CALIFORNIA AGRICULTURAL EXTENSION SERVICE Circular 137 Revised October 1949 GENERAL-CONTACT WEED KILLERS ALDEN S. CRAFTS / Spraying Diesel oil on roadside groWth to form a fire-control strip **& -f: i t * OF A OR I C ULTUR E LIFORNIA • BEttKllEY wsF^ General-Contact Weed Killers • • • are not selective. They destroy all kinds of vegetation — both weeds and crop plants. C^ These herbicides are used to control unwanted plant growth along highways, rights-of-way, and fence lines, and around farm and industrial buildings. Sodium arsenite has been the chemical most commonly used for such purposes, but it is dangerous to humans and livestock. It is now being replaced by new sprays which are non- poisonous, cheap, and easily applied. |^f There are three types of general-contact herbicides: 1 . Water-soluble materials, including common salts and corrosive chemicals, sodium chlorate, and salts of the phenol compounds. 2. Emulsions, made by combining water and oil. Additional toxic chemicals may be incorporated in either the oil or water base, or both. 3. Oils, including Diesel fuel, smudge-pot oil, stove oil, kerosene distillates, low-grade oils, proprietary weed-killer oils, and oils fortified by addition of phenol compounds or sulfur. f^} There are many conditions under which general-contact sprays are effective, and a variety of spray materials which are suitable. For this reason, the present circular is concerned with giving an over- all picture of these different sprays and suggesting possible uses, without making hard and fast recommendations. \^ Each grower will have to decide for himself which type of spray is best for his purpose, after he has studied the materials available and their advantages and disadvantages. General-Contact Weed Hitters • • • • A. S. CRAFTS Heavy weed growth on roadsides and ditchbanks encourages crop pests and diseases and is a fire hazard. Weed growth can be dangerous on highways where it cuts off the motorist's view. It can prevent access to pump houses, headgatss, store- houses, and other structures. Mowing or disking is often used to remove such weeds, but there are many places where these fairly cheap methods are not satis- factory. In such cases, general-contact herbicides (weed killers) are commonly used. General-contact weed killers are chemi- cals which kill all kinds of weeds and other plants. They are used to kill all un- wanted growth in and around areas such as the following: Roadsides Ditchbanks Railroad rights-of-way Fence lines Dry yards Parking areas Landing fields Firebreaks Highways Pole lines Billboards Farm buildings Warehouses Lumber yards Irrigation structures Industrial plants General-contact sprays are not selective. They are not ordinarily used on crop plants. However, they may be used as preplanting or preemergenee sprays, provided they do not carry readily available toxicants that will leach into the soil and injure the crop. They are also being used as postemergence sprays in corn, onions, bulbs, sugar cane, milo, and other crops that are robust and re- sistant to injury from sprays applied at the base of the plants. For many years sodium arsenite solu- tion has been the standard general- contact spray. The railroads have used millions of gallons of this solution. The results in general have been good, but many livestock have been killed. New weed killers have been found which will work as well as arsenic but which are not poisonous to stock. These are taking the place of the poisonous chemicals. The types of general-contact herbicides are: 1. Water-soluble chemicals, including common salts and corrosive chemi- cals (strong acids and alkalies) , and salts of the phenol compounds. 2. Emulsions of water and oil in which additional oil- or water-soluble toxi- cants may be included. 3. Oils, including straight oils, and oils fortified with phenol compounds or sulfur. This circular describes a number of these weed-killing materials and tells how to mix them. It also compares them so that the grower may judge which one will be best for his need. THE AUTHOR: Mr. Crafts is Professor of Botany and Botanist in the Experi- ment Station, Davis. [3] SPRAY SOLUTIONS MUST WET PLANTS THOROUGHLY In order to kill weeds, a contact spray must cover the plant surfaces and wet them thoroughly. Where grasses are part of the weed growth, the spray must creep down the stems of the plants and kill the plant crowns. Spray chemicals may be mixed with either water or oil. Oil is a better spray base than water, for killing grasses, be- cause it spreads and wets plant surfaces; water gathers into drops and rolls off. If a water spray solution is used, however, its wetting power can be increased by the addition of a wetting agent. Wetting agents may be used in concentrations of from about 0.1 per cent to 0.5 per cent or more by weight. They are also useful in mixing emulsions. APPLICATION METHODS Present use of general-contact sprays until the necessary killing power is ob- includes preplanting, pre- and post- emergence sprays in crops, and the older method of general weed spraying on road- sides, ditchbanks, fence lines, etc. Pre- tained. In some cases, this may be done rather easily; in others, the increase in dosage and hence in cost is such that use of oil alone is more economical. Only ex- emergence treatments are necessarily periments under his own conditions can made on very young weeds; postemer- help the grower decide which of these gence sprays are also usually limited to two types of contact herbicide is more young weed growth. Several requirements must be met in the killing of weeds by sprays. 