat animal were as follows : — Insoluble protein 38.7 Grms. Extractives : Protein 82.0 Grms. Non-protein 4.2 Grms. Ash —9.2 Grms. 77.0 Grms. Total 115.7 Grms. Somewhat similar results were obtained later by the same authors in experiments on fattening lambs. Evidently this increase in the soluble nitrogenous compounds of the muscles is one of the factors going to make up the observed gain of protein by fattening animals. 446. Object of fattening. — The fattening of animals as a commercial process is a practice based on experience, which has shown that the tenderness and palatability of the meat are materially increased thereby, so that the consumer is willing to pay a higher price for it. It is to this improvement in quality in the first instance, and only secondarily to the gain in weight, that the feeder looks for his profit. The facts as to the composition of the increase in fattening recorded in the foregoing paragraphs serve to show what are the principal factors in this improvement in the quality of the meat. They are, first, the deposition of the intermuscular THE FATTENING OF MATURE ANIMALS 359 and intramuscular fat, and, second, an increase in the muscular tissues themselves, due in part at least to an increase in the soluble protein and in the nitrogenous extractives. The dep- osition of fat adds directly to the nutritive value of the meat, materially increasing its fuel value. Moreover, its mechanical effect in separating the fibers may be presumed to render the meat more tender, while the products of its decomposition in some forms of cooking (roasting and broiling) probably add to the flavor of the meat. The increase of the soluble protein is also doubtless one cause of the tenderness of the meat of fat- tened animals, while the other nitrogenous matters, though of little or no direct nutritive value, are an improvement through the added flavor and palatability which they bring about. § 2. FEED REQUIREMENTS FOR FATTENING 447. Comparison with maintenance. — In the two preceding chapters it appeared that the maintenance requirements are determined substantially by the amounts of protein and of energy which are katabolized in fasting and which, therefore, must be made good from the feed in order to maintain the body. By analogy, the amounts of protein and energy stored up in the process of fattening may be taken as the measure of the require- ments for fattening — i.e., of the amounts which the feed must be capable of supplying in an available form. In other words, the requirements for fattening are equivalent to the tissue produced just as the requirements for maintenance are equiva- lent to the tissue whose loss is to be prevented. The total feed requirement of a fattening animal, then, is to be regarded as made up of the maintenance requirement plus the fattening requirement. In one important respect, however, the fattening require- ment differs from the maintenance requirement. The latter, while not invariable, is still more or less constant for the same animal. In fattening, on the other hand, there may be a varying rate of production up to the limit set by the in- dividuality of the animal and its capacity to eat and digest food. Accordingly, as slow or rapid fattening is anticipated or desired, the daily requirement of the animal may be higher or lower. NUTRITION OF FARM ANIMALS Net energy values for fattening 448. General conception. — Physiologically, the process of fattening may be regarded as a storing up by the animal, against a possible future scarcity, of feed energy supplied in excess of its immediate needs. This storage of .energy is not accomplished without some loss. As in maintenance feeding, so in fattening, a considerable portion of the feed energy escapes utilization for one reason or another. The conception of the net energy value as express- ing that part of the feed energy which remains available after these various losses have been deducted, has been considered in Chapter VIII. The same conception may be extended to fattening rations. Just as the net energy value of a feed for maintenance is measured by the loss of body energy which it prevents, so its net energy value for fattening is measured by the storage of body energy brought about. 449. Method of determination. — This conception, as well as the method of determining the net energy value for fatten- ing, may be illustrated by the following respiration experiment by Kellner upon a mature ox, in which meadow hay was added to a mixed basal ration already sufficient to cause some gain. The second column of the table shows the metabolizable energy of the two rations, the third column the computed heat pro- duction, and the fourth the energy contained in the observed gain of protein and fat. TABLE 72. — DETERMINATION OF NET ENERGY VALUE FOR FATTENING HAY ADDED TO BASAL RATION METABO- LIZABLE ENERGY OF RATION COMPUTED HEAT PRO- DUCED ENERGY OF FAT AND PROTEIN GAINED BY BODY Lb. Therms Therms Therms Basal ration + hay .... 7-7 23.14 18.90 4.24 Basal ration — 17.64 15.62 2. 02 Difference 7 7 5cjO ^ 28 2 22 Difference per Ib. of hay . . 0.714 0.426 0.288 Each pound of hay added to the basal ration resulted in a gain of protein and fat containing 0.288 Therm of energy. THE FATTENING OF MATURE ANIMALS1 361 This was its net energy value for fattening. A comparison with Table 37 in Chapter VIII (364), showing the results of a determination of the net energy value for maintenance, renders evident the identity of the method employed in the two cases, the only difference being that in one case the comparison be- tween the two rations is made below the point of maintenance and in the other case above it. It is evident that in fattening, as in maintenance feeding, there is a considerable expenditure of energy consequent upon the consumption of feed, so that, only part of the metabolizable energy is actually stored up in the gain by the oody. In the experiment given as an illus- tration, one pound of the hay contained 0.714 Therm of metabolizable energy, of which only 0.288 Therm or 40.7 per cent was recovered in the gain. 450. Relative values for maintenance and for fattening. - The same causes which were considered in Chapter VIII (367) are of course operative to bring about the increased expendi- ture of energy on the heavier rations of the fattening animal. In addition, it would appear that the chemical changes in- volved in the formation of fat from proteins and carbohydrates would result in more or less evolution of heat. Whatever expenditure of energy may be thus caused is additional to that caused directly by feed consumption under maintenance con- ditions and must evidently tend to reduce the net energy value of the feed by a corresponding amount ; in other words, the net energy values of feeding stuffs for fattening would tend to be lower than those for maintenance. Such data as are available, however, do not appear to indicate that this difference is a considerable one in the case of farm animals, and it would appear that, in the case of cattle at least and presumably in that of other species, the net energy values of feeding stuffs may be regarded as being substantially the same for fattening as for maintenance.1 Energy requirements for fattening 451. Energy content of gain. — Since the net energy value of a feeding stuff or ration for fattening, as explained in the foregoing paragraphs, is that part of its total energy which can be stored up by the animal in the increase, it follows that the 1 Compare Armsby and Fries, Jour. Agri. Research, 3 (1915). 435- 362 NUTRITION OF FARM ANIMALS ration of such an animal must supply an amount of net energy equal to the maintenance requirement plus the quantity of energy contained in the gain made. The latter quantity, how- ever, may be computed approximately from the data given in § i regarding the chemical composition of the increase in live weight during fattening. Estimating the energy content of protein at 2586 Cals. per pound (5.7 Cals. per gram) and that of fat at 4309 Cals. per pound (9.5 Cals. per gram), the energy content of one pound of increase was as shown in the last col- umn of Table 65 (442). Excluding two apparently questionable results,1 the range and average of the remainder are as follows. Although some- what variable they indicate that on the average of an entire fattening period a pound of increase in live weight in cattle, sheep and swine is equivalent to about 3.25 Therms. TABLE 73. — ENERGY PER POUND INCREASE IN LIVE WEIGHT Maximum 4.002 Therms Minimum 2.485 Therms Average 3-245 Therms 452. Influence of stage of fattening. — The results just cited are in most cases those of an entire fattening period. There can be little doubt, however, that the composition of the in- crease and its energy content vary materially as the fattening advances. This appears clearly from Henneberg, Kern and Wattenberg's re- sults upon sheep. The "fat" animal had been fed for 10 weeks and was regarded as fat according to local standards. The "very fat" animal had been fed for 29 weeks, or until no further gain in live weight occurred. As the table shows, the total gain by the "very fat" animal contained materially lower percentages of water, ash and protein and a higher percentage of fat, and had a 10 per cent higher energy content than the gain by the "fat" animal, while a com- parison between the "fat" and the "very fat" animals shows the gain made by the latter during the last 19 weeks of fattening to have 1 Pfeiffer and Friske's results appear exceptional, since the gain apparently con- sisted to an abnormally large extent of water, while the authors themselves point out that the gains of dry matter were notably less than should have been produced from the feed consumed. It would seem, therefore, that their omission is justified. Soxh- let's result upon swine No. i has also been omitted for a similar reason. THE FATTENING OF MATURE ANIMALS 363 contained nearly 92 per cent of fat and to have had an energy content of 4002 Cals. per pound. The same investigators also obtained en- tirely similar results in the fattening experiments on lambs cited in Chapter XI (458) although naturally the proportions of water and protein in the increase were greater than in the case of the mature sheep. During the earlier stages of fattening, especially with thin animals, the storage of fat is accompanied by a considerable gain of water and by more or less increase in body protein. As the fattening progresses, however, the gain comes to consist to an increasing extent of fat accompanied by very little protein and a relatively small percentage of water. The energy content of a unit of gain in live weight, therefore, in the later stages of fattening is materially greater than in the earlier stages of the process. Evidently, then, more net energy will be required in a fattening ration to produce a pound of increase in live weight toward the close of the fattening process than at its beginning, a fact which is entirely in harmony with the ex- perience of feeders that gains become increasingly expensive as the animals become fatter. So far as definite conclusions are warranted from the rather scanty data available, it would seem that in the earlier stages of fattening a ration supplying (in addition to maintenance) about 2.5 Therms of net energy would be sufficient to support a gain of a pound of live weight, while in the later stages the requirement may rise to 4.0 Therms or perhaps even more. Protein requirements for fattening 453. Protein unnecessary for fat production. — It was shown in Chapter V (247-249) that body fat, especially in the case of farm animals, is derived chiefly from the non-nitrog- enous nutrients of the feed, protein playing but a subordinate role in its production, and Kellner has shown (769) that the proportion of the energy of protein which can be stored up by mature fattening animals is distinctly less than the correspond- ing percentage for the non-nitrogenous nutrients. So far as simple fat production is concerned, therefore, it would appear that a surplus of protein over that required for maintenance would be unnecessary and possibly disadvantageous on account 364 NUTRITION OF FARM ANIMALS of its tendency to stimulate the general metabolism of the body (365). 454. Protein in increase. — As appeared in § i (444), how- ever, the actual increase in mature fattening animals has been found to contain a relatively small and rather variable propor- tion of protein, due in part to the growth of epidermal tissue, in part to an increase in the number of fat cells, and in part to an actual storage of protein and nitrogenous extractives in the muscular tissue (and in the internal organs?). It is to be remarked, however, that in most or all instances the rations consumed contained more protein than was necessary for main- tenance, with, of course, an abundant supply of non-nitrog- enous material, so that the conditions were favorable for such a storage of protein as that just mentioned. So far as the writer is aware, it has not yet been shown that the mature fattening animal actually requires any surplus of protein over the amount necessary for maintenance, although it can apparently utilize an excess, at least to some extent, to increase the stock of protein in its body. At most, the requirement of the fattening animal as meas- ured by the observed storage of protein is relatively small, one pound of increase in live weight containing in round numbers from 0.02 Ib. to 0.08 Ib. of protein. 455. Utilization of feed protein. — Assuming the observed gain of protein by the fattening animal to represent a real re- quirement, it is evident that a sufficient fattening ration must supply, in addition to the protein necessary for maintenance, an additional amount sufficient, after undergoing the various processes of digestion and metabolism, to yield the amount of body protein contained in the increase of body weight. As will appear more particularly in considering the subject of growth (470, 471), little is known regarding the amount of feed protein required to yield a unit of body protein. Doubtless this will differ as between different individual proteins, depend- ing, for one thing, upon the proportions of the different amino acids which they contain, but adequate quantitative data are as yet unavailable. 456. Protein in fattening rations. — In the absence of definite knowledge regarding the availability of the protein of the feed, the question of the amount of this nutrient which should be THE FATTENING OF MATURE ANIMALS 365 supplied to fattening animals may be approached much as was the question of- the amount necessary for maintenance in Chap- ter IX, i.e., by inquiring what is the least amount of digestible protein which, along with sufficient non-nitrogenous nutrients, has sufficed to support a satisfactory rate of fattening. If it appears that of two similar animals or lots of animals receiving equal amounts of feed, the one consuming the smaller amount of protein gave equally satisfactory gains, both as judged by the live weight and by the block test, it may be concluded that the smaller amount of protein was at least sufficient, although it cannot be determined whether it may not have been greater than was actually necessary. Unquestionably, the protein requirements of mature fatten- ing animals have been greatly overestimated in the past. Wolff's original feeding standards (791), published in 1864, recommended for fattening rations per thousand pounds live weight the follow- ing amounts of digestible protein : — Cattle 2.5-3.0 Ib. Sheep 3-0-3-5 lb- Swine 2.7-5.0 lb. Substantially these same figures have been repeated more or less uncritically from publication to publication, with a few exceptions, even up to the present time. It is clear, however, from Wolff's writings that his standards were based upon the then prevailing views of Voit and Pettenkofer (248) regarding the importance of protein as a source of animal fat rather than upon actual experimental results. Subsequent investigations, notably the respiration experiments of Kellner upon cattle (p. 367), have fully demonstrated that such large amounts of protein are neither necessary nor especially advantageous for fattening. Sheep. — Indeed, Wolff himself has demonstrated that his protein standard for sheep was unnecessarily high. In 1890, he published 1 the results of a comparison made in 1885-1886 of maize and beans as feed for fattening sheep, using two lots of two mature sheep each. After a preliminary feeding, the following results were obtained in 107 days' feeding : — 1 Landw. Jahrb., 19 (1890), 823. 366 NUTRITION OF FARM ANIMALS TABLE 74. — INFLUENCE OF PROTEIN SUPPLY ON GAIN BY MATURE FAT- TENING SHEEP LOT i, FED ON HAY AND BEANS LOT 2, FED ON HAY AND MAIZE Weight at beginning Weight at close Kgs. 99-93 118 71; Kgs. 98.61 118 56 Gain 1882 IQ Q^ Digestible matter eaten per 1000 kilograms live weight Protein 3.26 i 81 Total digestible (fat X 2 4) 18 19 in 20 Lot 2, receiving maize, produced about the same gain relatively to the digestible matter consumed as Lot i, notwithstanding the smaller amount of protein supplied. A block test tended to show a slight superiority on the part of Lot 2. Subsequent experiments l gave confirmatory results, barley being compared with beans on one animal each. The following table shows the actual digestible nu- trients, computed per 1000 kilograms live weight, and the total gain for each period : — TABLE 75. — INFLUENCE OF PROTEIN SUPPLY ON GAIN BY MATURE FAT- TENING SHEEP PERIOD NUMBER OF DAYS SHEEP No. i, FED ON BARLEY SHEEP No. 2, FED ON BEANS Total Gain Digested per 1000 Kilograms Live Weight Total Gain Digested per 1000 Kilograms Live Weight Protein Total Nutrients Protein Total Nutrients Kgs. Kgs. Kgs. Kgs. Kgs. Kgs. III ... IV ... V . . . Total . 29 20 38 1.6 1.4 4.0 1.62 1.63 2.03 14.69 I5-23 17.32 2.4 0.9 3-6 3-13 2-95 3-6l 16.12 16.02 16.32 7.0 6.9 1 Landw. Jahrb., 25 (1896), 175. THE FATTENING OF MATURE ANIMALS 367 In the final period of an experiment by Weiske with lambs cited in 'Chapter XI (487) the animals, when two years old, received an exclusive hay ration from which they digested 1.22 Ib. of protein per 1000 pounds live weight. While no material fattening was pos- sible on such a ration, there was still a gain of protein nearly as great per head as in earlier periods, thus rendering it probable that the pro- tein supply was at least nearly sufficient for a moderate rate of fattening. The foregoing results indicate that 1.5 Ib. of digestible protein per day and 1000 pounds live weight is at least sufficient for mature fattening sheep, while the experiments on cattle about to be mentioned suggest that the amount might even be reduced consider- ably below this limit. Cattle. — In Kellner's respiration experiments upon fattening cattle (443) , rations containing comparatively small amounts of protein produced as satisfactory a rate of fattening as those richer in that nutrient. Dividing the experiments into five groups according to the amount of digestible crude protein consumed gives the following averages : — TABLE 76. — INFLUENCE OF PROTEIN SUPPLY ON GAIN BY MATURE FAT- TENING CATTLE RATIONS PER 1000 GAINS PER 1000 KGS. LIVE WEIGHT KGS. LIVE WEIGHT NUMBER . AVER. GROUP LIVE MENTS WEIGHT Digestible Protein Metaboliz- able Energy Protein Fat Com- puted Energy Kgs. Kgs. Therms Grams Grams Therms I ... 7 656 0.523 3J-54 87.2 6lO.I 6.29 II ... 14 651 o-745 33.89 82.3 619.8 6.36 Ill ... 18 667 1.069 34-50 131.0 661.0 7-03 IV ... II 67l 1.332 35.32 154.0 756.8 8.07 V . . . IO 691 2.168 43.36 I57.0 754-0 8.06 The greater gains obtained in the experiments in which the larger amounts of protein were fed are not to be ascribed to this fact but to the greater consumption of total feed, since it has been shown that protein is no more available than non-nitrogenous nutrients for fat production. The point of the comparison is that rations containing amounts of protein little if at all greater than the maintenance require- ment gave relatively quite as large gains per unit of energy supplied as did those containing three or four times as much protein. 368 NUTRITION OF FARM ANIMALS The periods having been short in these experiments, the gain in live weight cannot be satisfactorily determined, but on the basis of Lawes and Gilbert's determinations of the composition of the increase (442) it may be estimated to have been approximately one pound per day. Loges l reports the results of experiments undertaken at Pomritz to test Kellner's conclusions, in which a nutritive ratio of i : 10.3 gave as great gains in weight with mature cattle as one of i : 5.7, but the absolute amounts of protein consumed are not stated in the abstract. Apparently from 0.75 to i Ib. of digestible protein per 1000 pounds live weight is sufficient to meet the requirements of fully mature fattening cattle. Swine. — Such experiments on the fattening of mature swine as are on record show that these animals, like cattle and sheep, need at most but a comparatively small surplus of protein over the amount necessary for maintenance. The respiration experiments by Meissl, Strohmer and Lorenz upon the sources of fat by swine (443) afford a general illustration of this. The following table shows the digestible protein and the metaboliz- able energy of the feed and the gain of energy by the animal. No distinct superiority of the high protein ration of Experiment IV over the low protein rations of Experiments I and III appears, while the greatest gain was realized in Experiment II with a moderate protein supply but relatively high energy content. TABLE 77. — INFLUENCE OF PROTEIN SUPPLY ON GAIN BY MATURE FAT- TENING SWINE RATIONS POUNDS Li PER 100 /E WEIGHT GAIN OF ENERGY ANIMAL LIVE WEIGHT Digestible Protein Metaboliz- able Energy Per 100 Lb. Live Weight Per Therm Metaboliz- able Energy I 1 4O Lb. o 074. Therms r ii Therms 2 GA Therms O ^O II 7O o. 161 IO.O2 c 06 o.q8 Ill 12 ^ o 098 4IO I 4.8 o ^6 IV . . IO4. o 4.10 •2 04. 2 <6 O 43 Soxhlet, in his experiments on the same subject (442) , fed two swine 1 6 months old and weighing about 200 pounds each at the beginning 1 Centbl. Agr. Chem., 31 (1902), 646. THE FATTENING OF MATURE ANIMALS of the experiment, a low protein ration consisting exclusively of rice for 75 and 82 days, respectively. The protein content of the ration and the average daily gain in live weight per head were as fol- lows : — TABLE 78. — INFLUENCE OF PROTEIN SUPPLY ON GAIN BY MATURE FAT- TENING SWINE INITIAL LIVE WEIGHT DIGESTIBLE PROTEIN (N X 6.25) DAILY GAIN IN LIVE WEIGHT PEK HEAD Per Day and Head Per Day and 100 Lb. Live Weight II Lb. 220 213 Lb. 0.265 0.269 Lb. O.I2I 0.126 Lb. I.I5 1.04 III These few results upon mature swine are of interest as showing the possibility of considerable fattening on low protein rations. In prac- tice the results are of comparatively little significance since the com- mercial fattening of swine is usually carried out upon the immature animal. The recorded experiments show that in the fattening of mature animals as satisfactory results have been obtained with rations containing 0.75 to 1.5 pounds of digestible protein per 1000 pounds live weight as with those containing a much more abundant supply. Even these amounts, however, are from 50 to 100 per cent higher than is necessary for maintenance, but with the exception of a small group of Kellner's experi- ments in which approximately the maintenance requirement of protein was consumed the results fail to show whether it is practicable or advisable to reduce still further the protein con- tent of fattening rations. As regards the simple question of protein supply, it appears likely that an amount of this nutrient but little superior to the maintenance requirement is all that is absolutely necessary. In practice, however, the inferior digesti- bility of low-protein rations (723, 724) as well as the fact that such rations are likely to be less palatable than those furnishing a more liberal supply have to be considered. The simple ad- dition of non-nitrogenous nutrients to a maintenance ration 370 NUTRITION OF FARM ANIMALS might furnish ample material for the production of body fat and yet not convert it into a practicable fattening ration. The economic aspects of the question, however, will be considered in connection with the subject of meat production (Chapter XII), the present chapter dealing more especially with the physiological aspects of the fattening process. CHAPTER XI GROWTH § i. GENERAL NATURE OF GROWTH 457. Cell multiplication. — The animal originates in a single microscopic germ cell. Its advance from this insignificant beginning to the size and complexity of maturity is~ effected by a multiplication of the number of cells, together with a progressive differentiation of function, the whole constituting the process of growth. Growth, then, may be characterized briefly as consisting in an increase of the structural elements of the body, chiefly by cell multiplication, resulting in a gain in size and weight. The increase during growth 458. Composition of increase. — As with fattening animals, so in a study of the feed requirements of growing animals, a prime factor to be considered is the amount and composition of the gain made at different ages. The nature of the gain made during growth may be investigated either by means of comparative slaughter tests or by means of respiration experi- ments. Of the former there are on record a study by Wilson l on the growth of pigs for the first 16 days after birth, an inves- tigation by Tschirwinsky 2 on pigs between the ages of 2 and 6 months, one by Kern and Wattenberg 3 on the growth of lambs between the ages of 6 and 28 months, one by Jordan 4 upon the growth of cattle between the ages of 23 and 33 months and one by Wellmann 5 on young pigs. Data regarding dogs and cats are also on record in investigations by Thomas 6 and by Gerhartz.7 1 Amer. Jour. Physiol., 8 (1903), 197. 2 Landw. Vers. Stat., 29 (1883), 3 Jour. Landw., 28 (1880), 289. 4 Maine Expt. Sta., Rpt. 1895, Vol. 2, pp. 36-77. 5 Landw. Jahrb., 46 (1914), 499. 6 Arch. (Anat. u.) Physiol., 1911, p. 9. 7 Arch. Physiol. (Pfliiger), 135 (1910), 163. 371 372 NUTRITION OF FARM ANIMALS Respiration experiments by Soxhlet1 on three young calves included determinations of the gain or loss of ash, while the live weights of the animals are also recorded. The feed being exclusively milk, the variations in the contents of the digestive tract were probably slight and a computation of the composition of the increase based upon the live weights seems justified. The results of both the slaughter and respiration experiments are contained in the following table, the energy content being computed from the fat and protein. TABLE 79. — COMPOSITION OF INCREASE OF LIVE WEIGHT IN GROWTH COMPOSITION OF INCREASE hr t/5 w j5 AUTHOR DESIGNATION or ANIMAL OR PERIOD SPECIES OF ANI- MAL AVER- AGE AGE DAYS Water Ash Pro- tein Fat Ot3 |3 w S % % % % Cals. f Skim milk 1 [ 8 80.08 0.032 18.40 1.49 540 Wilson . . .{ Lactose Swine \ 8 78.91 1.422 17.92 1-75 529 i Dextrose J ( 8 79-44 1.622 17-30 1.64 5i8 Wellman . . J VIII 1 IX Swine { 37 42 75-53 76.86 1.92 2.OO 15-52 3 15- 183 7-03 5-96 698 643 I • 4 67.96 1.84 13-08 3 17.12 1055 Thomas . . T\nr* 16 64.01 2.52 16.63 3 16.84 "Si LJQg 54 72.84 2.89 I7-593 6.68 736 • 101 66.56 4-49 22.31 3 6.64 818 4 80.61 1-63 11.66 3 6.10 544 Thomas . . Cat 18 68.29 2-73 18.93 3 10.05 935 101 4 64.16 3-52 24.01 3 8.31 982 Gerhartz . . Dog IO 72.93 2.90 13-98 3 10.20 800 f C 1 f 8 62.55 3-35 19.24 14.86 1136 Soxhlet . . .{ B.x Calf \ IS 61.28 3-63 19-15 15-94 1182 I B. 2 J I 21 62.13 3-50 I7-I5 17.22 1186 Tschirwinsky . ( No. 3 \ Swine < 114 46.51 3-73 9.10 40.66 1988 I No. 2 / I 134 34-23 2.24 9-73 53-80 2570 Lot I: Sheep Periods I and II 2QO 43-84 11.31 6 44.85 2226 Periods III and IV 521 27.27 7-03 6 65.70 3014 Kern and Wat- Period V 744 22.18 5.726 72.IO 3255 tenberg . . Lot II : Sheep Periods I and II 290 38.41 9.91 6 51-68 2484 Period III 458 16.03 4-13 6 79.84 3547 Jordan 6 . . Average Cattle 840 39.65 6.18 13-57 40.60 2IOO 1 ier Ber. Versuchs-Station Wien, pp. 101-155. 2 By difference. 3 Fat- and ash-free dry matter. 4 Two periods. 8 Computed from " fat-free body." 6 The figures differ slightly from those reported by the author. GROWTH 373 In spite of irregularities and gaps in the table two general facts are clearly shown; first, that the percentage of water in the gain decreases and that of dry matter increases with ad- vancing age of the animal, and second, that of the dry matter gained, an increasing proportion is fat as the animal matures. The latter fact becomes especially clear if the composition of the dry matter of the ash-free gain be computed. The result of investigations by Waters, Mumford and Trow- bridge as reported by Henry and Morrison l are quite in accord with the teaching of Table 79, the percentage composition of the first and the second 500 pounds gained by young fattening steers being as follows : — WATER ASH PROTEIN FAT First 500 Ib Second 500 Ib. 37°6 17.8 2.O 11.9 s-2 . 48^6 75- 459. Energy content of gain. — The amount of energy stored in a unit of increase in live weight shows a fairly regular and notable increase as the animal grows older, due to the smaller percentage of water and the higher percentage of fat which it contains. The rate of increase in the energy content per unit in those cases in which no considerable fattening of the animal was attempted seems to be fairly regular up to about 3.0 Therms per pound and the same thing is also true of most of the results upon fattening animals up to about 3.5 Therms per pound, although the actual energy content per unit at the same age is naturally greater in the fattening animal and the limit is therefore reached earlier in life. In both cases the limit seems to correspond in a general way with the average energy content of the gain made by mature fattening animals as estimated in Chapter X (451), viz., about 3.25 Therms per pound. The data, however, are few and further investigation is much to be desired. Relation of growth to age 460. The rate of growth. — If the successive weights or dimensions of a growing animal be platted, there are obtained 1 Feeds and Feeding, isth Ed., p.84. 374 NUTRITION OF FARM ANIMALS what might be called the curves of weight or of stature. These rise rapidly at first and afterwards more slowly as the animal approaches maturity. Or in like manner the increments of weight or size observed in successive equal periods (day, week, month or year) may be platted, showing at what periods the absolute growth is most rapid. It is evident, however, that an increase of a pound in weight by an animal weighing 500 pounds is relatively much less than the same increase in a loo-pound animal. For many purposes, a better expression of the relation of growth to age is afforded by a computation of the rate of growth, by which is meant the increment in a given unit of time expressed as a fraction of the amount present at the beginning of that time. Thus in the instance just supposed the rate of growth in weight per day would' be in the first case one five-hundredth and in the second one one-hundredth. In the second case the small animal, in proportion to its weight, is growing five times as fast as the larger and may be regarded as showing five times the energy of growth. An evident advantage of this manner of expression is that it permits of a comparison between animals of very dif- ferent weights, as, for example, of sheep with cattle. 461. Rate of growth at different ages. — Somewhat ex- tensive observations, both on man and the lower animals, show that the rate of growth as just defined diminishes from birth onward, the diminution being more rapid at first and slower as maturity is approached. This subject has been discussed in a most illuminating manner by Minot 1 on the basis of his own and others' observations on guinea pigs, rabbits, chicks and other animals as well as on man. Graphically the rate of growth is expressed by a descending curve, steep at first, but gradually becoming more and more nearly horizontal, while the same curve extends backward without material break into intrauterine life. Foster says: "It seems as if the im- petus to growth given at impregnation gradually dies out." In the early stages of growth, therefore, the anabolic processes, which tend to build up tissue, predominate, while as time goes on the katabolic processes gain more and more over the anabolic until at maturity the two tend to become substantially balanced. 1 C. S. Minot : Age, Growth and Death, Chapter III. GROWTH 375 462. The measure of growth. — The most familiar and obvious measure of growth is the increase in size or weight of the body. While for many purposes this is an entirely adequate standard, it is not a strictly accurate expression of growth proper. In the first place the facts regarding the composition of the increase in growth which have just been considered render it evident that a unit of gain in live weight has a very varying significance. In the very young animal as much as 80 per cent of it may consist of water, while its dry matter is chiefly protein. In the nearly mature animal, on the contrary, its percentage of water may fall to between 30 and 40, while its dry matter consists largely of fat. Moreover, a surplus of feed over the maintenance ration may lead to a deposition of fat in the young as well as in the mature animal, resulting in a greater increase in weight than that due to normal growth. On the other hand, as was shown in Chapter VIII (372), growth in the sense of increase in size may continue on a ration barely sufficient or even insufficient to maintain a stationary weight, i.e., growth when expressed in terms of weight may be masked by a loss of fat. The essential structural elements of the body, the increase of which constitutes growth proper, consist (aside, of course, from water) mainly of protein and mineral matter (98). Growth, therefore, in this view of it, is equivalent to a gain by the body of protein and ash, especially the former. The increase of protein, therefore, may be regarded as constituting a more accurate measure of growth in the narrower sense than mere increase in weight. 463. Rate of increase of protein. — What is true of the weight or size of the growing animal is true also of growth in the somewhat narrower sense of increase of protein tissue. The writer has elsewhere 1 collated the results of a number of experiments, including those whose results regarding the com- position of the increase are recorded in Table 79, in which the gain of protein by growing animals has been determined with more or less accuracy. In addition the results of experiments by Fingerling 2 on calves, of Ostertag and Zuntz 3 upon pigs, 1 U. S. Dept. Agr., Bur. Anim. Indus., Bui. 108 (1908), pp. 13-17. 2 Landw. Vers. Stat., 68 (1908), 141 ; 76 (1912), i. 3Landw. Jahrb., 37 (1908), 231. 376 NUTRITION OF FARM ANIMALS and of Just l on lambs have been included in the table which follows. In those cases in which the experiments were made by the method of comparative slaughter tests, the composition of the control animals gives an approximate measure of the initial protein content of the body. When no control animal was analyzed the initial protein con- tent has been estimated as well as possible from the live weight. Since this was the case in the majority of the experiments it seems desirable also to compute the gain of protein per 1000 live weight. Except in the case of very young or very fat animals, the results are likely to correspond substantially with those computed in the other way, while they have the advantage of being expressed in the manner usually adopted for formulating feeding requirements. TABLE 80. — RATE OF GAIN OF PROTEIN AVERAGE AGE Days DAILY GAIN OF PROTEIN Per 100 Body Protein Per looo Live Weight Cattle Soxhlet 8 Soxhlet 15 Soxhlet 18 Soxhlet 21 Fingerling 21 Soxhlet 32 De Vries Jzn 37 De Vries Jzn 38 Neumann 40 Neumann 45 De Vries Jzn 45 Fingerling 47 De Vries Jzn 50 Neumann 50 Neumann ..." 54 Neumann 57 Neumann 62 De Vries Jzn 63 De Vries Jzn 64 De Vries Jzn 65 Fingerling 68 Neumann 69 De Vries Jzn 74 2-347 2.076 1.644 1.722 1.974 1.693 '1-335 1.246 1-795 1.449 1.272 1.248 0.880 1.082 1.026 1.320 0-939 0.678 0.655 1.020 0.948 1.062 0.713 3-994 3-552 2.803 3.024 3-085 2-755 2.276 2.124 2-945 2.419 2.169 2.161 1.500 1.844 2.284 1.611 1.209 1.209 1.723 1.719 1.823 1.271 1 Landw. Vers. Stat., 69 (1908), 393, results of periods 3, 5, 7 and 9. GROWTH 377 AVERAGE AGE Days DAILY GAIN OF PROTEIN Per 100 Body Protein Per looo Live Weight Cattle De Vries Jzn 100 Fingerling 150 Fingerling 182 Fingerling 214 Fingerling 297 Jordan 840 Sheep Weiske 140 Weiske 177 Weiske 214 Weiske 254 Just 285 Kern and Wattenberg .... 290 Weiske 293 Just 315 Weiske 328 Just 360 Weiske 366 Just 390 Weiske 405 Weiske 436 Kern and Wattenberg .... 458 Kern and Wattenberg .... 521 Kern and Wattenberg .... 745 Swine Ostertag and Zuntz .... 5! Sanford and Lusk 7 Wilson 8 Ostertag and Zuntz .... 13 Ostertag and Zuntz .... 21 Ostertag and Zuntz .... 26 Tschirwinsky 114 Tschirwinsky 134 Dog Thomas 4 Gerhartz 10 Thomas 16 Thomas 54 Thomas 101 Cat Thomas 4 Thomas 18 Thomas 101 0.711 0.48 0.41 o-33 0.22 0.064 0.372 0.307 O.2I9 0.288 0.233 0.272 0.179 0.182 O.I 60 O.I 80 0.238 0.158 0.178 0-033 0.068 0.087 0.067 5-553 7.269 6.852 4.129 1.840 0-757 0.442 0.483 5-94 6.44 6.71 1.70 1.82 6.10 5-89 i. 60 1.192 0.83 0.76 0.64 o.47 0.089 0.651 0-499 0.360 0.449 0-475 0.303 0.284 0.370 0.264 0.360 0.382 0.315 0.301 0.06 1 0.074 0.096 0.069 9.029 5.621 5-757 6.675 3-257 1.470 0.663 0.740 7-73 7.67 8-73 2-35 2-93 7-57 7.91 3-05 378 NUTRITION OF FARM ANIMALS It is obvious that the error in single results obtained in this way may be very considerable, but the general teaching of the table is perfectly clear, viz., that the rate of growth of protein tissue, like the increase in size or in weight, whether expressed per unit of body protein or per 1000 live weight, is relatively high in the new-born animal and decreases rapidly at first and more slowly later, tending to be asymptotic to the zero line. Letting the g equal the gain of protein per day per 1000 live weight and a the age in days, a curve represented by the em- pirical equation 1 corresponds fairly well with the general average of the observed results on cattle and sheep. With swine, the few results ap- pear to indicate a greater rate of growth during the first three months. This is shown clearly in the accompanying graph (Fig. 38) in which the individual results on the different species are shown by the light lines, while the heavy curve is that repre- sented by the foregoing equation. Of course considerable individual variations are to be expected, and no particular significance attaches to the mathematical form of the curve, but it would seem that this formula may be used tentatively to express in a broad general way the average rate of protein growth of farm ruminants at different ages. The few results on the dog and cat seem to indicate a higher rate of growth in the young of these species. 464. Rate of gain of energy. — While the rate of increase of protein, as discussed in the foregoing paragraphs, may be regarded as the measure of growth in the more restricted sense, and while it is of importance as an indication of the amount of protein which must be supplied in the feed, the actual gain in normal growth includes more or less production of fat, as is clearly shown by the data regarding the composition of the in- crease already considered (458). Growth, therefore, in practice involves a storage of energy in the body not merely in the pro- tein gained but also in the accompanying fat laid on, while it is difficult to draw an exact line between the growth and the fattening of young animals. 1 The equation of a rectangular hyperbola. GROWTH 379 ill 38o NUTRITION OF FARM ANIMALS If it may be assumed that in those of the experiments re- corded in Table 79 in which no considerable fattening was at- tempted the increase in weight was approximately that due to normal growth, the amount of energy contained in the increase and the daily rate of gain of energy per 1000 pounds live weight may be computed. The following table shows the results of such a comparison, the figures per 1000 pounds being computed in direct proportion to the weight. TABLE 81. — ENERGY CONTENT OF DAILY GROWTH EXPERIMENTER ANIMAL AVERAGE AGE AVERAGE LIVE WEIGHT * ENERGY CONTENT OF GROWTH Per Head Penooo Lb. Live Weight Thomas Thomas Dog Cat Pig — Average Calf Dog Calf Dog Cat Calf Pig Pig Dog Dog Cat Pig Pig Sheep, Lot I Sheep, Lot II Pig Sheep Sheep, Lot II Sheep, Lot I Sheep, Lot I Cattle Cattle Days 4 4 8 8 10 15 16 18 21 23 34 54 101 IOI 114 134 290 290 300 1 4561 458 521 745 840 1460 l Lb. 0.79 0.38 4.12 106.87 0.98 138.60 i-55 0.864 I5I-53 I3-52 jy-93 3.38 5-95 1.91 39-51 34-66 67.02 73-42 181.88 135.58 106.70 102.51 130.07 826.10 1272.40 Cals. 58.13 14.67 73-58 2634.00 49-3 1 3i53.oo 113.40 39.20 3294.00 243-9 316.5 38.02 73-°4 27-34 568.6 705.6 328.0 608. i 5041.0 1185.4 712.9 434-5 520.4 1618.0 6378.0 Therms 73-35 38.47 17.87 24.66 50.47 22.74 73-35 45.36 21.74 18.04 17.65 11.25 12.28 14.32 14-39 20.36 4-90 8.28 27.72 8-74 6.68 4-24 4.00 1.96 5-oi Wilson Soxhlet Gerhartz Soxhlet Thomas Thomas Soxhlet Wellmann .... Wellmann .... Thomas Thomas Thomas Tschirwinsky . . . Tschirwinsky . . . Kern and Wattenberg Kern and Wattenberg Lawes and Gilbert Lawes and Gilbert Kern and Wattenberg Kern and Wattenberg Kern and Wattenberg Jordan Lawes and Gilbert 1 Approximate. 2 All data refer to empty weight, exclusive of hides. GROWTH 381 The rate of gain of energy as thus computed is notably greater for young carnivora (dogs and cats) during the first two or three weeks than that of pigs or calves. Aside from this, the results on farm animals, although more or less irregular, present in general the same picture as those on the rate of gain of pro- tein, viz., a diminishing energy of growth with advancing age. The few instances showing a wide divergence from the majority may probably be assumed to be due to rapid fattening. § 2. THE UTILIZATION or FEED IN GROWTH The utilization of protein 465. Relative values of proteins for growth. — A considera- tion of the utilization of protein in growth necessarily raises the question of the relative values of different individual pro- teins in this respect. As was pointed out in Chapter IX (398), it appears probable that the protein requirement for maintenance is essentially an amino acid requirement and that the relative values of proteins for maintenance may prove to depend largely or wholly on their ability to supply certain specific " building stones " required for the performance of specific functions. In the growing animal there is, in addition to this requirement for functional purposes, a demand for amino acids out of which new body proteins may be built up. In growth, therefore, the amino acid requirements may differ from those for maintenance not only in being quantitatively greater but in being qualita- tively different. A striking illustration of this is afforded by the investigations of Osborne and Mendel l on the relation of lysin to growth. In common with other investigators they have found that trypto- phan is indispensable for maintenance (399). Wheat gliadin con- tains tryptophan but only a minute amount of lysin. While they have repeatedly secured maintenance for long periods on rations con- taining gliadin as the sole protein, they have been unable to secure growth with such rations, but the simple addition of lysin enabled growth to proceed at a normal rate. The body proteins contain lysin, ox muscle, for example, yielding 7.6 per cent (50). Evidently this 1 Jour. Biol. Chem., 12 (1912), 473; 17 (1913), 325; 86 (1916), 293. 