1. Sufficient toxicant must be applied to kill the bulk of vegetation and allow considerable excess for unevenness of application and differences in plant thickness. 2. The spray solution must be dis- tributed so that the growing parts of the weeds are completely covered. useful to him. For satisfactory application of emul- sions and sprays mixed with water alone, pressure at the nozzles should be 100 pounds or more. The nozzles should be spaced so that the spray fans meet at ap- proximately the height of the vegetation. The boom should not be more than 18 inches above the vegetation ; the full driv- ing force of the spray is needed to wet the plants thoroughly. Most satisfactory results are obtained by using double 3. Where vigorous, weedy grasses are coverage from different angles. Where a p- present, the solution must creep down into the crowns to prevent re- newal of growth by tillers. The first two requirements may be met plication is made by hand, spraying should be slow and methodical so that all vegetation is thoroughly wet. If these precautions are observed, weeds should be controlled almost as ef- fairly readily by water solutions; the fectively as where straight oil sprays are third demands oil or a solution having the used. properties of oils. Although wetting agents The application methods given here will greatly increase the spreading and apply to the water-soluble chemicals and wetting properties of sprays mixed only to emulsions, since these mixtures involve with water, or of emulsions, the high special problems which do not apply to spreading and creeping qualities of oils straight oil sprays. can never be completely duplicated. (For detailed information on booms, One way to increase the killing power nozzles, and spray equipment, see Ex- of such sprays and emulsions is to in- crease concentration or volume, or both, tension Circular 389, "Chemical Weed- Control Equipment.") [4] WATER-SOLUBLE CHEMICALS There are two types of chemicals used in general-contact weed control that dis- solve readily in water, and that may be used in water solution as sprays. The first are well-known, common chemicals; the second are the new phenol compounds. The sodium or ammonium salts of these phenol compounds may be used directly in water solutions. Water-soluble chemi- cals are not ordinarily used for preplant- ing, preemergence, or postemergence treatments in crops because they may be leached into the crop roots by rainfall or irrigation water. In selecting a water-soluble chemical for weed control, several factors should be considered: (1) availability; (2) ease of use; (3) toxicity, as it relates to cost, dosage, and effectiveness; (4) hazards (poison, fire, and corrosion of equip- ment). Common chemicals. These include: sodium chloride, calcium chloride, so- dium chlorate, sodium arsenite, sodium hydroxide, sodium borate, sulfuric acid, and many others less commonly used. Although these chemicals have been widely used in general-contact sprays, they are not generally recommended be- cause many of them are extremely danger- ous to both humans and animals. Others may have injurious effects on the soil or leave toxic residues. Advantage: Some of these chemicals are manu- facturers' by-products. If the grower is near the place of production, he may buy them at very low cost. For example, one such material is bittern. Disadvantages: Sodium arsenite: Livestock are at- tracted to this chemical and, as a result, many have been killed. It is also dangerous to the spray operator, to pets, and to humans who may acci- dentally come into contact with it. It should only be used when such risks can be avoided. Sodium chlorate: If spray containing this chemical dries on clothing, dead plants, wood, or other surfaces, it be- comes highly inflammable, ignites by friction, and burns like gunpowder. It is hazardous to use as a spray. Strong acids and alkalies are corrosive to machinery and dangerous to use. New chemicals of high toxicity. Several new chemicals, the phenol com- pounds, have been recently introduced. They are extremely toxic to plants, and are now being used in place of the danger- ous, more common chemicals, for general- contact sprays. Among them are: penta- chlorophenol, dinitrocresol, dinitro sec- ondary butyl phenol, dinitro secondary amyl phenol, and their salts. Most of these compounds are sold under trade names. Their content in such commercial products is shown on the labels. Examples of these products are: Contax, Sinox General, and Dow General weed killers. Sodium