382 NUTRITION OF FARM ANIMALS amino acid cannot be synthesized in the body but must be supplied in the feed in order to permit the construction of the new protein molecules in the tissue, while for maintenance (399) it appears to be dispensable. Moreover, they have shown that the addition to in- adequate proteins like gliadin of other proteins containing lysin per- mits growth to take place and furthermore that the proportion of the second protein which must be added in order to support normal growth is less in proportion as it is richer in lysin. Osborne and Mendel's conclusions have been strikingly confirmed by the results obtained by Buckner, Nollau and Kastle l from feeding young chicks grain mixtures of high and low lysin content. It appears that the lack of lysin in a protein renders it in- capable of supporting growth, although it may still be adequate for maintenance (399), and that the proportion of lysin in those proteins containing it constitutes a limiting factor for the amount of growth which they can support. Tryptophan is obviously another limiting factor in this respect, while it must be regarded as altogether probable that other amino acids belong in the same category and may become limiting factors if the supply of them is deficient. In other words, the amount of some particular amino acid which is available may become the minimum factor which determines the rate of growth, just as the minimum supply of potassium, for example, may deter- mine the rate of growth of a crop. The unsatisfactory results obtained in practice with maize as the sole feed for young animals may well be due in large part to the poverty of the mixed proteins of this grain in tryptophan and lysin, it having been shown that as the sole source of protein they can support but slow growth (783). Unfortunately the knowledge available on these points is as yet chiefly qualitative in character and affords no sufficient foundation on which to base a quantitative discussion of the relative values of proteins in farm practice. Accordingly, in the case of growth as in that of maintenance it appears neces- sary for the present to consider questions regarding the protein requirement upon the basis of total protein, largely irrespective of its nature. (Compare Chapter XVII, § 4.) 466. Percentage retention of feed protein. — In the mature animal, the katabolism of protein substantially keeps pace 1 Amer. Jour. Physiol., 39 (1915), 162. GROWTH 383 with the supply in the feed (402), as indeed is really implied in the conception of maturity. By a mature animal is meant one which has completed its growth, and growth consists essen- tially in an increase of the nitrogenous structural elements of the body. Obviously, therefore, if the capacity for growth has been exhausted, no material storage of protein can occur and an excess of this material above the maintenance require- ment will serve chiefly or wholly as a source of energy to the organism. With the young animal the case is different. Its rapidly growing cells and tissues demand a liberal supply of protein, and if this is afforded by the feed it is largely utilized to build up tissue instead of undergoing nitrogen cleavage. Conse- quently, other things being equal, a much larger percentage of the feed protein is retained in the body. The investigations whose results have been considered on previous pages (463) , especially those upon the younger animals, afford striking illustrations of this fact, Soxhlet's experiments upon calves being the earliest and most familiar. Their results are summarized in the fol- lowing table, the feed consisting of fresh whole milk ad libitum. TABLE 82. — PERCENTAGE OF FEED PROTEIN RETAINED — SOXHLET ANIMAL AGE DIGESTED PROTEIN OF FEED PER DAY DAILY GAIN OF PROTEIN BY ANIMAL DIGESTED PROTEIN RETAINED A Days 1 i 6— 10 Grams 171 3 Grams Per Cent B 30-33 I C 1 / *"6 228.4 330 8 163.6 231 8 /b'6 7i.6 7O I C . . . . 21 8 317.5 262 4 216.1 2O2 O 68.1 77 O More recent and even more striking illustrations of the same fact are afforded by Fingerling's experiments. Thus, in one instance a calf averaging 9 days old received whole milk and in a succeed- ing period milk with the addition of butter fat and lactose, and retained the percentages of digested protein shown in the following table. 384 NUTRITION OF FARM ANIMALS TABLE 83. — PERCENTAGE OF FEED PROTEIN RETAINED — FINGERLING PERIOD AGE (AVERAGE) DIGESTED PROTEIN OF FEED PER DAY DAILY GAIN OF PROTEIN BY ANIMAL DIGESTED PROTEIN RETAINED I ....... Days Grams 24.O 4.2 Grams 214. 38 Per Cent 86 o II . . 19 254.94 207.48 81.4 With advancing age, a relatively smaller retention is observed. Thus Neumann obtained for calves 40 to 70 days old percentages vary- ing from 38.7 to 48.3, and Tschirwinsky, experimenting on pigs 100 to 120 days old, observed a retention of 20.7 to 33.6 per cent of the digested protein. With still older animals a yet smaller percentage retention has been observed, diminishing to nearly zero with fully mature animals. 467. Does not measure utilization. — On the basis of this greater percentage retention it has been customary to say that the utilization of feed protein is high in the case of the young animal and diminishes rather rapidly as it grows older. This statement is made essentially from a commercial standpoint and in that sense it is true. Only the growing animal is capable of using any large amount of feed protein to increase its stock of body protein and the ability to do this is the more marked the younger the animal. The percentage retention of the feed protein, however, is necessarily variable and neither affords a measure of the effi- ciency with which the animal converts it into body protein nor permits a comparison of that efficiency at different ages. The comparison is disturbed by two important factors to which attention has been especially called by Fingerling,1 viz., the influence of the total amount of protein supplied and the effect of a deficient energy supply. 468. Influence of protein supply. — As has already been implied, growth is primarily dependent upon biological factors. The feed supplies material for growth but does not determine its maximum rate. The rate of increase of protein as formu- lated in the previous section (463) represents (so far as the results are trustworthy) the capacity of the animal for protein 1 Landw. Vers. Stat., 74 (1910), i. GROWTH 385 storage at different ages, but the percentage of the feed protein which is retained will depend upon the relation between this capacity and the amount of protein actually supplied. For example, suppose a calf weighing 100 pounds to be capable of storing up per day 0.25 pound of protein and to require 0.05 pound for maintenance. If it receives 0.35 pound digestible protein in its feed and is able to store up the maximum amount of 0.25 pound on this ration, 71.4 per cent of the digestible protein would be retained, while 28.6 per cent would katab- olize and its nitrogen excrete in the urine. But if the feed of the animal supplied 0.45 pound digestible protein, the gain would still be 0.25 pound, since this is the maximum possible for the animal, but the percentage of the feed protein retained would be only 55.6, while 44.6 per cent of it would be katab- olized. The organism is unable to use the added one-tenth pound for constructive purposes and therefore it is katabolized as shown in Chapter IX (402-404) and serves simply as a source of energy. In other words, the greater the excess of protein supplied in the ration over the minimum required by the de- mands of growth and maintenance, the lower will be the per- centage retained in the body. On the other hand, with rations deficient in protein the percentage retention will increase with the protein supply up to the minimum amount necessary to utilize the growth capacity of the animal. Fingerling's experiments afford striking confirmation of the truth of the foregoing deductions from the general laws of protein katabolism. A calf received daily in one period 8 kgs. of whole milk with an addition of butter fat and lactose, while in the succeeding period whole milk alone was fed in amounts proportional to the age of the calf, averaging 11.875 kgs. per day. The results as regards protein, expressed in terms of nitrogen, were as follows : — TABLE 84. — INFLUENCE OF PROTEIN SUPPLY ON PERCENTAGE RETENTION OF NITROGEN DIGESTED NITROGEN OF FEED URINARY NITROGEN GAIN BY CALF PER CENT OF FEED PROTEIN RETAINED Grams Grains Grams June 2-5 June 25-30 42.49 62.97 7.91 28.77 34.58 34.20 81.4 54-3 2 C 386 NUTRITION OF FARM ANIMALS Evidently the protein supply was sufficient in the first period to ensure normal growth. The additional supply in the second period, therefore, had no effect on the gain but simply increased the protein katabolism, i.e., the added protein was used as a source of energy for maintenance or for the production of fat. On the other hand, a supply of protein notably insufficient to per- mit normal gain may yet show a comparatively high percentage re- tention. Thus the same calf received in an intermediate period only 4 kgs. per day of whole milk together with sufficient butter fat and lactose to supply the necessary energy. As compared with the first period only about one-half of the normal gain of protein was secured, yet the percentage retention is but slightly reduced. TABLE 85. — HIGH PERCENTAGE RETENTION OF NITROGEN ON INSUFFI- CIENT PROTEIN DIGESTED NITROGEN OF FEED URINARY NITROGEN GAIN BY CALF PER CENT OF FEED RETAINED Tune 2— t; Grams 4.2 4.Q Grams 7 OI Grams -1A eg 8l 4. June 13—18 ... ... IQ.CK (ft 14.00 74.7 469. Influence of deficient energy supply. — But not only may a surplus of protein be utilized as a source of energy in the manner just illustrated, but if the energy supply in the feed is inadequate protein may be diverted from growth to serve as fuel material, precisely as in the case of maintenance (412), thus lowering both the observed gain and the percentage re- tention. This effect is well illustrated by the following experiment by Fin- gerling upon a calf receiving in the first two periods a limited quan- tity (10 kgs. per day) of whole milk. As the animal grew -older the energy supply became insufficient and protein was diverted to fuel purposes so that the actual gain and the percentage retention both diminished. When, in a third period, one-half of the milk was re- placed by butter fat, the protein supply being kept at nearly the same level by the addition of egg albumin, the actual gain rose nearly to its original level and the percentage retention became even higher than at first on account of the somewhat reduced protein supply. GROWTH 387 TABLE 86. — INFLUENCE OF ENERGY SUPPLY ON PERCENTAGE RETENTION OF NITROGEN DIGESTED NITROGEN or FEED URINARY NITROGEN GAIN BY CALF PER CENT OF FEED PEOTEIN RETAINED Grams Grams Grams Sept. 2g-Oct. i 51.84 12.62 39.22 75-7 Oct. 7-9 5I-87 19.99 31.84 61.5 Oct. 19-27 45-76 8.10 37-66 82.3 470. Meaning of utilization. — The percentage of the digest- ible protein of the feed which is retained in the body of the growing animal, then, is not in itself a measure of the efficiency of the animal organism in converting feed protein into body protein, since the proportion retained is affected both by the magnitude of the protein supply in the feed and by the energy content of the ration. What then is the correct conception of the utilization of the feed protein ? As appeared in the previous section, the amount of protein which a growing animal can store up seems to be a function of its age (463), and the attempt was made to formulate approxi- mately the capacity for growth in this sense at different ages. The percentage utilization of the feed protein in the physio- logical sense, as distinguished from the percentage retention, is the ratio between the body protein thus stored up and the least amount of feed protein in excess of the maintenance require- ment which is necessary to support this growth under the most favorable conditions, especially as to energy supply. Suppose, for example, that an animal three months old actually has the capacity, as computed by the formula on page 378, to store up daily 1.23 pounds protein per 1000 pounds live weight, and that it has been shown that it can just reach this capacity on a ration supplying 2 pounds of digestible protein per day. De- ducting 0.5 pound for maintenance (415), there remains 1.5 pounds of protein in the ration out of which is produced 1.23 pounds of body protein. The utilization is therefore 1.23 -5- 1.5 = 82 per cent. If, on the other hand, it was found that 2.5 pounds of protein had to be supplied in the ration in order 388 NUTRITION OF FARM ANIMALS to bring the gain of protein up to the capacity of the animal, the percentage utilization would be only 1.23 -f- 2.0 = 62 per cent, while on the other hand if the maximum growth could be secured with 1.73 pounds of digestible protein, the utili- zation would evidently be 100 per cent. 471. Experimental results. — The writer is not aware of any exact determinations of the percentage utilization in the sense just denned, that is, of the maximum amount of protein tissue which can be produced either from single proteins or from the mixed proteins of feeding stuffs, but interesting data regard- ing the utilization of protein by growing animals are furnished in experiments by Fingerling 1 upon calves and by Just 2 on lambs in which the influence of a varying protein supply upon the nitrogen balance was determined. Reckoning the maintenance requirement for protein at 0.5 pound per 1000 pounds live weight, Fingerling's results for those periods in which the estimated capacity for growth appears to have been fully utilized were as follows : — TABLE 87. — COMPUTED UTILIZATION OF PROTEIN BY CALVES GAIN OF PRO- TEIN PER FEED PROTEIN IN EXCESS OF MAINTENANCE PERCENTAGE UTILIZATION AVER- PER 1000 ANIMAL PERIOD AGE AGE DAYS Capac- ity for Gain Ob- served Gain True Protein Crude Protein True Protein Crude Protein B. ./ i 172 0.70 0.67 1.94 2.00 34-5 33-5 I 2 175 0.63 0.63 0.62 0.84 101.6 75-o I 157 0.76 0.85 2.15 2-44 39-5 34-8 C . 2 184 0.66 0-75 0.79 1.04 94-9 72.1 } ( 3 211 0.58 0.68 2-59 2.85 26.3 23-9 i 237 o-53 0-57 1. 08 1.29 52.8 44.2 G 2 262 0.48 0-54 I-I3 1-37 47-8 39-4 4 309 0.41 0.50 0.3I 0.49 161.3 IO2.O 5 339 0.38 0.44 0.25 0.42 176.0 IO4.8 r Prelim- 135 0.87 1.07 2.42 2.81 44-2 38.1 H inary I 1-6 187 0.05 0.80 0.74 I.OI 108.1 79.2 1 Landw. Vers. Stat., 76 (1912), i. Ibid., 69 (1908), 393. GROWTH 389 From these figures it appears that in the low protein periods the estimated capacity of the animals for growth was fully uti- lized with a surplus of digestible true protein over the maintenance requirement equal to or even less than that actually recovered in the growth, while a much larger supply of protein failed to secure any additional growth but simply forced up the protein katabolism. In other words, if the estimate for the maintenance requirement is approximately correct, the utilization of the digestible protein in the low protein periods must have ap- proached 100 per cent. Indeed, in at least two cases it is neces- sary to admit either that the estimate for maintenance is too high or that non-protein was used for maintenance. TABLE 88. — COMPUTED UTILIZATION OF PROTEIN BY LAMBS FEED PROTEIN AP- GAIN OF PRO- IN EXCESS OF PERCENTAGE PROXI- TEIN PER 1000 MAINTENANCE UTILIZATION o MATE PER 1000 ANIMAL § AVER- Pi AGE DAYS Capac- ity for Gain Ob- served Gain True Protein Crude Protein True Protein Crude Protein I 262 0.48 O.OI —0.09 O.O2 — 50.0 2 280 o-45 0.40 0.17 0-54 235-3 74.1 3 296 o-43 0.48 0.42 0.56 II4-3 85.7 4 310 0.41 0.16 —0.14 0.26 — 61.5 I 5 324 0-39 0.36 0.29 0.44 124.1 8l.8 6 339 0.38 0.62 0.27 o-73 229.6 84.9 7 362 0.36 0.40 0.38 0.52. 105-3 76.9 8 377 0-34 0.24 — O.22 0.23 — 104.3 9 391 o-33 0.34 0.28 0.42 121.4 81.0 10 407 0.32 -0.03 — 0.21 -O.II — — . f i 262 0.48 o.oo — O.II O.O2 oo.o II 280 0-45 0.32 0.14 o-49 228.6 65.3 I 3 296 0-43 0.47 0.36 0.52 130.5 90.4 ' 4 310 0.41 0.07 — 0.07 0-37 — 18.9 5 324 0-39 0.38 0.28 0.42 135-7 90.5 6 339 0.38 0.61 O.2I 0.63 290.5 96.8 Ill x .... < 7 362 0.36 0-33 O.42 o-57 78.6 57.9 8 377 0.34 0.24 — O.22 0.24 — IOO.O 9 391 0-33 0.29 0.25 0.41 116.0 70.7 10 407 0.32 o.oo — O.2O -0.08 — — 1 Substituted for No. II. 3QO NUTRITION OF FARM ANIMALS Interesting data pointing in the same direction are contained in the investigation by Just, in which the nutritive value of non-protein for lambs was compared with that of protein. Estimating the maintenance requirement at 0.5 per thousand and computing the results of the protein periods as in Finger- ling's experiments, it appears that in nearly every case the actual gain of protein was only slightly less than the surplus of digestible crude protein above the maintenance requirement, while in many cases it was distinctly greater than the digestible true protein available. Apparently the non-protein must at least have contributed to the maintenance of the animal if not to its growth, while the utilization of the digestible true protein must have been very high (Table 88). Neither Fingerling's nor Just's investigations are adequate to solve the general problem of the maximum possible utiliza- tion of protein in growth, but their results indicate that it may be very high and should lead to caution in the interpretation of experiments upon the protein requirements for growth. Utilization of energy — net energy values for growth 472. General conception. — The conception of net energy values for growth is entirely analogous to that of net energy values for maintenance or for fattening. They represent that portion of the feed energy supplied in excess of the maintenance requirement which the animal is able to store up in the gain made. It is important to keep this conception clearly in mind when considering the utilization of feed in growth and not to be misled by the greater economic efficiency of the young animal as a producer of live weight increase. It is a familiar fact that the young animal gains in weight relatively much faster than when more mature and this has led to the general impression that the young animal utilizes its feed more perfectly than the older animal, or in other words, that the net energy value of a feeding stuff for growth is greater than that for maintenance or for fattening. It is true that the gain in live weight is different in character in the young animal, containing more water and protein and less fat and therefore less energy (458, 459) , but on the other hand the results recorded in § i show a greater rate of growth as regards both protein and GROWTH 391 energy (463, 464) in the young animal as compared with the more mature one. Is this difference to be ascribed to a specif- ically higher percentage utilization on the part of the younger animal, or is it due to a relatively greater consumption of feed or the relatively high net energy values which usually char- acterize the feeds given the young animal, particularly milk? The mere fact, for example, that a young animal consuming milk utilized a higher percentage of the feed energy than did the same animal later upon a mixed ration would not necessarily show any physiological superiority on the part of the younger animal but might be due solely to the difference in the kind of feed consumed. So, too, the mere ability to consume relatively large amounts of highly concentrated feed in the form of milk and thus to secure a large surplus above the maintenance requirement might (360, 510) give the younger animal a marked economic advantage without indicating any more efficient conversion of the surplus energy supplied than in the older animal. Unfortunately, investigations regarding the utilization of feed at different ages have been few in number and the avail- able data regarding net energy values for growth are exceed- ingly meager. To a large extent it is necessary to be content with comparisons of a very general nature, leading to proba- bilities only. 473. Experiments on suckling animals. — The experiments by Soxhlet on calves and those by Wilson on pigs cited on previous pages (458) and likewise an investigation by Rubner and Haubner 1 on infants afford some data for approximate estimates of the percentage of the metabolizable energy of milk utilized by growing animals. The computations involve a number of uncertain assump- tions, particularly as regards the maintenance requirement, and none of them afford a satisfactory basis for comparing the utilization of the metabolizable energy of milk at different ages. It is of some interest, however, to compare the average utilization computed from these experiments with that esti- mated by the use of Rubner's factors for the " specific dynamic action " of equal amounts of pure nutrients on mature animals. lZtschr. Biol., 36 (1898), i ; 38 (1899), 315. 392 NUTRITION OF FARM ANIMALS As explained in Chapters VIII and XVII (366, 759), Rubner's "specific dynamic action" is synonymous with the energy expendi- ture caused by the consumption of feed, and if it be subtracted from the metabolizable energy the remainder is the net energy. The per- centage utilization is of course the net energy divided by the metab- olizable energy. Estimated in this way, the percentage utilization would be as shown in the first column of the following table, the second and third columns of which show the utilization as computed by the writer both with and without a 10 per cent addition to the fasting katabolism as estimated from the data on mature animals. TABLE 89. — ESTIMATED UTILIZATION OF METABOLIZABLE ENERGY OF MILK COMPUTED BY WRITER COMPUTED, USING RUBNER'S FACTORS Fasting Katabolism Same as in Mature Animals Fasting Katabolism 10% Greater than in Mature Animals Rubner's experiments 84 08 % Soxhlet's experiments 86 18 73 77 7? ?6 Wilson's experiments 83.99 70.31 75-47 While it is clear that no final conclusions can be based upon small differences between figures obtained as these have been, it seems suggestive, nevertheless, that the actual experiments with growing animals show a lower average utilization than would be expected from Rubner's results upon mature animals. Moreover, Wilson's results are apparently lower than those which may be computed from Meissl's and Kornauth and Arche's respiration experiments upon mature swine consuming grain (757). Certainly these comparisons afford little sup- port to the notion that the utilization of energy in the physio- logical sense by young animals is much higher than that by mature animals. GROWTH 393 474. Experiments on older animals. — Of experiments upon older animals those of Kern and Wattenberg on lambs and of Tschirwinsky on pigs (458), permit an approximate computa- tion of the utilization of the feed energy during growth and afford data for some comparisons, although in neither case were very young animals employed, the lambs being between 6 and 7 months old at the beginning of the experiment and the pigs between 9 and 10 weeks. On the whole, the results of these experiments seem to indi- cate, if anything, a rather lower percentage utilization by the younger animals as compared with the older. At any rate they fail to show any superiority on the part of the former. The same is true of the results of experiments by Armsby and Fries * upon steers 10 to 27 months old in which the availability was deter- mined by the use of the respiration calorimeter. While not de- cisive, the results seem to indicate a slightly lower availability of the mixed grain and possibly of the hay for the younger animals. 475. Embryonic growth. — Several experimenters, especially Tangl and his associates 2 and Bohr and Hasselbalch,3 have determined the energy expended in the development of the embryo in oviparous animals, i.e., in the organization of the substances of the egg into embryonic tissue. These investi- gations have shown that a relatively large proportion of the chemical energy contained in the egg is evolved as heat during the process of development, so that the percentage recovered in the embryo, ranging from 60 to 68 per cent, is distinctly lower than the utilization of the energy of milk by suckling animals as computed in a previous paragraph (473). More- over, they show that the utilization of the energy of the egg is notably less in the earlier than in the later stages of incubation, as low a figure as 28 per cent having been observed after 10 days' incubation. The method may be illustrated by the results of two experiments by Tangl and Mituch, each upon three hens' eggs. From analyses of similar eggs from the same hen, it was computed that the three used contained respectively 229.72 Cals. and 291.38 Cals. chemical energy. 1 U. S. Dept. Agr., Bur. Anim. Indus., Bui. 128 (1911), 51. 2 Arch. Physiol. (Pfliiger), 93 (1903), 327 ; 98 (1903), 490; 104 (1904), 624; 121 (1908), 423 and 437. 3 Skand. Arch. Physiol., 10 (1900), 149 and 353 ; 14 (1903), 398. 394 NUTRITION OF FARM ANIMALS At the end of incubation the embryo * was separated from the yolk sack and its contents and the energy of each determined with the fol- lowing results : — TABLE 90. — UTILIZATION OF ENERGY IN INCUBATION EGGS PROM HEN VIII EGGS FROM HEN X Original energy of eggs 229.72 Cals. 291.38 Cals Remaining in yolk sack and contents . . . 63.95 Cals. 94.64 Cals. Used for production of embryo Recovered in embryo Percentage recovered . . . 165.77 Cals. 99.24 Cals. 59.87 Cals. 196.74 Cals. 125.66 Cals. 63.84 Cals In other words, 35 to 40 per cent of the energy of the egg substance used was not recovered but escaped as heat. Comparison with the loss of dry matter showed that the material thus katabolized consisted substantially of fat. No loss of nitrogen was observed. Experiments on mammalian and reptilian embryos, especially by Bohr,2 by Murlin 3 and by Carpenter and Murlin,4 appear in accord with the foregoing conclusion, since they show that the metabolism of the embryo per unit of weight is as great or greater than that of the mature animal, despite the fact that the maintenance requirement of the former must be decidedly less. The growth of the embryo consists essentially of the organ- ization of protein tissue. The fact that there is no loss of nitro- gen during incubation would indicate that chemically the process is effected by a cleavage and resynthesis of protein which appears to be a nearly iso thermic process (233, 367 d). Apparently the organization of the protein into structure is what calls for the large expenditure of energy. 476. Summary. — The experimental results mentioned in the foregoing paragraphs may be briefly summarized in the follow- ing statements : — In the case of suckling animals, while no direct comparisons of the same animal at different ages are available, the utilization of the metabolizable energy of milk for growth appears to be 1 Including the egg membranes. 2 Skand. Arch. Physiol., 10 (1900), 413 ; 15 (1904), 23. 3Amer. Jour. Physiol. , 26 (1910), 134. 4 Arch. Inter. Med., 7 (1911), 184. GROWTH 395 distinctly less than would be expected from Rubner's results on the utilization of pure nutrients by mature animals. In the case of swine, moreover, the utilization appears to be even less than that of the metabolizable energy of grain by mature ani- mals, although the contrary would naturally have been antici- pated. The results with older animals, while far from conclusive, seem, if anything, to indicate a lower utilization by younger animals as compared with older ones and at any rate fail to show that it is any greater in the former case. The results on embryonic growth show a relatively large expenditure of energy in development and indicate a compara- tively low utilization of energy. This large expenditure of energy in development seems to be required chiefly for the organization, in the broader sense, of the embryonic structure rather than for the mere chemical transformation of egg sub- stances, and it seems to be relatively greater in the young as compared with the more mature embryo. 477. Provisional hypothesis. — While it would be rash to draw any final conclusions from the foregoing data, it may be permissible to formulate a working hypothesis to the effect that the conversion of feed protein (including the protein of the egg) into tissue requires a considerably greater relative expenditure of energy than does the conversion of surplus feed into fat, the difference representing what might be called the work of organization, i.e., the formation of organized structure in the young animal and especially in the embryo. It has been shown (463) that the rate of growth decreases rapidly with increasing age. Accordingly, the work of organizing new protein tissue, so far as this is measured by the storage of pro- tein, must constitute a steadily diminishing proportion of the total energy expenditure of the organism, since as the animal grows older the increase consists to a diminishing extent of protein and to an increasing extent of fat. The percentage utilization of the feed energy would therefore, upon this hypothe- sis, tend to increase. It would be least immediately after birth and after two to four months would become relatively small, corresponding to the changing character of the gain. Probably by the time an animal has been weaned and is consuming the normal feed of its species, the percentage utilization of the feed energy might be assumed to be not much less than that ex- 396 NUTRITION OF FARM ANIMALS hibited by the mature animal and at any rate to be practically proportional to it. This would mean, of course, that the net energy values of feeding stuffs for maintenance and fattening might be used also to measure at least their relative if not their absolute net energy values for growing animals. The determination of the validity of this provisional con- clusion offers an interesting and profitable field for investigation. § 3. THE FEED REQUIREMENTS FOR GROWTH 478. Contrast with fattening. — la the case of fattening animals the conception of the feed requirement, particularly as regards energy, is somewhat artificial, since the extent of the fattening depends, within the limits of the animal's capacity, largely upon the amount of feed supplied. Growth, on the other hand, unless the feed fails to supply the necessary materials and thus becomes a limiting factor, goes on at a rate substan- tially determined by other conditions, the most obvious of which are the species, individuality and age of the animal. Indeed, it may be said that, within normal limits, the capacity for growth determines the feed consumption rather than the reverse. Heavy feeding may cause fattening but it does not appear, at least in the case of the higher animals, to materially accelerate growth, although Eckles 1 observed the growth of dairy calves to be somewhat more rapid upon heavy as com- pared with scant rations. In growth, therefore, as in mainte- nance, there is a real requirement to be satisfied, its measure be- ing the amount and character of th6 increase which the young animal is capable of making under normal conditions. Mention has been made (372) of the interesting results of experi- ments by Waters 2 upon growth under adverse conditions, while Osborne and Mendel 3 have shown that growth which has been sus- pended for a time because of inadequate feed supply may be resumed when this deficiency is made good (deferred growth). Neither of these possibilities, however, invalidates the statement just made that the continued maintenance of a normal rate of growth requires a definite supply of matter and energy. 1 Mo. Expt. Sta., Bui. 135, 1915. 2 Soc. Prom. Agr. Science, Proc. 2gih Annual Meeting, 1908, p. 71. 3 Jour. Biol. Chem., 18 (1914), 195; 23 (1915), 439; Amer. Jour. Physiol., 40 (1916), 16. GROWTH 397 479. Total increase in normal growth at different ages. — The feed requirements of the growing animal as regards protein and energy depend in the first place on the amounts which such an animal is capable of storing up in normal growth. From the data regarding the rate of growth recorded in § i of this chapter, even though they are somewhat fragmentary, it seems possible to derive average figures regarding the storage of protein and energy in growth at different ages which may be of some value as a guide in estimating the feed requirements of the growing animal. As regards protein, it was shown that the rate of gain per 1000 live weight apparently does not vary widely as between cattle, sheep and swine, and an empirical formula (463) was given by which its amount at any age may be approximately estimated. As regards energy, fewer data are available, es- pecially for farm animals, but the graphic representation in Fig. 39 of the results recorded in Table 81 (464) shows a diminish- ing rate of gain of energy as the animal grows older. In the following tabulation the daily gain of protein at differ- ent ages has been calculated by means of the formula just men- tioned and the gain of energy estimated from the smoothed graph of Fig. 39. The two together may be taken as an approximate expression of the normal increase in growth at different ages. TABLE 91. — DAILY INCREASE IN GROWTH PER 1000 POUNDS LIVE WEIGHT AGE PROTEIN ENERGY 4* 10 days Pounds 4^O Therms 24. S 20 days . . . . . 3.^8 21 8 30 days 2.70 2O.O 60 days I ^O 16 o QO days 1.23 J5 O 1 20 days 0.96 1 1.5 i <^o davs O 7Q 10 o 180 days 0.68 o.o 210 days O ^O 8 <; 240 days ... . . O ^2 7 S 270 days O.47 7.O 300 davs O 4.2 6 92° Cals. Energy evolved as heat 20,740 Cals. Computed maintenance requirement .... 15,060 Cals. Excess of heat 5, 680 Cals. Excess over maintenance 37-7% 538. Age and weight of animals. — The internal work of like animals of different sizes, under like conditions, appears to MEAT PRODUCTION 455 be approximately proportional to their body surface (345), and there is even good ground for believing that this law applies in a broad way to animals of the most diverse species and size. Since the action of external temperature is also approximately proportional to the surface, it would be expected that the size of the animal would not be an important factor. In fact, however, the other conditions are rarely alike. The young animal in particular is likely to be getting a relatively lighter ration than the animal which is being pushed for the butcher, and thus to have less surplus heat at its disposal, while the indefinable factor of " hardiness " would also seem to be in favor of the older animal. 539. Humidity. — The relative humidity of the air is an im- portant factor in the temperature relations of the animal. Moist air tends to increase the conductivity of the hair or wool, just as it does that of the clothing of man, thus facilitating the escape of heat and raising the critical temperature. Accord- ingly, it is to be anticipated that in a dry climate, like that of the northwestern United States, animals might be safely exposed to a greater degree of cold than in a damp climate, like the winter of the seaboard States. 540. Temperature of drinking water. — In general, the same considerations adduced in discussing the influence of the temperature of the air apply to that of the drinking water. Under heavy feeding, especially, unless in very cold quarters, the animal has a surplus of heat which it can apply to warming its drink. If, then, the latter is at such a temperature as to be consumed freely, there would seem to be no occasion for heating it further, except for one important consideration. The tem- perature of the air acts continuously and with approximate uniformity. That of the water, on the other hand, acts only at intervals, often only two or three times or even once per day. If, now, the animal consumes within a short time a large amount of cold water, a correspondingly rapid expenditure of heat is required to warm this water to the body temperature, and this demand may for a time exceed the supply of surplus heat and cause an increased oxidation of tissue or food material for the sake of heat production only. Such a loss can never be made good at a later hour since, once converted into heat, the energy has escaped from the grasp of the body. Other things being NUTRITION OF FARM ANIMALS equal, then, it will clearly be desirable to have the water con- sumption approximate as nearly as possible a continuous con- sumption by having it constantly accessible, while if the stock are watered only at intervals the temperature of the water may need to be rather higher than in the other case. Shelter A protection from rain or snow and from wind may be of quite as much importance as protection from low temperatures simply. 541. Precipitation. — An important factor in the case is the amount of precipitation (rain or snow) to be expected dur- ing the feeding period. In cold weather the low temperature of the water which penetrates to the skin of animals is the cause of a loss of heat which may be regarded as practically an ad- dition to that due to the cold air, the extent of both losses being affected by the thickness of the animal's coat. Far more im- portant than this, however, is the expenditure of heat re- quired to dry out the coat after it is wet, and this, as it would seem and as some of the experiments with sheep seem to indi- cate, would be greater with the heavier coated animal when it has once become thoroughly wet. Still greater, relatively, is the heat required to melt the snow falling on the animal or that upon which it is compelled to lie. These effects, it will be observed, are largely independent of the indications of the thermometer, and it is clear that the nature of the climate as regards humidity and precipitation is quite as important a factor as the temperature in its bearing on the question of shelter, and that in many localities a roof to shelter the animals from storms may be as efficient as a tight barn. One advantage of the roof, already mentioned inci- dentally, is that it provides the possibility of a dry bed, thus not only adding to the comfort of the stock but avoiding ex- penditure of energy in warming up or evaporating water or melt- ing snow or ice. 542. Wind. — All are familiar with the greater severity of a windy day as compared with a still one of the same temperature. A large part of the protective value of the clothing of man or the coat of an animal resides in the air entangled between the fibers of the material. Wind tends to replace this air with fresh, MEAT PRODUCTION 457 cold air and thus greatly reduces the protective effect. A wind- break, therefore, may have a distinct economic value in stock feeding. 543. Insolation. — The effects of the weather are appreciably modified by the exposure of stock to direct sunlight. Aside from any direct effect of the light as such, a not inconsiderable amount of heat is imparted to the body by the sun's rays. During cold weather this is likely to be a distinct advantage, but during the hot months the reverse is true. Since the animal cannot re- duce its heat production below that resulting from its internal work and the digestion and assimilation of its feed, it may se- riously tax its powers to dispose of the additional heat imparted by the direct sunlight. In this case shelter of some sort may be required for opposite reasons to those obtaining during the cold months. .For similar reasons a supply of cool, fresh water and exposure to the wind may be of great advantage in helping the animal to get rid of its surplus heat. Other conditions 544. Exercise. — The well-known fact that muscular exer- tion is accomplished at the expense of the katabolism of tissue and ultimately, therefore, at the expense of the feed, would seem at first thought to indicate that the activity of the meat-produc- ing animal should be restricted as much as practicable. In the case of the growing animal, however, another very important element enters into the case, namely, the fact that moderate exercise tends to stimulate the growth of the muscular system, or, in other words, the production of lean meat. Since this is the essential object sought, a normal and reasonable amount of muscular activity on the part of the growing animal should be allowed and encouraged, even though the muscular exercise involves the consumption of more feed. Accordingly, young stock should be given the freedom of the pasture or range to as great an extent as practicable, while at the same tune care should be taken to supply abundant feed containing a sufficient supply of protein in order that enough material may be present to supply the demand for growth stimulated by the exercise. In the case of breeding stock, especially, a most important consideration is that of the health and stamina of the animal, 458 NUTRITION OF FARM ANIMALS which can hardly fail to suffer through overconfinement. The above principles apply in a general way to all classes of stock. In particular, hogs should be given an opportunity for more movement and exercise than is frequently allowed. In the case of animals which have reached the fattening stage, on the other hand, there is comparatively little growth of pro- tein tissue, while it is only necessary to maintain sufficient health to ensure a normal appetite and assimilation of feed. In pro- portion, then, as this stage is reached, the endeavor should be to reduce the amount of exercise taken and to keep the fatten- ing animal as quiet as possible. To this end comfortable quar- ters should be provided, with plentiful bedding, and the animals should be kept -as undisturbed as possible, so that they may " eat and lie down." This is particularly important in the case of the sheep on account of its timid nature. For similar rea- sons it is desirable to have the water supply of fattening animals close at hand. 545. Water supply. — It should never be forgotten that rapid production, involving the utilization of relatively large amounts of feed, requires the consumption of a corresponding amount of water for the physiological purposes of the animal. For this reason, as well as for the one previously mentioned (540), it is desirable that stock should have ready access to water, if possible, at all times and that the water supplied should not be too cold to be consumed freely by the animals. CHAPTER XIII MILK PRODUCTION § i. THE PHYSIOLOGY OF MILK PRODUCTION 546. Components of milk. — In addition to water, milk contains representatives of the four great groups of nutrients, viz., proteins, fats, carbohydrates and ash. Proteins. — The principal protein of milk is casein, a sub- stance belonging to the group of phosphoproteins (65). This protein is peculiar to milk, not being found elsewhere in the body. In addition to casein, milk contains also a lact-albumin and a paraglobulin in small amounts. Their presence may be demon- strated by precipitating the casein by means of acid and heating the nitrate. Traces of peptones, possibly due to the presence of a proteolytic enzym, are also found in milk. According to Konig, the casein content of milk has been observed to vary from 1.79 per cent to 4.23 per cent and that of the other proteins from 0.25 per cent to 1.44 per cent. Fats. — Fats occur in milk in the form of microscopic glob- ules varying greatly in size and held in suspension in the col- loidal solution of casein. In cow's milk the diameter of these fat globules may be stated in a general way to range from 0.0016 to o.oi millimeter and in a single cubic centimeter of average milk their number runs into the millions. The fat globules were formerly described as surrounded by a membrane of a protein nature, but the supposed membrane is now re- garded as simply a condensation of the protein of the milk, due to surface tension. Milk fat, like other animal fats, is a mixture of a number of simple fats or triglycerids. As compared with body fats, the fat of milk is relatively rich in olein and consequently has a relatively low melting point. It is especially distinguished from 459 460 NUTRITION OF FARM ANIMALS body fat, however, by the presence of a considerable proportion of fatty acids of low molecular weight, as already noted in Chapter I (30), where a list of the principal constituents is given. The presence of these so-called " volatile fatty acids " (i.e., acids which can be distilled in a current of steam) affords an important means for the detection of adulterations of butter. The percentage of fat in milk varies widely. For the cow a minimum of 1.67 per cent is reported by Konig. Six per cent, on the other hand, is a high figure, although occasionally 7 per cent is reached. Babcock states that 9 per cent is the maxi- mum observed for a cow giving as much as 15 pounds of milk daily. The milk fat carries traces of lecithins and cholesterins and also varying amounts of coloring matter, derived, as Palmer and Eckles 1 have shown, chiefly from the carotin of the feed. Carbohydrates. — Milk contains in solution a disaccharid peculiar to itself, namely, lactose, or milk sugar (13). In distinction from fat, the percentage of lactose in fresh milk shows comparatively small variations, averaging about 5 per cent in cow's milk. The souring of milk is brought about by a fermentation of the milk sugar by which its molecule is split into four molecules of lactic acid. Among the organic ingredients of milk should also be men- tioned citric acid, which occurs in appreciable quantities in the form of calcium citrate. Ash. — The total mineral matter in cow's milk averages about 0.7 per cent according to Van Slyke.2 Qualitatively, the ash of milk contains the same ingredients found in all animal substances. Its quantitative composition, however, as compared with the blood serum, on the one hand, and with that of the tissues on the other, shows some interesting relations. Bunge 3 gives the following figures for the composition of the ash of the serum of cattle blood and of the ash of cow's milk. To these have been added Lawes and Gilbert's figures for the ash of a calf for the sake of comparison. 