pentachlorophenate or sodium dinitro cresylate may be used as general- contact sprays in many situations if their toxicity is increased by activating them with an acid salt, such as ammonium sul- fate, aluminum sulfate, or sodium bisul- fate. These salts, at concentrations of 1 to 2 per cent (or even less when no grasses are present) will kill weeds that have been protected from full effects of sun and wind and, consequently, do not have thick leaf surfaces. Such protected areas are found, for example, in deep drainage ditches, shady orchards, and lathhouses. Higher concentrations up to 5 per cent may be used on hardier weeds. If grasses are present, a wetting agent should be included in the spray solution. The ammonium salt of dinitro second- ary butyl phenol is so toxic that it will kill even grasses in their seedling stages. When used at concentrations of 0.5 per cent and over, with a good wetting agent, this herbicide has proved effective in kill- [5] ing mixed weeds on roadsides, ditch- banks, etc. On state highways in central California, fire strips sprayed with this material in April could be burned suffi- ciently within two to three weeks to pro- vide adequate fire protection for adjoin- ing crops and pastures. Advantages: 1. The salts of the phenol compounds are soluble in water. 2. They are highly toxic ; so little chem- ical is required that hauling is re- duced to a minimum. 3. If used in large quantities, they can be bought at prices that compare favorably with those of fuel oils. 4. They have no serious poison hazard. 5. They form true solutions and re- quire no agitation once they are dis- solved. 6. They kill certain oil-resistant weeds, such as sweet fennel, yellow star thistle, mayweed, and pineapple weed. Disadvantages: They are not economical for use on coarse, vigorous grasses, such as wild oats, foxtail, ripgut, and the like. On these weeds, an oil spray is necessary, because it will creep down the grasses and penetrate the crowns, where a water spray, unless applied in very large amounts, will run off. All water-soluble herbicides require thorough application. Formulas for mix- ing them will be found on the labels. OIL SPRAYS A description of oil fractions and their toxic effects on weeds will be found on pages 12 to 14. Growers who are not familiar with the prop- erties of oils should consult that section before reading about oil sprays. In contrast to sprays in water solution, oils wet plant surfaces readily and tend to spread as thin films and run down the stems. They penetrate the crowns of grasses where growing tissues that form new shoots are located. If an oil spray wets the tops of grasses thoroughly, the film may creep from 4 to 6 inches down the stems and kill all tissue from which new shoots might grow. This accounts for the satisfactory results usually obtained with Diesel and smudge-pot oil sprays, both of which are standard materials for weed killing. The content of aromatic compounds in general-contact weed oils should be above 25 per cent and may be as high as 50 per cent or more. Oils are often used as pre- planting, preemergence, and, occasion- ally, as postemergence sprays because they are not readily leached into soils. Diesel oil to be used for making firebreaks along California highways must meet the following specifications: Specific gravity at 60° F, not less than 27° A.P.I. Flash point (Pensky-Martens closed cup), not less than 150° F. Viscosity (Saybolt Universal) at 100° F, not over 50 seconds. Distillation (90 per cent point), not over 61 0° F. Water and sediment, not over a trace. Diesel meeting these specifications is Diesel No. 1, is less desirable as a weed readily available from oil dealers, and oil because, being more highly refined should be bought under the name of than Diesel fuel oil, it has less herbicidal Diesel fuel oil. Automotive Diesel oil, or toxicity. [6] Advantages: Diesel combines a low acute toxicity with a medium chronic toxicity. Be- cause it clings to the waxy plant sur- face, it wets plants easily and covers them well. It is very effective against mixed weeds, including grasses, and will kill completely even where growth is so dense that spray cannot be forced down into the bases of the plants. Being low in cost (in many areas), noncor- rosive, and safe to handle, Diesel oil is an excellent herbicide for general weed control. Disadvantages: Because Diesel is relatively low in toxicity, a large volume per acre is re- quired. This makes transportation costs relatively high. It also makes Diesel unfit for low-volume application by air- plane unless fortified. Plants which are resistant to oil require too large an ap- plication to make use of Diesel eco- nomical. Because of its value as a fuel, widespread use of Diesel as a weed killer seems unwise. Smudge-pot oil. This may be used in place of Diesel because its properties are similar. It is often less refined and hence may be more toxic. Stove oil. Although this oil may be used as a contact herbicide, it is largely limited to preemergence spraying in cropped areas. Advantages: It may be used