1 Jour. Biol. Chem., 17 (1914), 191-264. 2 Jordan, The Feeding of Animals, 1908, p. 305. sZtschr. Biol., 10 (1874), 301 ; 12 (1876), 191. MILK PRODUCTION TABLE 1 19. — PERCENTAGE COMPOSITION OF ASH 461 SERUM or CATTLE BLOOD Cow's MILK BODY OF CALF K2O % 32 % 22 I % Na2O C C I I 3 O * 82 CaO I 6 2O O A-3 nr MgO 06 2 6 Fe2O3 .... O I O Od. o r 2 Cl 4.7 i 212 O 1 2 P2O5 •3 A 24 8 With smaller animals, having a shorter period of growth, the rela- tions are even more striking. Thus, for the rabbit Bunge l reports the following results. TABLE 1 20. — PERCENTAGE COMPOSITION OF ASH SERUM OF RABBIT BLOOD RABBIT MILK BODY OF 14 DAYS OLD RABBIT K2O •2 2 IO I IO 8 Na2O ^4. 7 7 Q 6 o CaO I A •2 C 7 •7 C O MeO o 6 2 2 2 2 Fe2O3 O O O I O 2 Cl 4.7.8 r 4 4.Q P2O5 2 o •2Q Q 4.1 Q It appears that while sodium and chlorin are the predominant ingredients of the blood serum, these elements are present in milk in relatively small proportions, while potassium, calcium and phos- phorus predominate in the latter, the ash of milk closely resembling that of the body of the same species. 647. Average composition. — Wing 2 cites the following figures as showing approximately the average composition of cow's milk 3 according to various authorities. 1 Quoted by Sellheim in Nagel's Handbuch fiir Physiologic, II, 188. 3 Milk and its Products, 1897, p. 17. 3 For data regarding the composition of the milk of other species than cattle, see Schaefer's Text Book of Physiology, Vol. I, p. 125. 462 NUTRITION OF FARM ANIMALS TABLE 121. — AVERAGE COMPOSITION OF Cow's MILK AMERICAN (Babcock) ENGLISH (Oliver) GERMAN (Fleisch- mann) FRENCH (Cornevin) Water Fat Casein 87.17 3-69 302 87.60 3-25 •2 AQ 87.75 3-40 2 80 87-75 3-30 Albumin Sugar Ash 0-53 4.88 O 71 °-45 4-55 O 7C 0.70 4.60 O 7 C 4.80 O 7 e IOO.OO IOO.OO IOO.OO 99.60 548. Milk glands. — The milk glands, properly speaking, are two in number, one on each side of the median line of the body, although in many animals each gland is subdivided into two or more lobes having separate outlets or teats. Thus in the horse and sheep each gland has two lobes, in the cow two or three, and in the hog from ten to fourteen. The milk gland is classified as a compound tubulo-acinous gland. Its structure may be roughly compared to that of a bunch of grapes. It consists of a great number of acini or alveoli, three of which are shown schematically FIG. 40.— Lobule of milk in Fig. 40, corresponding to the single berries of the grape cluster. Each alve- olus consists of an outer layer of con- nective tissue carrying capillary blood vessels, nerves and lymphatics. These alveoli are about ^ of an inch in diameter and are united in groups of 3 to 5 to form lobules having a common outlet as shown in the figure. Internally, the alveoli are lined with a single layer of epithelial cells (Fig. 41), which are the active agents in secreting milk. The ducts or passages leading from the alveoli are also lined with epithelial cells but of a different sort and which do not produce milk. These ducts unite to form larger ones, as shown in Fig. 42, which lead finally to the teat, emptying first into the so-called " milk cistern," a cavity lying near the base of the teat. In compound MILK PRODUCTION 463 milk glands there is more or less connection through these milk ducts between the several lobes, but none between the two glands on either side of the body. The milk gland, therefore, consists of a framework of connective tissue carrying more or less fat, of alveoli, milk ducts, veins, arteries, lymph vessels and nerves, the whole forming a reddish gray spongy mass. In the cow the -y two glands constituting the udder are separated by a band of fibrous tissue which serves to support the organ. The udder may vary widely in the proportion of connective and fatty tissue on the one hand and of true , . ,.N , FIG. 41. — Alveoli of milk secreting tissue (alveoli) on the other. gland. (wikkens, Form und A large proportion of the former gives Leben der Landwirthschaft- what is commonly known as a fleshy Uchen Hausthiere-) udder. The size of the udder, therefore, is not the sole criterion of its capacity as a milk pro- ducing organ. At the branches of the milk ducts are located sphincter muscles which are more or less under the control of the animal and the contraction of which interferes with the flow of milk, enabling the animal, as the phrase goes, to " hold up " her milk. 549. Development of milk glands. — In the young animal, the milk glands are rudimen- tary and in the male remain so during life, except in extraor- dinary cases. In the female, however, as sexual maturity approaches, a considerable formation of glandular tissue wirthschaftlichen Hausthiere.) takes place, but the glands 464 NUTRITION OF FARM ANIMALS reach their full development only in the later stages of preg- nancy. At that time, a rapid growth of the alveoli and per- haps the formation of new ones occurs, the stimulus to this growth being, according to Bayliss and Starling, the formation of certain stimulating substances (Hormones) in the fetus which .pass into the blood of the mother and so reach the milk glands. That other causes may at least cooperate, however, is shown by the apparently well-established fact that the regular re- moval of the fluid found in the glands of the virgin animal, or even mechanical stimulation, may lead to the formation of considerable quantities of milk, in some instances even in the male. 550. The secretion of milk. — That milk formation is a true secretion and not a mere nitration of material from the blood is clearly shown by the facts already stated regarding the com- position of milk. As was pointed out, all the principal organic ingredients of the milk are peculiar to it. Casein and lactose are not found elsewhere in the animal body, and while the prin- cipal simple fats of milk are also found in the body fat, their proportions are different in the milk fat and the latter is specially characterized by the presence of glycerids of the lower acids of the aliphatic series. Furthermore, even more marked quan- titative differences exist between the mineral elements of the milk and those of the blood serum. From all these facts, it is clear that the milk gland is a producing or secreting organ and that the solid ingredients of the milk are largely manufactured in it out of materials derived from the blood. A theory of milk secretion first propounded by Virchow found wide acceptance. According to this theory, milk pro- duction consists essentially of a physiological fatty degeneration of the epithelial cells of the alveoli. The microscope shows that the cells of the actively secreting gland are larger than those in the resting gland and more or less filled with fat globules, especially on the side toward the cavity of the alveolus. It was held that while this process went on the cell divided, forming two or more, and that finally the cell next to the cavity liquefied, setting free the fat globules which it contained' and, perhaps with the addition of more or less water, constituted the milk. Milk production was thus regarded as a form of the growth of tissue. MILK PRODUCTION 465 Subsequent investigation, however, has generally failed to show satisfactory evidence of cell division. A modification of Virchow's theory still held is that while there is no cell division, the outer portion of the protoplasm is sloughed off and dissolved, forming the milk, and is again renewed by the growth of new protoplasm. The weight of opinion, however, regards milk production as a true secretion, entirely analogous to that ob- served in other glands. It is not believed that there is normally a breaking down of cells, but that the latter extrude their secreted materials into the alveolus precisely as do the secreting cells of other glands. This is held to apply to the fat globules as well as to the other ingredients of milk. The process is in many ways analogous to that of the resorption of digested material by the epithelial cells of the small intestine, the obvious difference being the direction in which the materials move. The secretion of milk in the active udder is a more or less continuous process, the product accumulating in the cavities and passages of the gland. Fleischmann long ago showed, however, that the cavities of the udder cannot possibly contain the amount of milk produced in a single milking by a reason- ably productive cow, and it is well recognized that a rapid secre- tion of milk occurs during suckling or milking. In other words, the milk gland, like other glands, reacts to a specific stimulus. 551. Sources of ingredients of milk. — While the ultimate source of the material contained in the milk is of course the feed, the milk gland draws its supply of material for milk pro- duction immediately from the blood, while at the same time it brings about extensive chemical transformations in the sub- stances thus supplied. Probably all the ingredients of the milk should be regarded as products of the chemical activity of the epithelial cells of the glands, although the extent to which the original material is modified varies. 552. Origin of milk proteins. — The albumin and globulin of milk are quite similar to the corresponding substances in the blood. The casein, on the other hand, is radically different. In the first place, it is, as already stated, a conjugated protein containing some phosphorus-bearing radicle. Whether the latter is derived exclusively from the organic phosphorus com- pounds of the feed has not been demonstrated, although it appears probable that inorganic phosphorus compounds (phos- 2 H 466 NUTRITION OF FARM ANIMALS phates) may be utilized as sources of the phosphorus of the milk (257, 258, 497). The production of casein, however, is not simply ,a conju- gation of a simple protein with a phosphorus group. The constitution of casein is markedly different from that of the proteins of the blood serum or of the muscles, as is shown by the proportions of its various cleavage products as given in Chapter I (50), so that if casein is formed from the protein of the blood or tissue, a considerable reconstruction of their mole- cules is necessary. On the other hand, if the casein of the milk is built up in the epithelial cells of the udder, in the manner suggested in Chapter V (226, 227), from the simpler cleavage products in the blood, the process is specific for the milk gland. 553. Origin of milk fats. — It was stated in Chapter V (247- 249) in discussing the sources of body fat that although the latter may be derived in part from the fat of the feed and show some of its characteristics, nevertheless, the production of fat must be regarded as due essentially to the activity of the fat cells, and not to a simple deposition. In the first place, it has been demonstrated by the researches of Jordan and others that milk fat as well as body fat may be formed from the carbohydrates of the feed. In Jordan's l experiments cows were fed either with an ordinary ration or with one very poor in fat and the production of fat in the milk determined. After deducting the maximum amounts of fat which could possibly be accounted for by the protein and fat of the feed, a considerable balance was left which could only have been pro- duced from the carbohydrates. The following table gives a summary of the results : — TABLE 122. — PRODUCTION OF FAT BY Cows NUMBER or DAYS TOTAL PRO- TEIN 2 METAB- OLISM EQUIVALENT FAT FAT OF FEED TOTAL FROM FAT AND PROTEINS FAT ACTUALLY PRODUCED Grams Grams Grams Grams Grams 59 15,109 7,766 1,490 9,256 17,585 74 34,661 17,816 2,211 20,027 37,637 4 2,209 1,131 1,504 2,635 3,289 1 N. Y. (Geneva) Expt. Sta., Buls. 132 and 197. 2 Digested protein of feed less gain of protein by the animal. MILK PRODUCTION 467 It should perhaps be pointed out that the formation of fat from carbohydrates in these experiments may not necessarily have occurred in the milk gland itself. It is entirely conceivable that the main portion of the synthesis of the fat may have taken place elsewhere and that the fat or its precursors were simply transferred to the milk gland. Second, it has also been shown by a considerable number of experiments that, as in the case of body fat, the fat of the feed may sensibly affect the properties of the milk fat. Not only have changes in the melting point, iodin number, and other properties of butter fat been found to follow in a general way similar changes in the feed fat, but characteristic ingredients of foreign fats given in the feed have been detected in the milk. While it is not necessary to conclude, and is indeed unlikely, that the feed fat is simply transferred, as it were mechanically, to the milk, it is clear, on the other hand, that relatively large fragments of the fat molecule are able to pass through the epithelial cells into the milk. These facts render it evident that feed fat is a source of milk fat. Not only so, but experi- ments by Morgen and his associates, to be mentioned later (613), seem to show that a certain amount of fat in the feed (in her- bivorous animals at least) conduces to the most efficient pro- duction of milk fat. The idea that the fat of milk is produced synthetically to a considerable extent is perhaps supported also by the presence in it of the lower acids of the aliphatic series, which may be intermediate steps in the synthesis of fat from simpler carbon compounds, or, on the other hand, may arise during the partial breaking up of the carbon chain in the feed fat which probably precedes its transformation into milk fat. As a general conclusion, therefore, it may be stated that the fat of milk may have its origin either in the fat or in the carbo- hydrates of the feed, or in both. Whether it may also be pro- duced from protein has not been demonstrated experimentally, but reasoning by analogy with the formation of body fat, it must be regarded as at least very probable. 554. Origin of lactose. — The lactose of milk is a disaccharid yielding upon hydration dextrose and galactose. Dextrose or its derivatives are abundant in the feed of herbivorous animals and it is also a constant ingredient of the blood. On the other 468 NUTRITION OF FARM ANIMALS hand, while the ordinary feed of herbivora contains carbohy- drates yielding galactose, the latter is apparently transformed into glycogen quite promptly and at any rate has not been found in the blood, while animals receiving feed containing no galactose (carnivora, e.g.) produce lactose in their milk. The probability seems to be that the galactose half of the lactose is manufactured in the milk gland from the dextrose of the blood. 555. Sources of ash. — The ash ingredients of the milk, including its sulphur and phosphorus, are, of course, derived ultimately from the corresponding ingredients of the feed. In liberal milk production on ordinary winter rations containing a sufficiency of organic nutrients, however, it appears from in- vestigations by Forbes 1 that considerable amounts of calcium, magnesium and phosphorus may be drawn from the relatively large store contained in the body, presumably to be replaced in later stages of lactation. 556. Character of milk production. — While the statement that milk production is a form of tissue growth is probably incorrect anatomically, it is essentially true so far as the chemical composition of the product and the demands which it makes on the feed supply are concerned. This is clearly shown by comparing the ratio of protein to fat in the organic matter of milk and in that of the increase in weight of growing animals. In the solids of milk, it is evident that in order to make a fair comparison its milk sugar should be reduced to the equivalent amount of fat. Taking Babcock's figures (547) as representing the average composition of milk, the 4.88 per cent of sugar di- vided by 2.25 is equivalent to 2.17 per cent of fat, which added to the 3.69 per cent of fat present as such makes a total fat equivalent of 5.86 per cent, while if milk sugar were thus re- placed by fat the total organic matter would amount to 9.41 per cent. On this basis, 100 parts of organic matter would contain 37.73 per cent of protein and 62.27 per cent of fat. Comparing these figures with those given in Chapter XI (458) for the composition of the increase in growth, it appears that the proportion of protein to fat is greater than that computed for young animals except in the earliest stages of growth. The computed energy content of average milk solids is 2620 1 Ohio Expt. Sta., Bui. 295 (1916). MILK PRODUCTION 469 Cals. per pound. This is greater than the energy content of the dry matter gained by very young animals, but less than that computed in later stages of growth. In a general way, then, it may be said that milk solids correspond in proportion of protein and in energy value per pound to the gains made by growing animals when in the neighborhood of three months old. 557. Rate of production of milk solids. — A beef calf three months old may be assumed to make a growth of approximately 1.5 pounds per day, containing perhaps three-fourths of a pound of dry matter with an energy content of about 2200 Cals. The very moderate yield of 15 pounds of average milk per day would contain about 1.92 pounds of total solids equiva- lent to 5030 Cals. of energy. In other words, considerably more than twice as great a production would be effected by the relatively small bulk of the secreting cells in the udder as by the whole body of the calf. When it is further considered that the product of the dairy cow is all edible, her great economic value as a producer of human food becomes ob- vious. On this point Jordan says : l " A cow yielding 6000 pounds of average milk per year is not regarded as an unusual animal. This means, however, the annual produc- tion of not less than 780 pounds of milk-solids, an amount at least double the dry matter in the body of a cow weighing 900 pounds. When we consider that this manufacture of new material is carried on not only during a single year, but through the entire adult life of the animal, we begin to realize how ex- tensive are the demands upon the food supply. Still more striking is the case of high-grade cows yielding annually over half a ton of milk solids, and when we remember the perform- ance of Clothilde, whose 26,000 pounds of milk produced in a year certainly contained more than 2500 pounds of solid matter, we must regard the cow as possessing wonderful powers of transmutation. Her capacity for the rapid and economical production of human food of the highest quality is not equaled by any other animal." 558. Factors of milk production. — Milk production differs from meat production in one very essential particular. In the latter, broadly speaking, an increase in the whole body of the animal is what is sought, and while the product may vary in 1 The Feeding of Animals, 1908, p. 308. 470 NUTRITION OF FARM ANIMALS market quality, all the feed consumed in excess of the main- tenance requirement is available for the production of gain. In milk production, on the contrary, what is desired is the secretion of a single set of glands. An increase in weight in the mature dairy cow is not sought. At best it represents a diversion of feed to other purposes than the one in view, while any considerable fattening tends to check the activity of the milk glands. In feeding for milk production, therefore, it is necessary to consider not only the surplus feed above the main- tenance requirement but the factors affecting the distribution of that surplus between milk production on the one hand and growth or fattening on the other hand. The art of feeding for milk consists in stimulating the milk production to the greatest economically possible extent and in supplying the feed material necessary for this production, while avoiding, in the mature animal, any material increase of body tissue. The factors governing milk production are essentially the same as in other branches of animal production, viz., the ani- mal, the environment and the feed supply. In milk production, however, the relative importance of the first and second conditions is greater than in other forms of production for the reason that they may materially influence the distribution of the excess feed between milk production and tissue increase. § 2. THE ANIMAL AS A FACTOR IN MILK PRODUCTION 559. The prime factor in successful dairy production is the animal. Unless the latter possesses abundant secreting tissue which is capable of being stimulated to a normal rate of activity and of yielding a secretion of good quality, the most scrupulous care and the most abundant feeding will inevitably fail to yield satisfactory returns. Individuality 560. Includes breed differences. — The influence of in- dividuality may be said to include that of breed, since a breed is simply an aggregate of more or less similar and genetically related individuals. It is outside the scope of this work to discuss problems of breeds and breeding, and this branch of the MILK PRODUCTION 471 subject will therefore be considered mainly from the point of view of individual differences. 561. Influence on yield of milk. — While the actual quantity of milk produced is affected by feed, care and other circum- stances, the capacity of the animal as a milk producer is an individual characteristic. Just as the maximum speed of which a horse is capable is dependent primarily upon his con- formation, spirit and other individual characteristics, while the actual rate at which he travels at any given time is largely dependent upon his driver, so the maximum capacity of the milk cow constitutes an individual limit beyond which she can- not be pushed by any amount of care or feed. Striking illustrations of the importance of individuality are afforded by the various public tests of dairy cows. For example, in the World's Columbian Exposition of 1893, the conditions of the so-called ninety-days test were such as to induce liberal feeding and the best of care on the part of the exhibitors. The cows, numbering 74, were of three different breeds and presumably represented the best avail- able specimens of each breed. The following table shows the average daily product of the best 1 and the poorest cow of each breed in that test. TABLE 123. — AVERAGE DAILY YIELD OF Cows IN NINETY-DAYS TEST, WORLD'S COLUMBIAN EXPOSITION 2 MILK FAT OF MILK TOTAL SOLIDS OF MILK Best Jersey 4.0.4. Ib. 1.98 Ib. 5.67 Ib. Poorest Jersey 22 9 Ib i 09 Ib 3 21 Ib Poorest in per cent of best . Best Guernsey 56.7% •2Q o Ib 55-i% i 70 Ib 56.6% r 20 Ib Poorest Guernsey Poorest in per cent of best Best Shorthorn 19.3 Ib. 49-5 % 40.9 Ib. 0.97 Ib. 57-1 % 1.49 Ib. 2.75 Ib. Si.o% 4>GIutencells- S(Starch resemble in composition, THE FEEDING STUFFS 585 while the proportion of light oats is not sufficient mate- rially to raise the value. Oat hulls are rarely offered as such in the market but are usually disposed of in one of two ways. First, they are made the basis of various proprietary feeds, cheap by-products of various sorts being added, usually including a small amount of the protein-rich by-products shortly to be described. These feeds are offered under various names and with abundant advertising testimonials. While they are by no means worthless, it is evident that the oat hulls themselves are no more valuable because of the addition to them of other materials, while the consumer ultimately pays the cost of mix- ing, transportation and advertising. The second use to which oat-hulls are put is the adulteration of the mixed feeds, es- pecially corn and oat feeds, which are freely offered on the market. Since it is difficult to recognize even a considerable adulteration of this sort, such mixed feeds should be purchased only from manufacturers of known integrity or under a satis- factory guarantee as to purity. Barley feed, a by-product of the manufacture of pearled barley, is similar in feeding value to oat hulls. Hominy feed. — In the manufacture of hominy from corn, the hull, the germ and the more starchy parts of the kernel are rejected and constitute hominy feed, or hominy chop, which is similar to the whole kernel in composition and digestibility, except that its percentage of fat is greater. Consequently it has a somewhat higher feeding value, although the fat is likely to become rancid on long keeping and thus lower its quality. 695. By-products of the fermentation industries. — The manufacture of alcoholic liquors consists essentially in the conversion of the starch of grains or potatoes into sugar and the subsequent fermentation of this sugar by means of yeast. The resulting liquor may be consumed directly (beer, ale) or it may be distilled, yielding the more concentrated distilled liquors or commercial alcohol. M alt sprouts. — The first step in the process is the prepa- ration of malt, by allowing moistened barley to germinate. The growth of the sprouts is stopped by drying when they are about one-third inch long, and these dried sprouts, sepa- rated from the grain, constitute malt sprouts. Being young roots of barley, they have the general properties of all young 586 NUTRITION OF FARM ANIMALS plant growth, containing a high percentage of nitrogen, much of it in the form of non-protein, and a low percentage of crude fiber. Brewers' grains. — The next step in the process is the mash- ing of the ground malt and other grain with warm water. In this process, the diastase of the sprouted barley acts on the starch of the grain, transforming it into sugar. In the manu- facture of beer or ale, the resulting liquid is drawn off and fer- mented separately, leaving a residue known as brewers' grains, which is used extensively as a dairy feed. In the fresh state it is valuable, but is subject to the disadvantage of fermenting or souring very readily, and tending in this state to injure the quality of the milk. Somewhat recently, economical pro- cesses for drying it have been perfected, and the dried brewers' grains constitute a valuable feed which can be shipped like any other dried feed. Distillers' grains. — In the preparation of distilled liquor or alcohol, the liquid is fermented in contact with the grains and the alcohol then distilled off, leaving a residue known as dis- tillers' grains or distillery slop. This residue is much wetter than brewers' grains, but is less subject to fermentation, since the sugar has been more completely removed. Large quantities of it are now put on the market in the dried form, both under its own name and various trade names, some of which contain no suggestion of the real nature of the material. It constitutes a valuable source of stock feed. The grains produced from rye are regarded as the poorest and those from maize as of the best quality. In all these processes the object is to convert the starch of the grain as completely as possible into sugar and then into alcohol. This results in increasing the percentage of all the other ingredients in the residues. They contain accordingly a high percentage of protein with also a somewhat greater percentage of crude fiber than the ordinary grains. They serve, therefore, not only to supply digestible matter as a whole but also to correct a deficiency of protein in the ration. 696. By-products of oil extraction. — The extraction of com- mercial oils from various oil-bearing seeds leaves by-products, called oil cake or oil meal, some of which have a high feeding value. Of these, cottonseed and linseed meal are the only ones extensively used in the United States and are typical of the THE FEEDING STUFFS 587 others. The seeds of cotton and flax are rich in both fat and protein. Hulled cottonseed contains about 30 per cent of each and flaxseed about 22 per cent protein and 35 per cent fat, the latter percentage, however, being somewhat variable. The oil is extracted from the seeds either by pressure or by the use of solvents, leaving a residue still containing some fat and very rich in protein. Cottonseed meal. — At present cotton oil is extracted only by pressure, the resulting hard cake being ground to cottonseed meal. The highest grade of cottonseed meal is made from the hulled seed and contains 40 to 44 per cent of crude protein and 8 to 9 per cent of fat. It should be nearly free from the hulls and therefore contain little crude fiber. Cottonseed meal is adulterated extensively with the tough, black hulls of the cottonseed, which have a very low feeding value. This is es- pecially true of the inferior grades of commercial cottonseed meal, which are sold at a lower price than the standard grade. Linseed meal. — Linseed oil is extracted from the flaxseed both by pressure and by means of naphtha, the latter being com- pletely removed from the resulting oil-meal and recovered for use again. The " new process " of extraction removes the fat more completely than the " old process " of pressure, and the resulting linseed meal is somewhat poorer in fat and contains somewhat more protein than the old-process meal. The pro- cess of extraction by pressure has been so far perfected in recent years, however, that the difference between the old-process and new-process meal is distinctly less than formerly. The protein of the new-process meal appears to be slightly less digestible than that of the old-process meal, which tends still further to reduce the difference between the two. Other oil meals. — Oils are also manufactured commercially from the seeds of the common peanut, the soybean, the oil palm and the cocoa palm. The resulting oil cakes or meals are extensively used as feeding stuffs in European countries but do not appear to have as yet found access to the feed market of the United States to any considerable extent. The corn-germ meal mentioned in connection with the gluten feeds may also be classed as an oil-meal. 697. By-products of starch and glucose manufacture. — Starch and glucose are made in the United States chiefly from 588 NUTRITION OF FARM ANIMALS maize. The starch is separated by coarse grinding and the use of water, the starch being carried off in suspension and al- lowed to settle out. Glucose is manufactured by further treat- ment of the starch with acid. In the preparation of the starch, the parts of the kernel which are rejected are the hull, the germ and the more glutinous part of the interior of the grain from which the starch cannot be completely separated. Corn (maize) bran. — The hulls are comparatively low in protein and contain con- siderable fiber. When sold separately they are called corn bran, although the com- position of commercial sam- ples indicates some admix- ture of the germs. FIG. 45. — Partial section of maize kernel. Germ meal. — The germ (Bailey's Cyclopedia of American Agricul- contains about 30 per Cent of oil, which has a com- i. Outer layer of skin. 2, Inner layer of skin. -11 i • i 4, Gluten cell. 5. Starch cells. (Jordan.) merCial Value and IS SCCUred by pressing the germs. The residue constitutes germ meal, which still contains about 7 per cent of oil, and in the neighborhood of n per cent of crude protein. Gluten meal and feed. — The glutinous residue of the kernel constitutes gluten meal, containing, in general, 30 to 40 per cent of crude protein with a comparatively low percentage of fat and fiber. Some factories mix the gluten meal and the hulls, and sell the mixture under the name of gluten feed, which con- tains approximately 24 per cent of crude protein, 6 per cent of crude fiber and 6 per cent of fat. Sometimes the hulls and germs are sold together under the names " sugar feed " or " starch feed," either wet or dry. In fact, various mixtures of the three main products are made and sold under diverse commercial names. These various glucose products should invariably be purchased on a guarantee as regards composition and purity. 698. By-products of sugar manufacture. — Sugar has come to be manufactured from sugar-beets to a considerable extent in THE FEEDING STUFFS 589 the United States, while in certain regions the manufacture from sugar cane is an important industry. Sugar-beet pulp. — The sugar is extracted from the finely cut beets by means of water in what is known as the diffusion process. The residue from this constitutes what is commonly known as beet pulp, which is essentially sugar beets minus the sugar and some of the other soluble substances. In the fresh state it contains 90 to 95 per cent of water, which may be re- duced to about 85 to 87 per cent by pressing. Its general properties are similar to those of roots and it occupies much the same place in the ration. Its digestible matter consists chiefly of carbohydrates belonging to the group of pectins and gums, somewhat inferior to the sugar of the beets but, according to recent investigation, fully as valuable as the digestible matter of mangels. The wet beet pulp is too heavy to bear long trans- portation, but may be preserved in the neighborhood of the factory by ensiling. It is now, however, dried and put on the market as dried beet pulp, containing not more than 5 to 10 per cent of water. The dried pulp is relatively about equally valuable with the wet pulp, especially if soaked in water, as it should be before feeding. Molasses. — In the further manufacture of sugar either from sugar beets or sugar cane, there remains, as a final residue, the molasses. This contains 20 to 25 per cent of water, approxi- mately 50 per cent of sugar, scarcely more than one-half per cent of true protein, and 8 to 10 per cent of non-protein, along with other substances of doubtful nutritive value. It is essen- tially a source of easily soluble carbohydrates, principally sugar. Beet molasses, in particular, has a marked laxative action, commonly ascribed to the potassium salts present in it but perhaps due quite as much to the sugar. For this reason, care is required to accustom animals to it gradually and not to overfeed with it. Its laxative qualities are said to be valuable when used in small amounts for horses in preventing attacks of colic. Molasses feeds. — Owing to its physical properties, molasses is an inconvenient material to handle. To avoid this difficulty, the so-called molasses feeds have been put on the market. These consist of molasses dried down on some suitable material. A large number of concentrated feeding stuffs have been used 5QO NUTRITION OF FARM ANIMALS for this purpose, and it has also been dried together with the beet pulp, forming molasses pulp. All these feeds are of value in proportion to the materials out of which they are made. 699. By-products of the packing house. — The slaughtering of meat animals on a large scale in the modern packing house yields a number of highly nitrogenous by-products which are of especial value in the feeding of swine and poultry. Dried blood is especially rich in protein, of which it contains over 80 %, practically all of which is digestible. It contains a small amount of fat and but little ash. Tankage consists essentially of the residue left after the rendering of the meat scraps, trimmings and scrap bones of the packing house. Tankage contains much less protein than dried blood but, on the other hand, contains a considerable per- centage of fat, while the bone which it contains renders it rela- tively rich in ash ingredients, especially calcium and phos- phorus. As is obvious from the method of its manufacture, tankage is likely to vary widely in composition and should always be bought on a guarantee. CHAPTER XVI RELATIVE VALUES OF FEEDING STUFFS As soon as live stock husbandry emerged from the pastoral stage and man began to store up forage for the winter or to utilize the products of his cultivated land for feeding his do- mestic animals, the question of the relative values of the dif- ferent feeding stuffs necessarily arose. As agriculure has gradually become more intensive and as the variety of natural materials and of technical by-products available has increased, the question has grown in importance, the traditions of prac- tice based on the experience of earlier investigations have been recognized to be insufficient guides, and much effort has been put forth to replace these traditions by exact knowledge. § i. DIRECT COMPARISONS OF FEEDING STUFFS 700. Hay values. — A natural and logical method of inves- tigation was to feed the materials in question to animals and compare the amount of increase or of milk which was secured. Good meadow hay was universally regarded as a complete feed, suitable for practically all purposes. Hence it was naturally taken as the standard and the effort was made to establish from the results of experience and experiment what amounts of dif- ferent feedstuffs would replace a unit weight of hay. In this way arose the tables of so-called hay values.1 The first of these was that published by Thaer in Germany in 1809, based chiefly on the early chemical analyses of Einhof in which the con- stituents soluble in water, alcohol, dilute acids and dilute alka- lies were determined. The sum of all these ingredients, with- out distinction as to kind, was taken to represent the nutritive value, and the hay values were computed in proportion to them. 1 Compare Henneberg, Uber den Heuwert der Futterstoffe ; Beitrage zu Fiitter- ung der Wiederkauer, Heft i, 1860, pp. 1-16; and von Gohren, Naturgesetze der Fattening, 1872, pp. 286-305. 59 ! 5Q2 NUTRITION OF FARM ANIMALS The system had the advantage of simplicity. Experience had afforded a fairly definite idea of the quantity of hay required for a given amount of production. It was only necessary to compute from the hay values what weights of the available feeding stuffs would produce equal effects. The simplicity of the calculations, due especially to the fact that the relative value of a feed was expressed by a single fixed number, led to a rapid adoption of the system. " To each feeding stuff a defi- nite hay value was assigned and in a short time one had a beautiful table constructed which gave the most exact infor- mation regarding the value of the most diverse feeding materials in comparison with hay. Anything which appeared in any way suited for feeding found its place in the table and each new feeding stuff which the progress of agronomy provided, directly or indirectly, was likewise quickly incorporated. It went so far that even the salt supplied to the animals was computed in hay values." l Thaer himself based his figures in part on the results of prac- tical experiments. Numerous subsequent investigators carried out direct comparisons of feeding stuffs on an extensive scale and not one but several tables of hay values were formulated. Unfortunately, these tables differed widely from each other, some of them giving two or three times as great a hay value as another to the same feed. It was evident also that the un- limited substitution of different classes of feeds, as for instance of grain or roots for hay, was impossible. Such discrepancies and limitations led to various modifications of the methods of estimating the hay values. Boussingault regarded the protein content of the feed as the principal factor, while Nathusius took into account also the content of crude fiber and Wolff z worked out a somewhat elaborate method in an attempt to retain the convenience of reckoning with a single number for a feed. The impossibility of this, however, gradually came to be recognized, and the hay values have now only a historical interest. 701. Practical feeding trials. — But while the system of hay values has become obsolete the idea of determining the relative nutritive values of feeding stuffs on the basis of direct comparisons of the results obtained in practice has survived in 1 Settegast, Die Fiitterungslehre, 1879, p. 4. 2 Die landwirtschaftliche Fiitterungslehre, 1861, pp. 455-456. RELATIVE VALUES OF FEEDING STUFFS 593 full vigor. A very considerable share of the investigations in stock feeding during the last two decades, especially perhaps in the United States, has consisted of experiments intended to determine the effects of the substitution of one feed for another in a ration. Undoubtedly the so-called practical trial has an important part to play in the development of a sound theory of feeding as well as in relation to the economic aspects of the subject. Re- garded, however, simply as a means for the quantitative deter- mination of the relative values of feeding stuffs it is subject to precisely the same limitations and uncertainties as the old at- tempt to determine hay values, and in this respect has in general led to scarcely more satisfactory or concordant results. It is as true in the later as in the earlier experiments that the effect of a feeding stuff may vary widely with the com- bination in which it is fed and the conditions under which it is used. 702. Feed units. — An interesting attempt to revive the fundamental conception of hay values in a modified form and within a restricted field, and thus to retain the advantage of expressing the relative value of a feed by a single number, is found in the so-called feed unit system devised by Fjord and his associates in Denmark and extensively used also in Sweden.1 The feed unit system, like that of hay values, is essentially a system of empirical equivalents according to which feeding stuffs may replace each other. Instead of hay, the basis of comparison is a unit weight of grain (corn, barley, wheat or rye or a mixture of grains). This is called a feed unit and the amounts of other feeds required to equal the feed unit have been determined in very extensive cooperative feeding experi- ments by the group system (572) with swine and especially with dairy cows. The experiments themselves have been executed with every precaution to ensure accuracy. The results for dairy cows, as revised by Woll for American feeding stuffs, and the Danish values for swine and for the horse are given by Henry and Morrison 2 as follows : — 1 For a more complete discussion of the feed unit system compare Woll ; Wis- consin Expt. Sta., Circular No. 37. 2 Feeds and Feeding, isth Edition, p. 127. 2Q 594 NUTRITION OF FARM ANIMALS TABLE 167. — AMOUNT OF DIFFERENT FEEDS REQUIRED TO EQUAL ONE FEED UNIT1 FEED FEED REQUIRED TO EQUAL i UNIT Average Range FOR DAIRY COWS Concentrates Corn, wheat, rye, barley, hominy feed, dried brewers' grains, wheat middlings, oat shorts, peas, molasses beet pulp, dry matter in roots . Cottonseed meal Oil meal, dried distillers' grains, gluten feed, soy- beans Wheat bran, oats, dried beet pulp, barley feed, malt sprouts Alfalfa meal, alfalfa molasses feeds Hay, and straw Alfalfa hay, clover hay Mixed hay, oat hay, oat and pea hay, barley and pea hay, red-top hay Timothy hay, prairie hay, sorghum hay . . . Corn stover, stalks or fodder, marsh hay, cut straw Soiling crops, silage and other succulent feeds Green alfalfa Green corn, sorghum, clover, peas and oats, can- nery refuse Alfalfa silage Corn silage, pea vine silage Wet brewers' grains Potatoes, skim milk, buttermilk . . . . . . Sugar beets Carrots Rutabagas Field beets, green rape Sugar beet leaves and tops, whey Turnips, mangels, fresh beet pulp The value of pasture is generally placed at 8 to 10 units per day, on the average, varying with kind and condition . .... i.o 0.8 0.9 i.i 1.2 2.0 2-5 3-o 4.0 7.0 8.0 S-o 6.0 4.0 6.0 7.0 8.0 9.0 IO.O 12. 