as a preemergence spray in row crops even after crop plants have started breaking through the soil if the crops are tolerant of the oil. Some tolerant plants are carrots, celery, pars- nips, and parsley. (For use of stove oil on carrot crops, see Extension Circular 136.) Disadvantages: It is lighter and less concentrated in chronic toxicants than Diesel oil, and is therefore less effective for general weed spraying, especially where there are large grasses. Some refined stove oils are so low in toxicity that they are almost worthless as herbicides. Kerosene distillate. This is useful, if obtainable, as are other petroleum distillates in the same general boiling range. The gravity of such oils should range between 27° and 38° A.P.I. Below 27° they are heavy and may be difficult to apply in cold weather; above 38° they lack the persistent action of the chronic toxicants of heavier oil fractions. They are somewhat selective and are therefore ineffective on many oil-tolerant plants. Low-grade oils. In certain oil- producing regions, low-quality oils that are perfectly satisfactory for killing weeds may be bought from local re- fineries. Usually such oils are high in sulfur or aromatic compounds and may be unsuitable for use as lubricants or fuels. The properties that make them unfit for these uses may make them especially good weed killers from the standpoint of toxicity. Where such oils are available, they prove very useful in weed control. Oil extracts. In the refining of medic- inal oil, kerosenes, lubricating and other finer oils, solvent extraction is used to remove the more reactive unsaturates. Lighter grades of these by-products are recovered from the extract and used as paint thinners, cleaning fluids, and sol- vents. When these materials have been re- moved there may still remain a dark, foul- smelling substance of little commercial value. This residue is high in aromatic compounds and is extremely toxic to all plants. It is sold in limited quantities as a weed killer. (Avon Weed Killer, of the Tidewater Associated Oil Company, and 4060 distillate, of the Union Oil Com- pany, are examples.) Such oils are very useful as herbicides, but only a limited amount is available. They may be used to fortify Diesel oil when it has been so highly refined that it does not have enough killing action. [7] Fig. 1. — Unsprayed alfalfa field in which foxtail is beginning to spread. Cracked oils. As demands for gaso- line increase, stove, Diesel, and smudge- pot oils will be used for its manufacture, and new types of oils will be made avail- able in their place. These new oils will be largely fractions resulting from the cracking operations used in gasoline manufacture and, if unrefined, may be higher in unsaturates than the present types. This should increase their toxicity. Nontillage programs in orchards, and an increased use of oil in weed control are creating a great demand for weed killing oils. It seems likely that the large refiners will offer special products combining maximum spreading and wetting proper- ties with a balanced toxicity sufficient to kill all weeds without wasting toxic com- pounds. Many oils of this type are now available. Fortified oils. Oils of low toxicity may have their killing power increased by the addition of certain phenol com- pounds, or sulfur. They are then known as fortified oils. In addition to increasing the killing power, fortification provides extra toxicity where an oil is being used in emulsion or in low-volume application by airplane. It also makes oils more toxic to certain oil-tolerant weeds, such as sweet fennel, wild carrot, yellow star thistle, and mayweed. Most of these com- pounds are sold under trade names, with their content in the product shown on the labels. Pentachlorophenol and dinitrocresol are about equally toxic in oil solution. For most purposes in the field, a V2 per cent solution— roughly, 4 pounds per 100 gallons of oil— is enough; the same dosage rate applies to sulfur. Dinitro secondary butyl phenol (active ingredient in Dow General) is about three times as toxic, and only about 1% pounds per 100 gallons are required. Dinitro secondary amyl phenol (active ingredient in Sinox General) has similar properties. Where oil-tolerant weeds, such as sweet fennel, yellow star thistle, and mayweed, predominate, the fortifying agent may be increased. (These [8] special weed types, while resistant to oils, are very susceptible to 2,4-D, and dusts or sprays of this chemical may be used as separate treatments, preferably before the oil spray. Except for the special case described on page 11, the 2,4-D should not be combined with a toxic oil because the oil will defeat the purpose for which the 2,4-D is used. For further information on 2,4-D, see Extension Circular 133.) In addition to the fuel oils, several less toxic oils are useful in weed control if they are fortified. Examples are wash and slop oils, reclaimed solvent and lubricating oils, old crankcase oil, and gas-drip oil. In fact, the wetting and creeping prop- erties of any oil may be combined with the high toxicity of a fortifying agent to produce a weed killer. Viscosity, as it affects flow through nozzles and the spread of the oil on plants, is the most important factor in specifications of such oils. The viscosity of any herbicidal oil, as determined by the Saybolt Universal test at 100° F, should not be over 50 seconds. Advantages: Fortified oils are more toxic than straight fuel oils, and the volume used may be cut to the smallest amount which will give thorough coverage. In one case, where 150 gallons of straight Diesel oil per mile were required to spray ditches in the Sacramento River delta, 75 gallons of Diesel containing 3 pounds of dinitrocresol or 1 pound of dinitro secondary butyl phenol pro- duced better results. In another in- stance, 14 gallons of fortified oil per acre were applied by airplane to kill small weeds in a field where onion bulbs had been planted for seed pro- duction. The field was too wet for ap- plication by ground sprayer, or for cultivation or hoeing. A similar appli- cation has proved effective for elimi- nating small annual weeds in alfalfa. (For information on weed control in alfalfa, apply to the Botany Division, Davis, for the mimeographed circular describing the methods.) Fig. 2. — Alfalfa field which was sprayed with fortified oil to remove annual weeds. [9] Fortified oils are more rapid in their action than straight Diesel or heavier fuel oils. In spraying grass along the water line of irrigation ditches, for con- trolling the breeding of mosquitoes, only 24 hours were required before the ditch was restored to normal use. By killing quickly, fortified oils provide a longer period for burning. This makes them especially useful on highways where it is necessary to burn while the surrounding vegetation is still green. Fortified oils will kill oil-tolerant weeds. They are useful for spot treat- ments on clumps of sweet fennel, poison hemlock, artichoke thistle, St. John's wort (Klamath weed), or other shallow-rooted perennials. They kill all weeds, including grasses, that come up through cracks in sidewalks, pavement, and concrete ditch lining. They are use- ful for treating Bermudagrass and similar pests in gravel walks and rail- road ballast, and for treating in- dividual weeds, such as dandelions, plantains, and chicory in lawns and permanent pastures. EMULSIONS Emulsions. The extreme toxicity of Since Diesel oil weighs about 8 pounds the recommended fortifying agents makes per gallon, a satisfactory approximation it possible to prepare fortified oils having can be made by adding the oil on a volume much greater killing power than they basis-2 per cent would be 2 gallons of have spreading ability. Such solutions oil an^ 98 gallons of water; 6 per cent, may be applied in low volume from air- planes. Similar applications can be made by ground rigs if high-pressure, small- orifice nozzles are used. Low-volume ap- plications can also be made by vapo- dusting equipment, or by means of aero- sol, smoke, or steam. Although low-volume application is ef- fective on small weed growth, more vol- ume is required when plants have grown to the stage where they cover the soil and protect their own crowns. When this point has been reached, the fortified oil must be applied in emulsion. In fortified oil emulsions it is usually necessary to use wetting agents. Agitation in the spray tank is usually required because fuel-oil emulsions are not stable enough to hold throughout the period required for appli- cation. Emulsion formulas. The formulas given below should be mixed on a weight basis. One hundred gallons of emulsion weigh roughly 800 pounds; 1 per cent would represent 8 pounds per 100 gal- lons; a/2 per cent, 4 pounds; % per cent, 2 pounds; and Yq per cent, 1% pounds. 6 gallons of oil and 94 gallons of water. A satisfactory emulsion may be made using: 1 per cent sodium pentachlorophenate (8 pounds) V2 per cent aluminum sulfate, as an ac- tivator (4 pounds) V2 per cent wetting agent (4 pounds) 2 per cent Diesel oil (2 gallons) 98 gallons of water. In making up this formula, dissolve the sodium pentachlorophenate and the wet- ting agent in the water, then add the aluminum sulfate, with agitation. Add the oil last, with continuous, vigorous agita- tion. This formula has proved effective against broad-leaved annuals and such grasses as annual bluegrass and soft chess. A better formula from the standpoint of cost includes: Vi per cent pentachlorophenol (4 lbs.) 8 per cent aromatic oil (8 gallons) V2 per cent oil-soluble wetting agent (4 lbs.) 92 gallons of water. [10] In preparing this formula, add the penta- chlorophenol to the aromatic oil and agi- tate until it is all dissolved, then add the oil-soluble wetting agent and agitate until a uniform solution is obtained. This makes a stable concentrate that may be stored. To prepare the spray solution, mix equal volumes of concentrate and water, agitate vigorously until a stable emulsion is formed, then add the remaining water, with agitation. A similar emulsion can be made by using % per cent dinitro sec- ondary butyl phenol in place