0 12-5 2.0- 3.0 2-5- 3-5 3-5- 6.0 6.0- 8.0 7.0-10.0 5.0- 7.0 8.0-10.0 10.0-15.0 1 The values for pigs and horses are those given in the Danish valuation table and those for dairy cows the values as revised by Woll for American feeding stuffs in Wisconsin Circular, No. 37. RELATIVE VALUES OF FEEDING STUFFS 595 TABLE 167. — AMOUNT OF DIFFERENT FEEDS REQUIRED TO EQUAL ONE FEED UNIT (Continued) FEED FEED RE EQUAL QUIRED TO i UNIT Average Range FOR PIGS Indian corn, barley, wheat, oil cakes .... Rye wheat bran I.O I 4. — Boiled potatoes 4.O Skim milk 6 0 Whey ... . ... 12 O FOR HORSES One pound of Indian corn equals one pound of oats or one pound of dry matter in roots . . 703. Logical basis of feed unit system. — The Scandinavian feed unit values have a broad experimental basis. The re- sults of the experiments have been reasonably consistent and in general the feed unit values correspond well with the relative net energy values discussed in the following chapter except that they ascribe somewhat higher values to protein- rich feeds. Nevertheless, the logical basis of the system has the same defect that is inherent in all such systems. As was shown in Chapter V (263), feed has two distinct functions and these func- tions are incommensurable. It is as impossible to combine the value of a.feed as a source of protein or other structural material with its value as a source of energy, and to express the result in a single number, as it is to compare the relative values of food and water to a starving man. A protein-rich feed like cotton- seed meal, for example, will necessarily produce a greater effect when added to a ration deficient in protein than when added to one containing an abundance of that ingredient; with a material deficient in protein precisely the reverse would be true. As a matter of fact the feed units are only claimed to be equiva- lent values, " under ordinary conditions of feeding these animals, when fed in mixed rations that would contain over a certain 596 NUTRITION OF FARM ANIMALS minimum of digestible protein." 1 As Henry and Morrison have pointed out, " The feed unit system has been evolved in a com- paratively small region where similar crops are grown on the different farms and the price of purchased feeds does not vary widely throughout the district." 704. Comparison of feed units and net energy values. — The writer is not able to agree with those who would introduce the feed unit system in this country with its wide variety of feeding stuffs and conditions. The applicability of the feed units, as just pointed out, is conditioned upon the presence of sufficient protein in the rations. As thus limited, however, they practically attempt to measure the relative values as sources of energy, and for this purpose the use of the net energy values to be considered in the next chapter is just as simple arithmeti- cally and equally accurate, while it has two immense advantages. First, the net energy values are rational and not empirical values. They are based on physiological investigations and their very imperfections tend to stimulate further investigation which may lead to their great improvement or to the discovery of new and still better methods of comparison. The feed unit, on the other hand, constitutes a dead end so far as investigation is concerned, leading to nothing beyond some increase in numerical accuracy, while it is far inferior in pedagogic value. Second, the feed units are purely relative values, based on direct comparisons of the results with different materials with no attempt to discover the causes of the observed differences. They show to what extent one feeding stuff is better or worse than others, but es- tablish no relation between feed and product. Energy values, on the other hand, aim to show the amount of product which may be expected from a unit weight of the feeding stuff -r- i.e., the amount of energy which it can contribute to the maintenance of the body or to the building up of new tissue. Thus, if aver- age maize meal, for example, has an energy value of 85 Therms per hundred pounds, this means that one hundred pounds of it, fed as part of a maintenance ration, would conserve in the body of the animal an amount of fat and protein having an energy value of 85 Therms, which would otherwise be burned up to support the vital activities. Furthermore, it means that, if added to the maintenance ration, the maize will furnish ma- 1 Woll, loc. cit. p. 13. RELATIVE VALUES OF FEEDING STUFFS 597 terial sufficient to produce a quantity of milk or of meat having an energy value of 85 Therms. Still further, the investigations by which these facts are established also show that out of the approximately 187 Therms gross energy of 100 pounds of maize meal, about 50 escape unused in the various excreta, while about 52 are expended in the various processes connected with the consumption and assimilation of the feed. In other words, they show the nature of the losses suffered as well as the final amount of product to be expected. Such data as these have an inde- pendent value and are of an entirely different nature from those expressed in the feed units. § 2. RELATIVE VALUES BASED ON COMPOSITION AND DIGESTIBILITY 705. Chemical composition. — Even before the rise of the system of hay values, attempts were made by Davy, Einhof, Sprengel and others to compare feeding stuffs on the basis of chemical analyses, and indeed the earlier hay values were based in part on such comparisons (700). The methods for the chemical analysis of feeding stuffs were gradually improved, although they still remain quite imperfect, but along with this improvement came a clearer recognition of the fact that the problem of relative values is at bottom a physiological and not a chemical question. 706. Physiological functions of nutrients. — In particular the teachings of Liebig and the investigations of Bischoff and Voit l on the nutrition of carnivora served to establish those basal facts regarding the functions of proteins, carbohydrates, fats and ash in nutrition which have been confirmed and ex- tended by later inve*stigations and have been outlined in Chap- ter V. Haubner appears to have been the earliest to suggest the application of these principles to comparisons of feeding stuffs and the feeding of farm animals, while to Grouven 2 belongs the credit of having first formulated the requirements of animals and the values of feeding stuffs in terms of the different classes of nutrients. His tables, however, were based on the total nutrients found by chemical analysis and were comparatively 1 Gesetze der Ernahrung des Fleischfressers, 1860. 2 Vortrage uber Agriculturchemie, 1858. 598 NUTRITION OF FARM ANIMALS soon replaced by more accurate data based on determinations of the digestible nutrients. 707. Henneberg's and Stohmann's investigations. — It is to the fundamental investigations of Henneberg and Stohmann 1 Sit the Weende Experiment Station, near Gottingen, that we are indebted for the inauguration of a system of comparing the values of feeding stuffs which has endured with little material change up to the present time. These investigators were the first to apply systematically in studying the nutrition of herbiv- ora the physiological principles already demonstrated for other classes of animals and to base their determinations upon the outgo as well as upon the income of the body. Their earlier experiments deal chiefly with the digestibility of feeding stuffs and rations. Later a comprehensive scheme of investigation, including determinations of the gaseous excreta, was laid out 2 and begun but never completed. TABLE 168. — EXAMPLE OF COMPUTATION OF DIGESTIBLE NUTRIENTS CLOVER HAY MAIZE MEAL Chemical composition Water Ash . . . iS-03 5-49 13-73 1.25 Protein 10.24 8.80 Non-protein Crude fiber Nitrogen-free extract Ether extract 1.36 28.61 36.98 2.29 0.25 1.89 70.44 3-64 Percentage digestibility Ash Protein Non-protein .... ... Crude fiber IOO.OO 46.48% 53.19% 100.00% 50.27% 0 IOO.OO 18.40% 66.43% 100.00% 32.40% Nitrogen-free extract . ... Ether extract 68.94% 65.02% 97-75% 95-74% Digestible nutrients Ash . . 5.49 X 0.4648 = 2.55% 1.25 X 0.1840 = 0.23% Protein 10.24 X 0.5319 = 5.45% 8.80 X 0.6643 = 5.85% i 36 X i ooo = 1.36% 0.25 X i. ooo = 0.25% Crude fiber 28 61 X 0.5027 = 14.38% 1.89 X 0.3240 = 0.61% Nitrogen-free extract Ether extract 36.98 X 0.6894 = 25.49% 2.29 X 0.6502 — 1.49% 70.44 X 0.9775 = 68.85% 3.64 X 0.9574 = 3-48% 1 Beitrage zur Begrundung einer rationellen Fiitterung der Wiederkauer, 1860 and 1864. J Neue Beitrage, etc., 1870. RELATIVE VALUES OF FEEDING STUFFS 599 708. The digestible nutrients. — The methods of digestion experiments as used by Henneberg and Stohmann and modified by later experimenters were outlined in Chapter III (157-161). A vast number of determinations of digestibility have been made, upon a great variety of materials, and the results have served as the basis for computing the relative values of feeding stuffs. The method of comparison may be illustrated by means of the digestion experiment on clover hay and maize meal used in Chapter III to illustrate the method. (Table 168.) Simplified statement. — Since the digestible crude fiber and digestible nitrogen-free extract have been shown (168, 169) to have the elementary composition of starch, they have been commonly added together and called carbohydrates. Con- sidering the digestible ether extract to be substantially fat, and omitting the ash on the assumption that an average ration contains a sufficient supply, the amounts of the three principal groups of digestible nutrients may be stated more concisely as follows : — TABLE 169. — SIMPLIFIED STATEMENT OF DIGESTIBLE NUTRIENTS CLOVER HAY MAIZE MEAL Digestible protein r 4r% r.8c% Digestible non-protein I l6 % O 2Z °7n Digestible carbohydrates 30 87% 60 4.6% Digestible fats 1.4.0 % 3.48% This statement may be still further simplified. A pound of fat produces when burned about 2.25 times as much heat as the same weight of carbohydrates. The non-proteins have ap- proximately the same heat value as the carbohydrates, while it is still questioned whether they help to build up protein tis- sue. By multiplying the digestible fat by the factor 2.25 and adding the digestible carbohydrates and non-protein we obtain the carbohydrate equivalent for the digestible matter other than protein and the digestible nutrients may be expressed in the following still more concise form : — 600 NUTRITION OF FARM ANIMALS TABLE 170. — DIGESTIBLE NUTRIENTS REDUCED TO CARBOHYDRATE EQUIVALENT CLOVER HAY MAIZE MEAL Digestible protein r AC% * 8be safely assumed that the average for cattle is substan- tially applicable to these species also. In the horse the principal seat of the methane fermentation is the colon and ccecum (128). Since the more soluble carbo- hydrates of the feed are largely or entirely digested before reach- ing these organs, methane is much less copiously produced than in the case of ruminants and may be regarded as derived chiefly from the fermentation of crude fiber. In respiration experiments on mixed rations of oats, hay and straw, Lehmann, Zuntz and Hagemann 3 observed as the result of eight rather discordant experiments an average total excretion of methane of 4.73 grams per 100 grams di- gested crude fiber and in addition an average excretion of 0.203 gram of hydrogen per 100 grams digested crude fiber. In more recent experiments, Von der Heide, Steuber and Zuntz,4 using a Regnault-Reiset respiration apparatus (298), obtained for the methane excretion per 100 grams digested crude fiber 9.06 grams on hay and 2.28 grams on straw pulp. Using the average of these rather discordant experiments, the fer- mentation losses in the case of the horse may be approximately computed from the amount of crude fiber digested. Swine with their simpler alimentary canal suffer but small losses from fermentation in the digestive tract. Fingerling, Kohler and Reinhardt 5 found the amounts of combustible gases excreted too small to be determined with their form of Pettenkofer apparatus. Von der Heide and Klein 6 in three experiments with a Regnault-Reiset apparatus obtained the following results : — 1 Ernahrung landw. Nutztiere, 6th Ed., p. 94. 2 Jour. Agr'l Research, 3 (1915), p. 450. 3 Landw. Jahrb., 23 (1894), 125. 4 Biochem. Ztschr., 73 (1916), 161. 5 Landw. Vers. Stat, 84 (1914), 197. 6Biochem. Ztschr., 55 (1913), 195. THE PRODUCTION VALUES OF FEEDING STUFFS 639 TABLE 185. — EXCRETED BY SWINE PER 100 GRAMS DIGESTED CARBOHYDRATES METHANE GRAMS HYDROGEN GRAMS Period I 0.62 O.I I Period II 06^ O O7 Period III 0.68 o 04 Average o 6^ O O7 Although there is considerable range in the results of in- dividual experiments, and while those on non-ruminants are few in number, nevertheless, the foregoing figures afford a basis for an approximate estimate of the losses of chemical energy in the combustible gases. Summarizing the available data and computing the equivalent quantities of energy, it appears that the following average deductions may be made from the gross energy of the feed for the fermentation losses. TABLE 186. — FACTORS FOR COMPUTING FERMENTATION LOSSES EQUIVA- WEIGHT LENT ENERGY Per 100 grams digested carbohydrates Grams Cals. Ruminants — IMethane . .... 4-5 60. 1 Swine — JVIethane 0.65 8.7 Hydrogen 0.07 2.4 Total 0.72 II. I Per 100 grams digested crude fiber Horse — IVTethane 4.7 62.7 Hydrogen O.2 7.0 Total 4-9 69.7 Metabolizable energy 746. Definition. — The difference between the chemical energy of the feed and that lost in the excreta shows how much of the former is capable of being converted into other forms in the body, either during the changes which the feed undergoes in 640 NUTRITION OF FARM ANIMALS the digestive tract or in the course of metabolism in the tissues. As stated in Chapter VI (323), this convertible portion of the feed energy has been given various names by different investi- gators, such as " physiological heat value," " fuel value," " avail- able energy," etc., but following a suggestion made earlier by the writer it is here designated as " metabolizable energy." 747. Method of , determining. — • As is apparent from the foregoing paragraphs, the direct determination of the metab- olizable energy of a feeding stuff or ration requires the meas- urement of the amounts and heats of combustion of the feed and of the solid, liquid and gaseous excreta by the methods outlined in Chapter VI. These quantities being known, a simple subtraction gives the metabolizable energy. Thus the results of the experiment used as an illustration in Chapter VI (322), put in a somewhat more detailed form, were as follows : — TABLE 187. — EXAMPLE OF DETERMINATION OF METABOLIZABLE ENERGY HEAT OF FRESH WEIGHT DRY MATTER COMBUSTION or DRY MATTER PER ENERGY OF FEED ENERGY OF EXCRETA GRAM Grams Grams Cals. Cals. Cals. Daily feed Timothy hay .... 6,988 6,086 4,556 27,727 — Linseed meal .... 400 354 5,111 1,811 — Daily excreta Feces ....... 16,619 2,948 4,831 — 14,243 Urine 4-2^7 O 23O1 I,2IO Methane ,00 / 142 142 w, •6OV"' 13,344 — I,896 Metabolizable energy By difference .... — — — — I2,l89 29,538 29,538 748. Correction for gain or loss of protein. — In the foregoing experiment the animal gained 15.2 grams of fat and 66.6 grams of protein and therefore stored up in its body equivalent amounts of energy, viz., In protein, 5.7 Cals. X 66.6 = 380 Cals. In fat, 9.5 Cals. X 15.2 = 144 Cals. 1 Per gram fresh urine. THE PRODUCTION VALUES OF FEEDING STUFFS 641 The 144 Cals. of energy contained in the fat, however, although not actually transformed into other forms of energy, were capable of such transformation had the demands of the organism required it, and therefore constitute part of the metabolizable energy of the feed. With the 380 Cals. contained in the protein stored up, however, the case is different. Had these 66.6 grams been katabolized, part of their energy would have escaped in the resulting nitrogeneous meta- bolic products. According to Rubner each gram of urinary nitrogen derived from lean meat is equivalent to 7.45 Cals. of chemical energy. The katabolism of the 66.6 grams of protein, therefore, would have in- creased the chemical energy of the urine by 83 Cals., while only 297 Cals. would have been transformed. This amount of 83 Cals. must consequently be added as a correction to the urinary energy as measured in computing the metabolizable energy. In case of a loss of protein from the body a similar correction must evidently be subtracted. When a respiration apparatus for the determination of the combustible gases is not available, their amount may be esti- mated from the digestible carbohydrates in the manner al- ready outlined (745) , so that it is possible to estimate the metab- olizable energy with a considerable degree of accuracy from the results of an ordinary digestion experiment to which has been added the collection of the urine and determinations of the heats of combustion of the visible excreta. The additional labor thus required is so small that it is to be hoped that in future digestion experiments it may be undertaken whenever possible and that in this way more extensive data may be secured re- garding the metabolizable energy of feeding stuffs. While such results do not show the production values of the rations (750), they constitute an important step toward their more exact determination. 749. Experimental Results. — There are on record a some- what limited number of experiments with cattle and a few with swine in which the losses of energy in the feces, urine and methane respectively have been determined directly, while in a considerably larger number the losses of methane have been estimated from the digestible carbohydrates (crude fiber plus nitrogen-free extract) in the manner just described. The re- sults of these experiments are recorded in Table iSS,1 which shows the percentages of the gross energy which were carried 1 This table is not claimed to be an exhaustive compilation of data, but is be- lieved to be fairly complete. 2 T 642 NUTRITION OF FARM ANIMALS off in the several excreta and, by difference, the percentages which were metabolizable. The metabolizable energy per gram of digestible organic matter is also added, since, as will appear subsequently, it forms a convenient basis for the com- putation of metabolizable energy when direct determinations of it are not available. TABLE 188. — APPARENT METABOLIZABLE ENERGY PERC ENTAGE Lc SSES w %8 £ W r£ AUTHOR w . In Feces In Urine In Meth- ane f 15 CATTLE Roughages % % % % Cals. Meadow hay Kellner 40.96 5-71 6.77 46.56 3-Soi Meadow hay Tangl, et al. 44.6 5-5 6.8i 43-1 3-437 Timothy hay Armsby and Fries 46.4 , 8 7-3 42.5 Red clover hay .... Armsby and Fries 41.9 O-° 6.8 6.5 44-8 3^486 Mixed timothy and red clover hay Armsby and Fries 43-9 5-2 7-4 43-5 3-390 Alfalfa hay Armsby and Fries 44.1 5-8 6.2 43-9 3-605 Hay from irrigated meadows Tangl, el al. 47-5 3-o 6.61 42-9 3.600 Ensiled hay Tangl, et al. 62.5 o.4(?) 4.91 32-2 3-698 Oat straw Kellner 56.8 2.1 5-3 35-8 3-740 Wheat straw Kellner 58.2 2.4 8-3 3I-I 3-310 Straw pulp Kellner 12.8 -0.8 12.5 75-5 3-640 Maize stover Armsby and Fries 42.8 4.2 7-9 45-1 3-450 Average 3-529 Concentrates Maize meal Armsby and Fries 13-3 S-i IO.O 71.6 3-797 Wheat bran Armsby and Fries 31-8 5-4 7-4 55-4 3-954 Hominy chop Armsby and Fries 12.2 3-8 9-2 74-8 4-075 Mixed grains No. i . . . Armsby and Fries 19.2 7-2 8.2 65.4 3.910 Mixed grains No. 2 ... Armsby and Fries 22.7 4-4 8.1 64.8 3.879 Millet Tangl, et al. 34-6 3-4 7-71 54-3 3-787 Palmnut meal Voltz, et al. 19-3 - 2.02 6.91 75-8 4-849 Distillers' slop (from potatoes) Voltz, et al. 61.1 4.5 2 5-41 29.0 2.703 Beet molasses Voltz, et al. — 46.5 -3-02 I3-71 135-8 5-361 Beet molasses Kellner 9-9 2-9 "•3 75-9 3-473 Distillers' residue from grapes + beet molasses . Tangl, et al. 59-1 3-4 3-8' 33-7 4-519 Pumpkins Tangl, et al. 20.1 2.8 6.91 70.2 4-287 Starch . . Kellner 17.6 — 0.7 9-2 73-9 3 '603 Wheat gluten Kellner 20. 2 I3-I O.I 66.6 4.792 Average 4.078 1 Estimated. 2 Not corrected to N. equilibrium. THE PRODUCTION VALUES OF FEEDING STUFFS 643 TABLE 1 88 — APPARENT METABOLIZABLE ENERGY (Continued) - PERCI ,NTAGE LOS SES . $• I |P AUTHOR ap In Feces In Urine In Meth- ane o « SHEEP Roughages % % % % Cals. Meadow hay Tangl, et al. 46.6 4.8 6.2 i 42.4 3-559 Meadow hay Voltz, et al. 43-5 4-82 6.3 1 45-4 3.611 Hay from peat meadows . Tangl, et al. 59-4 4.1 4.81 31-6 3-544 Hay from alkali soil . . . Tangl, et al. 52.9 4-1 5-51 37-5 3.601 Hay from same, irrigated . Tangl, et al. 40.4 4.1 n.41 44.1 3.288 Alpine hay Tangl, et al. 39-2 4.1 6.9l 49.8 3-765 Average 35-6i Alfalfa hay Tangl, et al. 32.5 4.1 5-4 1 58.0 4-467 Average 3.691 Dried potato vines . . . Voltz, et al. 43-2 3-5 2 5-3 l 48.0 4. 182 Same with fruit .... Hay and dried potato vines Voltz, et al. Voltz, et al. 45-9 41-3 3-82 4.81 6.3l 45-5 47-2 4-319 •2 62O Hay and ensiled potato £.\JHJ vines Voltz, el al. 42.2 S-92 5-9 * 46.0 g Wheat straw Voltz, et al. 74-9 4-1 2 4.6! 16.4 2.378 Average 3.655 Concentrates Oats Tangl, et al. 35-5 4.1 6.61 53-8 3-973 Millet Tangl, et al. 30.2 7.1 8.2 i 54-5 3.405 Corn-and-cob meal . . . Tangl, et al. 31-0 2-9 8.2! 57-9 , 8*6 Palmnut meal Voltz, et al. 35-o 4-5 2 6.3 i 54-2 O-°OL' 3-977 Lentils . Voltz, et al. 7.O 12.92 8.7! 7I.J. Distillery slop from pota- 4.079 toes Voltz, et al. 23-2 7.62 6.3! 62.9 4.383 Beet molasses Voltz, et al. 18.6 17-3 7-9 56.29 3-124 Average 3-825 HORSES Roughages Meadow hay Tangl, et al. 55-1 3-6 1.9! 39-4 3.707 Hay from peat meadow Tangl, et al. 66.1 3.7 o.8i 29.4 3.854 Hay from alkali soil . . . Tangl, et al. 59-3 3-7 i.6i 45-4 3-803 Hay from same, irrigated . Tangl, et al. 50.4 3-7 2.0 ! 43- 3-741 Alpine hay Tangl, et al. 50.1 3-7 i.6i 44.6 3.915 Sour meadow hay . . . Tangl, et al. 66.4 3-7 I-51 28.4 3.607 Silage from same .... Tangl, et al. 70.0 3-7 i.6l 24-7 3-352 Average 3-712 Concentrates Oats Tangl, et al. 41.4 3.7 O.2 1 54-7 4.493 Distillery residue from grapes and beet molasses Tangl, et al. 66.9 1.4 o.8i 30.9 4-76i Average 4.627 Estimated. 2 Not corrected to N. equilibrium. 