of the % per cent pentachlorophenol. If an emulsion of great stability is re- quired, the above formula should be homogenized. This breaks up the oil droplets to such an extent that they will remain in suspension. These formulas are satisfactory for broad-leaved weeds and grasses grown in sheltered locations. Where more vigorous grasses are present, the percentage of oil may be increased or the volume of water decreased. It usually does not pay to use over 4 pounds of pentachlorophenol (or 1% pounds dinitro secondary amyl or butyl phenols) per 100 gallons of spray. If more oil is required to handle grasses, it may be mixed with the fortified oil of the above formulation, and the whole mixture may be emulsified. For this sup- plementary oil, the least expensive oil available may be used provided it has the proper viscosity (not over 50 sec). Mix- tures up to 25 per cent oil have been used against large, rapidly growing wild oats, barley, and foxtail in dormant alfalfa. The grower will learn by experience how much oil is required and how to adjust for the height and vigor of the vegetation. Density of growth and plant maturity, with respect to possible tillering and late germination of additional seed, must also be considered. A few situations have been found where it pays to include 2,4-D in a general- contact spray treatment. One is where Commelina, or day lily, and grasses occur together in sugar cane. Another is where wild morning-glory and grasses occur in corn or milo. Where an aromatic oil is used, 2,4-D acid may be dissolved directly in it. An ester form of 2,4-D may also be placed directly in the emulsion. Advantages: Emulsions kill oil-resistant weeds at a minimum cost. Chickweed, pineapple weed, and yellow star thistle have been killed in experimental tests in an onion crop where selective herbicides failed and where straight oil would have seri- ously harmed the crop. Although the emulsion injured the onions, they re- covered rapidly. Yellow star thistle and mallow, both of which are resistant to Diesel, have been killed on roadsides by applications of emulsions. The composition of emulsions may be adjusted to meet almost any condi- tion. The oil content may be held to 1 or 2 per cent on tender weeds in or- chards or in ditches, or may be in- creased to 15, 20, or 25 per cent in fighting heavy, vigorous infestations of grasses. Within this wide range there may be an appreciable saving of oil. And in all cases, oil-tolerant weeds are killed. As the oil content is raised above a level of about 25 per cent, a point of diminishing return is reached where the relative costs of oil, fortifying agent, and wetting agent must be bal- anced with the inconvenience of mixing and effectiveness of treatment. Under most conditions, emulsions containing fortified oil are cheaper than straight oil because the toxicants are so much more effective than those present in oil alone. For example, the second formula on page 10 can be pre- pared for about one half the cost of a straight oil spray. Use of emulsions cuts the cost of long-distance hauling of fuel oils. This becomes a real factor in regions far re- moved from oil refineries. For preemergence or postemergence use of general-contact weed killers, [in those having toxicants insoluble in water are least likely to cause injury to the crop. Fortified oil emulsion sprays may be used as preplanting or as post- planting preemergence treatments to reduce competition by weeds during the early stages of crop growth. Disadvantages: In using emulsions, the spray tank must be equipped with a mechanical agitator. Also, where large, vigorous grasses are present, results may never be quite so good as with oil because of the difference in creeping ability. PROPERTIES OF OILS To use oil sprays in weed control, the grower must know something about oils and their effects on plants. In this way he will be able to choose the best oil for his own needs. In the oil trade, all oils are described by sets of specifications. These are either re- quired by law or used by the manufac- turer as a standard of quality for his own products. Every product must meet the specifications which its manufacturer has set up for it. These standards are intended to show a product's ability to do the job for which it was made. Certain oils now being tested as weed killing sprays were not really intended for this use. Thus specifications listed for these oils do not necessarily show how well they will act as weed killers. There are no specifica- tions for weed killing oils. The only sure way to find out if an oil is useful as a weed killer is by tests in the field. Hence, in buying oils for weed killing, the grower will have to rely on the ability of the oil dealer to supply a satisfactory product. This circular lists some of the words commonly used to describe oils and oil sprays, and tells what they mean. It also gives a simple account of the way in which oil is refined. The terms should be helpful to the grower when choosing his oil sprays. Refining. Two main processes are in- volved in oil refining: distillation and separation. An example of simple distilla- * The four drawings are based on illustrations supplied through the courtesy of the Shell Oil Company, Incorporated, to whom acknowledge- ment is also made for editorial advice in connec- tion with the preparation of material used in this section. tion is the boiling of a teakettle. The water in the kettle is heated to boiling tempera- ture. At this point it turns to steam or vapor. If the steam touches a cold surface, it condenses. This condensed steam is a distillate. Since water has only one boiling point, the condensed steam is the only product resulting from the boiling of water. Unrefined (crude) oil, on the other hand, is made up of many parts (frac- tions) which have different boiling points. The oil is put into a container, or tower, and heated. The fractions which have the lowest boiling point (gasoline) vaporize first and the vapors rise to the top where RESIDUE Fig. 3. — As the vapor passes up the tower, the part with the highest boiling point condenses and drops back. The frac- tions with the lowest boiling points are run off at the top. [12] n v O >- X /\ DEGREE MARKINGS ON TUBE BULB Fig. 4. — The hydrometer floats in oil. Gravity measurements are indicated by the markings on the tube. they condense. These fractions which evaporate quickly are the light (more volatile) ones. Other, less volatile frac- tions, such as stove oil and Diesel fuel, have higher boiling points, and condense later. Thus each batch of crude oil con- tains some gasoline, some stove oil, and some Diesel fuel, etc., and each fraction may be removed within its own boiling range. (See fig. 3.) Distillates. The liquid oil fractions re- sulting from distillation are called distil- lates. Not all the materials in crude oil can be distilled; tars and asphalt are left. The distillates are not pure compounds but contain a mixture of all compounds which turn to vapor during distilling. Some of these are called unsaturated com- pounds. There are more of these unsatur- ated compounds in an oil such as Diesel fuel, that has not been highly refined, than there are in kerosene or spray oils that have received more treatment. When a re- fined oil is wanted, some or all of the unsaturated compounds may be separated from the oil by use of chemicals. Sulfur dioxide is one commonly used for this purpose. It is the unsaturated compounds which are important in weed killing oils. They determine, in part, how well the oil will kill plants. (See "Toxicity.") Gravity. The gravity, or density of an oil has to do with its weight. It is ex- pressed in degrees A.P.I, because the gravity of oils in the United States is determined by tests set up by the Ameri- can Petroleum Institute. The gravity of an oil is found by use of an instrument called a hydrometer. This is a glass tube with degree markings on the side and a bulb at one end. (See fig. 4.) The bulb floats in the oil sample to be tested, and the depth to which it sinks, as indicated by the marks on the tube, is a measure of the gravity of the oil. The bulb does not sink so far in heavy oils as in light ones. The degrees are marked on the tube in such a way that gravity readings of heavy oils are lower than those of light ones. Gravity is important in choosing a weed killing oil. Heavy oils, which fall be- low 38° A. P. I., will kill crop plants as well as weeds. For use as a selective herbi- cide on crop plants, therefore, a weed killing oil should not fall much below 38° A.P.I. Flash point. Flash point is a measure of the inflammability of an oil. One of the means for testing the flash point is the FLAME APPLIED HERE CLOSED CUP VAPORS CONFINED Fig. 5. —The closed cup test for determin- ing the flash point of oils. [13] Pensky-Martens closed cup test. The oil is heated in a closed container, or cup. (See fig. 5.) A slide covers a small opening in the cup. This is opened at definite inter- vals, and a flame is passed over the oil. The temperature at which the oil ignites is its flash point. Highly volatile oils ignite at fairly low temperatures. All gasolines flash at ordi- nary temperatures (for instance, 70° F) ; in fact, they will flash at freezing tempera- ture for water. From the standpoint of safety for the operator, it is dangerous to use gasoline for weed spraying. Only less volatile fractions, such as some of the thinners and solvents, are fairly safe to use. However, the spray operator must remember that all these products are in- flammable. The spray mist and surround- ing air may ignite and burn with great heat. Viscosity. This relates to the flowing quality of an oil. To find the viscosity, 60 cc (about 2 ounces) of oil are put into an instrument called the Saybolt Universal viscosimeter. The oil is heated to 100° F. It is then timed as it runs through a small opening in the instrument. (See fig. 6.) The viscosity of an oil to be used as a spray determines somewhat the amount of pressure needed, and the size of the spray orifices. The heavier oils will not break up into drops easily, nor flow as fast as will the lighter