644 NUTRITION OF FARM ANIMALS TABLE 1 88 — APPARENT METABOLIZABLE ENERGY (Continued) PERCE NTAGE LOS SES W wO PH & AUTHOR In Feces In Urine In Meth- ane li CJ O o w Oil W X METABOLIZABLE GRAM DIGESTIBLI GANIC MATTI HORSES Mixed Rations Oats, hay and straw Oats, hay and straw . . Oats, hay and straw . . Oats, hay and straw . . Oats, hay and straw . . Oats, hay and straw . . Average . Zuntz and Hagemann Zuntz and Hagemann Zuntz and Hagemann Zuntz and Hagemann Zuntz and Hagemann Zuntz and Hagemann % % % % % 4-474 3-236 3-403 3-803 3-980 5-052 3 991 SWINE Concentrates Millet Tangl, et al. 28.8 3-4 0.2 1 67.2 4-335 Pumpkins Barley and a little flesh meal Flesh meal Wheat gluten Tangl, et al. Fingerling, et al. Fingerling, et al. Fingerling, et al. 25-6 16.3 6.9 7-4 3-9 3-3 8.9 10.9 I.41 69.1 80.4 84.2 81 7 3-460 4-521 5-629 4 908 Starch Straw pulp Sugar Fingerling, et al. Fingerling, et al. Fingerling, et al. 2.6 14.4 2.7 — 2.0 - 1.8 0.4 - 99-4 87-4 96.9 4.076 3-952 3-75° Peanut oil Barley, dried potatoes and dried yeast Same + palm oil . . . . Same + dried potatoes . . Computed for oil ... Computed for dried pota- toes Fingerling, et al. V. d. Heide and Klein V. d. Heide and Klein V. d. Heide and Klein V. d. Heide and Klein V d Heide and Klein - 0.4 - 0.5 - 100.9 8-997 4-237 4-442 4.160 9-552 Whole milk Skim milk and saccharified starch Skim milk and raw starch . Skim milk and fat ... Averages Flesh meal and wheat gluten Whole milk . . . . Peanut oil .... Other rations . . . GEESE Wellmann Wellmann Wellmann Wellmann Tangl et al 4-7 3-4 3-8 3-5 9.8 n-3 10.5 7-5 g - 85-5 85.3 85-7 89.0 76 i 5467 4.5I9 3-825 5-994 5-269 5-467 8.997 4-055 3 753 Millet DUCKS Millet Tangl, et al. Tangl et al 4: .8 i 6 — 56.2 46 4 2-723 1 Estimated. THE PRODUCTION VALUES OF FEEDING STUFFS 645 750. Significance of metabolizable energy. — By metab- olizable energy, as already explained, is meant simply the energy capable of transformation in the body, with no impli- cation as to the proportion of the energy thus transformed which can be utilized by the organism. The heat evolved during the methane fermentation, for example, constitutes part of the metabolizable energy as thus denned, although it does not enter into the tissue metabolism. The metabolizable energy of a feeding stuff does not meas- ure its production value, since it takes account of only one of the two classes of losses to which its chemical energy is sub- ject. Obviously, however, it is an essential factor in fixing that value, since frequently from one-fourth to one-half or more of the feed energy is thus rejected unused. The determination or estimation of the metabolizable energy of a feeding stuff is, therefore, an important step in ascertaining its production value as regards energy, and constitutes an advance over the simple determination of digestibility, since it takes account of the losses in urine and methane as well as of those in the feces. 751. Real and apparent metabolizable energy. — The metab- olizable energy of a feeding stuff as determined experimentally in the manner illustrated in a preceding paragraph (747) is the aggregate effect as regards energy of all the influences which the feeding stuff exerts on the digestive processes. For example, in one of Kellner's experiments' beet molasses added to a basal ration diminished the amount of energy car- ried off in the methane by 135.8 Cals., while at the same time it so depressed the digestibility of the basal ration that the amounts lost in the feces and urine were increased by 1865.9 Cals. and 272.3 Cals. respectively. By the method of com- putation here used, the algebraic sum of these amounts is vir- tually regarded as representing the losses of energy from the molasses and is subtracted from the gross energy of the latter to obtain its metabolizable energy. The metabolizable energy as thus computed expresses the net increase in the amount of energy available for conversion in the body and may be called the apparent metabolizable energy. On the other hand, the results for the metabolizable energy of the digestible nutrients recorded in the next paragraph in- clude corrections for these secondary effects. They aim to show 646 NUTRITION OF FARM ANIMALS the actual amounts of metabolizable energy supplied by the digested portions of the feed irrespective of its secondary effects — i.e., to express its real metabolizable energy. Such figures give a more accurate idea of the store of metabolizable energy contained in the feeding stuff regarded by itself, while the ap- parent metabolizable energy is better adapted for use in a dis- cussion of questions of feeding.1 The distinction is similar to that already discussed in Chapter III (167) between real and apparent digestibility. 752. Computation of metabolizable energy from digestible nutrients. — While, in the absence of a respiration apparatus, the metabolizable energy of a feeding stuff or ration may be estimated with a fair degree of accuracy by the method out- lined in previous paragraphs, not every experimenter is equipped to determine the heats of combustion of the feed and the visible excreta, and no satisfactory method of computing them is avail- able. Various attempts have accordingly been made to compute the metabolizable energy of feeding stuffs from chemical data. One such method is that employed by Rubner and by At- water for estimating the metabolizable energy of the food of man and of carnivora as described in Chapter VI (324), their factors for protein, carbohydrates and fat being applied directly to the digestible nutrients of feeding stuffs, and several tables of energy values as thus computed have been published. Later investigations, however, showed that the results thus obtained were much too high in the case of herbivorous animals, es- pecially of ruminants. To cite but a single instance, experi- ments on cattle by the writer 2 gave the results shown in Table 189 for metabolizable energy as compared with those computed by the use of Rubner's factors, and Kellner's somewhat earlier results 3 led to the same general conclusion. There are two principal reasons for this discrepancy. The first is the extensive fermentation of the carbohydrates in the digestive tract of ruminants, leading to a relatively larger loss of energy in the combustible gases excreted. The second rea- son is the fact that the urine of herbivora carries off much more non-nitrogenous material (224) than is the case with man or carnivora. The results of direct determinations on swine 1 Compare Armsby, Principles of Animal Nutrition, pp. 291-293 and 333-335. 2 Penna. Expt. Sta., Bui. 71, p. 7. 8 Landw. Vers. Stat., 53 (1904), 440-449. THE PRODUCTION VALUES OF FEEDING STUFFS 647 show much smaller differences between the observed and com- puted results, the fermentation losses in particular being notably less with swine than with cattle or sheep (745). TABLE 189. — COMPARISON OF METABOLIZABLE ENERGY PER POUND COMPUTED BY RUBNER'S FACTORS DIRECTLY DETERMINED Timothy h«iy Calories 875 Calories 777 QOI 742 Muizc meal 1^2^ 1308 Kellner has attempted to secure factors for cattle similar to those of Rubner for men and carnivora by means of experiments in which approximately pure nutrients (starch, sugar, oil, gluten) were added to a basal ration. In the case of starch, for ex- ample, the increase in the amount of nitrogen-free extract di- gested was compared with the increase in the total metaboliz- able energy of the ration, the losses of energy in feces, urine and methane being determined with the aid of a respiration ap- paratus by the method of indirect calorimetry (329). The re- sults are corrected for the effects of the starch upon the digest- ibility of the several nutrients of the basal ration and upon the losses from the latter in urine and methane, i.e., the real metabolizable energy is computed. A few similar de£ermina- tions on other species have also been reported. In an earlier publication 1 the writer has discussed in con- siderable detail the recorded experiments regarding the metab- olizable energy of the nutrients digested by farm animals with the results summarized in the following table. To the extent to which satisfactory factors can be selected, this table may be used to compute the metabolizable energy of feeding stuffs or rations whose digestibility is known, but it should be noted that the results will include no allowance for the secondary effects of the feed on the digestive processes and will prob- ably be higher than the " apparent " metabolizable energy obtained by direct experiment. 1 Principles of Animal Nutrition, pp. 302-335. 648 NUTRITION OF FARM ANIMALS TABLE 190. — METABOLIZABLE ENERGY OF DIGESTIBLE NUTRIENTS PER GRAM Protein (N X 6.25) : From wheat gluten . . ' . . . From wheat gluten (N X 5.7) . . From beet molasses From mixed grain From mixed ration of oats, hay and straw From meadow riay From timothy hay From straw Fat: From peanut oil From hay (ether extract) . . . Carbohydrates : Starch, Kellner's experiments . . Starch, Kiihn's experiments . . Nitrogen-free extract (assumed) . Crude fiber, of straw pulp . . . Crude fiber, of hay fed alone . . Crude fiber, of hay added to basal ration Crude fiber, of oat straw . . . Crude fiber, of wheat straw . . . Crude fiber, of mixed ration . . CATTLE Cals. 4.894 3.984 1.272 8.821 8.322 3.648 3.606 3-3II 3.606 3-437 3.001 HORSE SWINE Cals. j Cals. — — 3.228 4.185 3-523 4.083 753. Computation of metabolizable energy from digestible organic matter. — A more simple and direct method of compu- tation may, however, be employed, based on the total digest- ible organic matter of the ration. As already pointed out, the differences shown in Table 188 between the percentages of the gross energy of different feeding stuffs which are metabo- lizable are due chiefly to differences in the proportion of the chemical energy carried off in the feces, while the losses in urine and methane are far more uniform. Accordingly, the metabo- lizable energy per unit of digestible organic matter necessarily exhibits much smaller variations than that per unit of dry matter, and in fact shows a striking degree of uniformity. Selecting those averages which appear most trustworthy, the results may be summarized as follows : — THE PRODUCTION VALUES OF FEEDING STUFFS 649 TABLE 191. — METABOLIZABLE ENERGY PER KILOGRAM DIGESTIBLE OR- GANIC MATTER NUMBER OF SINGLE TRIALS MAXIMUM MINIMUM MEAN Roughage Cattle . . Sheep Horse 73 33 12 Therms 3-74 3-77 3.Q2 Therms 3-31 3-2Q •2.2C Therms 3-53 3-56 •2.71 Concentrates Cattle 31 4.8< 3-70 4 O4. Sheep Horse 25 8 4.08 A 76 3-41 4 40 3-85 462 Swine 1 36 5-63 3-46 4.40 A similar degree of uniformity appears when the results on mixed rations are compared, as the following summary shows : — TABLE 192. — METABOLIZABLE ENERGY PER KILOGRAM DIGESTIBLE ORGANIC MATTER IN MIXED RATIONS NUMBER OF SINGLE TRIALS MAXIMUM MINIMUM MEAN Therms Therms Therms Cattle Kellner and Kohler .... 38 3-72 3-48 3.60 Armsby and Fries 26 I 89 •j 5 [36.7 29 6 Barley Fingerling et al AC 2 Dried potatoes Flesh meal Mixed rations Rice, flesh meal and whey Cockle, barley and maize Rape cake, barley and maize .... Skim milk and flour . V. d. Heide and Klein Fingerling, et al. Meissl, et al. Kornauth and Arche Kornauth and Arche \Vellmann 49.76 47.90 41.1 24.4 27.9 60 9 Single nutrients Starch . . . Fingerling et al 51 Q-J Cane sugar Straw pulp Wheat gluten Peanut oil Palm oil Fingerling, et al. Fingerling, et al. Fingerling, et al. Fingerling, et al. V d Heide and Klein 47.22 60.56 SI-67 30.35 IOC 02 1 Biochem. Ztschr., 74 (1916), 161. THE PRODUCTION VALUES OF FEEDING STUFFS 657 758. Experiments on the horse. — No experiments on this animal have been reported in which the energy expenditure due to the consumption of a single feeding stuff has been deter- mined. Practically the only data available are those derived from the extensive investigations of Zuntz and Hagemann, the results of which regarding the fasting katabolism have been considered in Chapter VIII (385). On the basis of their ex- periments they compute the energy expenditure and the net energy value from the composition and digestibility of the ration by a method identical in principle with that employed in the experiments on cattle already described. The experi- ments were conducted so differently, however, as to consti- tute practically a distinct method and they may be more conveniently considered in connection with the computation of net energy values discussed in subsequent paragraphs (775-778) . 759. Results on carnivora. — Mention was made in Chapter VIII (365, 366) of the fact that in carnivora, as well as in herbivora and omnivora, the consumption of feed stimulates the heat production, the increase having been called by Rubner the specific dynamic action. It is evident that experiments like those of Rubner and of Lusk were virtually determinations of net energy values for these species. While having no direct bearing on the question of the nutri- tive values of feeding stuffs for farm animals, these data have been extensively quoted in related physiological writings and it seems de- sirable to include them here. Rubner's later experiments were made at about 33° C., or considerably above the critical temperature for the dog, a fact which is of importance in the interpretation of the results (395-397). A balance experiment with a respiration calorimeter in which nearly enough fat was fed to supply the requirement for energy gave TABLE 199. — INCREMENT OF HEAT PRODUCTION BY DOG ON FAT DIET METABOLIZABLE ENERGY OF FEED GAIN BY BODY HEAT PRODUC- TION Fat fed Cals. C-2 A Cals. — 7 r Cals. 60 9 Fasting Q — o 02 ^ O O 27 6? Rye Q2 Q O 7 O 'J 17 ^Q Wheat 91 6 7 22 U«O APPENDIX v 717 TABLE VII. — VALUES PER 100 POUNDS FOR RUMINANTS (Continued) DRY MATTER DIGESTIBLE NET ENERGY VALUE Crude Protein True Protein FRESH GREEN ROUGHAGE Green cereals, etc. Barley fodder Pounds 23.2 23-8 36.4 43-6 36.6 8.9 14.1 23.1 14.9 19.9 25.1 26.2 34-8 20.7 10.6 15.0 21. 0 27.9 10.0 20.3 21.5 27.6 26.1 29.2 16.7 21.3 24.9 24.2 32.1 46.4 27.4 19.9 25-9 29.8 24.3 17.4 Pounds 2-3 3-7 2.8 1.9 2.2 1.9 i-7 I.O i.i I.O 1-3 i.i i-5 I.O 0.9 0.9 I.O 1.2 0.8 1.2 I.O 1.9 2-3 i-7 2.6 2.1 0.7 1.8 i-3 i-5 2.8 3-5 3-3 2.1 2-7 2-3 Pounds 2.O 2.8 2.2 1.6 1-5 i-3 i.i 0.8 0.8 0.8 I.O 0.8 i.i 0.8 0.7 0.7 0.8 0.9 0.6 0.9 0.8 i.i 2.0 X.I 1-7 1.4 0.4 I.I 0.8 I.O 1.9 1.9 1.8 i-3 i-S 1.6 Therms 14.08 14.82 17.77 2I.OI 17.78 8.87 7-05 14.60 9-52 13.64 17-35 16.74 22.48 13-53 6.89 10.39 13-49 17.84 7.82 13.38 14.26 17.24 14.06 IS.Sl 13.07 15-99 15-37 18.36 18.89 26.36 18.75 9-2O 11.50 II. IO 14.56 10.83 Blue grass, Kentucky, before heading . . . Blue grass, Kentucky, headed out .... Blue grass, Kentucky, after bloom .... Buckwheat Japanese . ... Cabbage Cabbage waste outer leaves Corn (maize) fodder, dent, all analyses . . Corn (maize) fodder, dent, in tassel . . . Corn (maize) fodder, dent, in milk .... Corn (maize) fodder, dent, dough to glazing . Corn (maize) fodder, dent, kernels glazed Corn (maize) fodder, dent, kernels ripe . . Corn (maize) fodder, flint, all analyses . . Corn (maize) fodder, flint, in tassel . . . Corn (maize) fodder, flint, in milk .... Corn (maize) fodder, flint, kernels glazed Corn (maize) fodder, flint, kernels ripe . . Corn (maize) fodder, sweet, before milk stage Corn (maize) fodder, sweet, roasting ears or later ... Corn (maize) fodder, sweet, ears removed IVlillet Hungarian Oat fodder Orchard grass Rape . . . ... . . . . Rye fodder Sweet sorghum fodder Timothy, before bloom Timothy, in bloom Timothy, in seed Wheat fodder Green legumes Alfalfa before bloom Alfalfa in bloom Alfalfa after bloom Clover alsike Clover crimson .... 7l8 APPENDIX TABLE VII. — VALUES PER 100 POUNDS FOR RUMINANTS (Continued) Thjv DICES >TIBLE NET MATTER Crude Protein True Protein ENERGV VALUE FRESH GREEN ROUGHAGE Green legumes Clover, red, all analyses Clover red in bloom ... Pounds 26.2 27 ? Pounds 2-7 2.7 Pounds 1-7 1.8 Therms 15.87 16.74 Clover, red, rowen Cowpeas . 34-4 16.3 3-3 2.3 2.2 1.7 17.30 10.42 Peas, Canada field Soybeans, all analyses Soybeans, in bloom Soybeans, in seed . 16.6 23-6 20.8 24. 2 2-9 3-2 3-o 3.1 2.1 2.4 2-3 2.5 9.78 12-53 10.44 12.70 Vetch, hairy SILAGE Corn (maize), well-matured, recent analyses . Corn (maize) immature ... . 18.1 26.3 21 O 3-5 i.i I.O 2.4 0.6 0.4 11-95 15.90 11.96 Corn (maize), from frosted ears Corn (maize), from field-cured stover . . . Clover 25-3 19.6 27.8 1.2 0-5 I. a 0.6 0-3 0.8 14.27 8.98 7.26 Cowpeas 22 O i 8 i.i H.O5 Soybeans . 27 I 2.6 1.5 11.59 Sugar beet pulp IO.O 0.8 0.5 9.32 ROOTS, TUBERS, AND FRUITS Apples 18 2 O.4 O.I 15.92 Beets, common I^.O O.O O.I 7.84 Beets, sugar 16.4 1.2 0.4 11.20 Carrots • II. 7 O.O 0.5 9.21 Mangels 0.4 0.8 O.I 5.68 21.2 i.i O.I 18.27 Potato flakes 87.0 3.6 0.4 72.68 Potato flour 8o.4 1.4 O.I 80.09 Pumpkins field 8.3 i.i 0.6 6.O5 Rutabagas IO.O I.O 0.3 8.46 Turnips Q.C I.O 0.4 6.16 GRAINS Cereal grains Barley ........ 00.7 9.0 8-3 89.94 B uck wheat 87.0 8.1 7.2 59-73 Corn (maize) dent 80.5 7-5 7.0 85.50 Corn (maize), flint Corn (maize) and cob meal .... 87.8 89.6 7-7 6.1 7-2 5-7 84.00 75.80 APPENDIX 719 TABLE VII. — VALUES PER 100 POUNDS FOR RUMINANTS (Continued) DRY MATTER DIGESTIBLE NET ENERGY VALUE Crude Protein True Protein GRAINS Cereal grains Corn (maize) meal Oats Pounds 88.7 90.8 92.1 90.4 90.6 87-3 89.8 89.1 89.9 86.6 88.4 90.8 89.1 93-5 94.0 90.1 90.6 90.8 95-5 93-i 9.4 13-6 9.9 9.6 91.7 6.6 92.5 91.8 24.1 93-4 92.8 Pounds 6.9 9-7 12.8 4-7 9-9 7-5 9.2 8-7 9.2 18.8 19.4 19.0 19.8 19.4 24.1 30.7 13-3 2O.6 23-3 13-5 3-4 3-3 3-6 3-i 34-4 0.8 21.5 18.7 4.6 22.4 13-6 Pounds 6.4 8-7 n-5 4-5 9.0 6-7 8.1 7-7 8.1 16.4 16.9 16.6 17.2 16.9 22.2 27-3 II.9 I9.2 20. 2 11.7 3-4 3-3 3-6 3-1 34-4 0.8 20. 2 17-5 4-4 18.3 ii. i Therms 85.20 67.56 86.20 77-33 93-71 89-7S 91.82 91.66 91.41 73-29 79-46 78.72 77.62 83-15 109.04 81.29 78.33 83-17 95-77 92.49 I3-32 29.01 i4-3i 15-43 103.91 10.39 53.38 50.93 14-53 85.08 56.01 Oatmeal Rice rough Rve Sorghum grain W^heat all analyses \Vheat winter Wheat, spring Leguminous seeds Beans, navy Cowpeas Peas field . Pea meal Peanuts with hull Peanut kernel Oil seeds Cottonseed. Flaxseed Sunflower seed . Sunflower seed with hulls DAIRY PRODUCTS Buttermilk . ... Cow's milk Skim milk — centrifugal . . . Skim milk — gravity Skim milk — dried • Whey BY-PRODUCTS Fermentation industries Brewers' grains, dried Brewers' grains, dried, below 25 per cent protein Brewers' grains wet Distillers' grains, dried, from corn .... Distillers' grains, dried, from rye .... 720 APPENDIX TABLE VII. — VALUES PER 100 POUNDS FOR RUMINANTS (Continued] DRY MATTER DIGESTIBLE NET ENERGY VALUE Crude Protein True Protein BY-PRODUCTS Fermentation industries Distillers' grains, wet Pounds 22.6 94-2 92.4 88.8 89.7 88.0 89.9 88.9 89.9 QO-5 90.0 88.6 89.9 89.3 89.6 90.4 92.3 90-3 92.5 92.2 91.1 90.4 90.9 89.6 89.3 94.4 88.2 90.0 9i-3 90.9 90.7 33-4 74-7 74.2 Pounds 3-3 15-8 20.3 10.5 0.4 24.6 7.0 14.8 7-9 7-3 8.0 12.2 12-5 15-7 13-4 18.8 18.4 o-3 37-0 33-4 16.5 31-7 30.2 12.4 42.8 20. 2 38.1 32.0 21.6 30.2 II. 2 4.1 I.I I.O Pounds 2.8 n.8 12.5 9.1 ? 20.8 6.5 13.2 7.0 6.4 * 7-i 10.5 10.8 14.0 I2.O I8.3 18.0 ? 35-4 32.0 14-3 30-9 28.5 12.0 41.4 I9.S 37-3 29.1 20.1 28.1 9.2 3-7 o.o o.o Therms 22.05 87.82 72.72 30-59 -7.69 72.19 88.78 78.80 45-29 65.24 77-7° 79-35 53-00 75-02 59.10 83-49 100.31 9.92 93-46 90.00 83.88 85.12 88.91 94.18 93-55 42.57 99.65 88.87 80.72 84-iS 77.46 30.45 57-10 55-38 Malt Malt sprouts Milling Buckwheat bran . Buckwheat hulls Buckwheat middlings Hominy feed Red dog flour Rice bran, high grade Rice meal ... Rice polish Rye bran Wheat bran ... ... Wheat middlings, flour \Vheat middlings standard Oil extraction Cocoanut meal, low in fat Cocoanut meal, high in fat Cottonseed hulls Cottonseed meal, choice Cottonseed meal, prime Germ oil meal maize Linseed meal, new process . ... Linseed meal, old process Palmnut cake Peanut cake from hulled nuts .... Peanut cake, hulls included Soybean meal, fat extracted Sunflower seed cake Starch manufacture Gluten feed ... Gluten meal Starch feed dry Starch feed, wet . . . Sugar manufacture Molasses beet Molasses, cane or black strap APPENDIX 721 TABLE VII. — VALUES PER 100 POUNDS FOR RUMINANTS (Continued) DRY DICES TIBLE NET MATTER Crude Protein True Protein ENERGY VALUE BY-PRODUCTS Sugar manufacture Molasses beet pulp Pounds Q2.A Pounds r.O Pounds •i e Therms 76 28 Sugar beet pulp dried 91 8 A6 O 7 75 8? Sugar beet pulp, ensiled IO O 08 O 5 O 12 Sugar beet pulp, wet O.7 O "? o.c 8 oo Packing house Dried blood QO.l 60 I 686 68 12 Tankage Over 60 per cent protein 02 6 *8 7 <;<; 6 Ql O4. 55—60 per cent protein Q2.C 1:4. o CI.I ST. 58 4S~5S per cent protein Q2 5 A8 I AS e 72 06 Below 45 per cent protein . . . . • . Q-2 C 17 6 is 6 CA 16 TABLE VIII. — VALUES PER 100 POUNDS FOR THE HORSE • DICE 5TIBLE NET MATTER Crude Protein True Protein ENERGY VALUES Alfalfa hay Pounds OI 4. Pounds IO Q Pounds 7 d Therms 48 82 Red clover hay .... . 87 I 7 2 4 5 •7Q Q4. Timothy hay 88.4 i.3(?) o.5(?) 26.64 Wheat straw oi 6 08 O 4. — 2O QO Beans 10 "? 17. 1 IOQ 4.O Corn (maize) dent 80 «; r Q 5 A 112 80 Corn (maize), meal 88 7 7 I 66 112 7O Oats 00.8 0 Q 8.Q 01.44 Peas . . QO 8 18 7 16 i IO5 2O Linseed cake QO.Q 20 5 27.8 101.60 Carrots 117 I 2 08 16 60 Potatoes 21 2 I O O Q I1? 7O 722 APPENDIX TABLE IX. — VALUES PER 100 POUNDS FOR SJSONE DRY MATTER DIGESTIBLE NET ENERGY VALUES Crude Protein True Protein Grains Barley . . Pounds 90.7 89.5 88.7 89.6 89.1 90.4 90.6 87.3 89.8 88.9 89.9 89.5 90.9 88.2 90-3 92.6 21.2 9.9 Pounds 8.8 7.6 7-i 6-5 21.4 6-5 9.9 5-5 9.9 14.8 12.0 14.4 28.8 34-8 59-2 44-8 1.8 3-8 Pounds 8.1 7-1 6.6 6.1 18.8 6.3 9.0 4-7 8.8 13.2 10.3 12.7 27.1 34-o 58.7 41.7 0.8 3-8 Therms 106.08 118.82 120.25 103.30 122.43 110.98 123.68 100.59 108.85 107.02 74-95 103.73 110.85 108.42 116.89 109.39 24.69 14.74 Corn (maize), dent Corn (maize) meal Corn (maize) and cob meal .... Pea meal Rice, rough Rve . Sorghum seed Wheat Milling products Red dog flour Wheat bran Wheat middlings, standard .... Oil meals Linseed meal, old process .... Soybean meal .... Sundries Dried blood Tankage, over 60 per cent 'protein . . Potatoes . ... Skim milk APPENDIX 723 § o % « g JJ I s n s *§ cig as so <>d> N M H izSrt b?<^2'^|cSo?>f?^2>^c'°"M>^'^'^*~"l?2 ^ M M ^- t-M M C\^f^J'\O MO MOO « TJ- PO d\ PO 10 M 10 10 d> M rotOTfOvvO O 't 0\ ON N 10 10 PO W O *O O t^t*» q N *7^o ro VOOO f>OQ M ONO«t-O ^g n • §ss 5d£ t^OOfOO^.COwc^ .vOt1* q^qoq^l & »? ^ I o1? (sr-iOMnwriMMMtNO NOO A 3 OO'OOO M in P« »0 -OO OOlN»Ot^.O ^frow PO S io(» ^oq" f^c» <» 5 ro 10 O O CO O O