ones. Viscosity is also a factor in determining how much of the oil soaks into the plant surfaces. A heavy oil will stay on the plant longer than will a lighter, more volatile one. Thus it may soak in in larger amounts and be more toxic. For use as a weed spray, an oil's viscosity should be about 50 seconds or less. Toxicity. An oil's toxic effect on plants depends in part on how volatile it is and on the amount of unsaturated compounds it contains. Oils vary in their toxicity. Some kill all plants; some are selective, and kill only weeds, leaving certain crop plants undamaged. Light unsaturated compounds cause a rapid burning of leaves called acute toxicity. Heavy un- saturated compounds injure the growing parts and cause a chlorosis (yellowing of leaves) . This injury comes on much more slowly and is called chronic toxicity. Very light unsaturated compounds, such at those from gasoline stock, cause burn- ing of the leaves. Injury is not complete, however, if the spray incompletely satur- ates the plant, because these oils may evaporate before all tissues are killed. Unsaturated compounds of medium weight are very toxic to grasses and most weeds. They do not kill plants of the car- rot family, except at high concentrations. They are usually found in unrefined petroleum distillates, such as stove oil, at concentrations between 20 and 30 per cent. Heavy unsaturated compounds, such as those in Diesel and other heavy fuels, kill plants slowly by chronic toxicity. Crop plants as well as weeds are killed by such oils. Fig. 6. — The viscosimeter operates on the principle shown below. S STARTED WITH APPARATUS FILLED WITH 1 GALLON OF LIGHT (THIN) OIL p «N AFTER ONE MINUTE OIL DRAINED STARTED WITH APPARATUS FILLED WITH 1 GAL. HEAVY (VISCOUS) OIL' SAME SIZED OPENINGS T3 AFTER ONE MINUTE SMALL AMOUNT DRAINED 6 [14] OILS FOR WEED CONTROL Selective oil sprays are ones which do not damage certain oil-tolerant crop plants but kill many common weeds. General-contact oil sprays kill all kinds of plants, both weeds and crops. They should contain enough toxicants to kill all plants sprayed. Emulsions are physical mixtures of oils and water. Most emulsions have a water base in which droplets of oil are suspended. Invert emulsions, however, have an oil base in which droplets of water are suspended. Weed killing emulsions may have additional chemicals dissolved in either the oil or water, or both. Fortified oils are oils which have had their toxicity increased. This is done by adding chemicals (fortifying agents) such as phenol compounds or sulfur. It usually costs less for these fortifying agents, per unit of toxicity, than for an amount of unfortified oil of equal toxicity. Fortified oils are used as general-contact sprays. The oil itself acts mainly as a car- rier for the toxic fortifying agent. Oils serve two purposes in chemical weed control. They act as toxicants (kill- ers) and as carriers. In selective weed oils, the toxicants (medium-weight unsatur- ated compounds) should be present at concentrations that kill weeds without killing crops. For carrots, this concentra- tion may be fairly high (around 25 per cent) . To spray safely in flax, it should be considerably lower, and for onions, still lower. The oil should be so light that all toxicity is of the acute type. This means that it should have a gravity above 38° A.P.I. (Shell Weedkiller No. 10, for ex- ample, has a gravity of about 43° A.P.I.) This guarantees that the toxicity is acute. At the same time, such oils evaporate fast, so that the oil flavor soon leaves the crops. The nontoxic part of the selective oil acts as a carrier for the toxic parts. It cannot be replaced by water because the selective toxicants are oils and will not act in emulsion. Also, the wetting properties of straight oil are required to kill grasses in the crop. Oil is a better carrier than water be- cause it has a low surface tension and high wetting ability. This means that in- stead of running off the plants as water does, the oil creeps over them. In doing this, it soaks into the growing parts and kills the tissues. This wetting ability is especially important where the oil is to be used on weed growth containing rank, vigorous grasses. In order that the information in our publications may be more intelligible it is sometimes neces- sary to use trade names of products or equipment rather than complicated descriptive or chemical identifications. In so doing it is unavoidable in some cases that similar products which are on the market under other trade names may not be cited. No endorsement of named products is intended nor is criticism implied of similar products which are not mentioned. Cooperative Extension work in Agriculture and Home Economics, College of Agriculture, University of California, and United States Department of Agriculture cooperating. Distributed in furtherance of the Acts of Congress of May 8, and June 30, 1914. J. Earl Coke, Director, California Agricultural Extension Service. 20m-10,'49(B5979) [15] AGRICULTURE • • . 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