Division of Agricultural Sciences

UNIVERSITY OF CALIFORNIA

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SELECTIVE

CHEMICAL
WEED

CONTROL

F. M. Ashton
W. A. Harvey

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CALIFORNIA AGRICULTURAL
Experiment Station
Extension Service

CIRCULAR 558

Selective chemical weed control uses a herbicide to control weeds in a crop with-
out injuring the crop. The chemical "selects" the weed and destroys it because
of either a quality inherent in the herbicide, or because of the way a nonselec-
tive herbicide is used. Selectivity is relative: it depends on proper use of the
herbicide. It will work only within a given range of concentration of the herbi-
cide and under particular conditions.

This circular explains the principles of how herbicides are used on foliage
and soil (pages 4 and 5), indicates how herbicides work selectively to kill
weeds while keeping crops alive (pages 5 to 14), and suggests how to apply
selective herbicides (page 15). Also included is a glossary of terms used in
selective weed control (pages 16 to 17).

SEPTEMBER, 1971

THE AUTHORS:

Floyd M. Ashton is Professor of Botany and Botanist in the Experiment Station,
Davis. W. A. Harvey is Extension Environmentalist, Agricultural Extension,
Davis.

This publication replaces circular 505, Principles of Selective Weed Control, by F. M.
Ashton, W. A. Harvey, and C. L. Foy, and draws heavily on information contained in
that circular.

[2]

«

SELECTIVE CHEMICAL WEED CONTROL

Weeds are costly. They take water, fer-
tilizer, and light from the crop plants,
and often they reduce yield and quality
of crop and livestock products. They also
can increase labor and equipment costs,
harbor insect and disease organisms, and
reduce land values. Weeds cost the Amer-

ican farmer an estimated $7,500,000,000
annually, and losses in California alone
have been estimated at $1,500,000 per day
— in certain crops the yearly losses due to
weeds can exceed $150 per acre. In many
crops such losses can mean the difference
between success and failure to the grower.

HOW LOSSES ARE REDUCED

Losses can be reduced by three principal
means — weed prevention, eradication,
and control.

Weed prevention. Weed prevention
means primarily good farm sanitation.
You start out with weed-free fields, and
prevent weeds from being introduced and
from spreading. Preventive methods in-
clude the use of clean seed, cleaning con-
taminated equipment, keeping fence row
and ditch bank weeds from seeding and
spreading, and spot-treating small weed
infestations within the field.

Weed eradication. Weed eradication
completely destroys or removes all weed

plants including regenerating plant parts.
Eradication is sometimes justifiable, as in
the case of small new infestations of par-
ticularly troublesome weeds, even at rela-
tively high costs per unit area or loss of
selectivity.

Weed control. Weed control is usually
the most reasonable approach. Many
weeds are so widespread that eradication,
though desirable, becomes economically
impractical. But you can reduce the in-
festation to a level that will enable you
to produce a crop profitably in spite of
the weeds.

CONTROL METHODS

Weeds can be controlled by one or more
of four methods: mechanical, cropping,
biological, or chemical. But no single
method is most efficient — a long-range
program combining all four methods pro-
vides maximum results.

Mechanical control

Mechanical weed control includes cultiva-
tion, mowing, burning and hoeing; even
hand pulling is justified under some con-
ditions.

Cropping control

Cropping control methods take advan-
tage of crop rotation to obtain changes in
the environment which will keep weeds
down. Often the new crop successfully
competes with the weeds from the previous

crop. Sometimes other methods of weed
control combine with cropping, as for ex-
ample the use of 2,4-D in grain following
a 2,4-D-sensitive crop in which broadleaf
weeds have become a problem.

Biological control

Biological methods use living organisms,
mostly insects, to control weeds. Examples
are the Chrysolina spp. beetle to control
Klamath weed of California ranges, and
the use of geese in cotton for control of
grasses.

Chemical control

Weed control by chemicals offers the
greatest single potential. Many selective
commercial herbicides for use in specific
crops against specific weeds are available.

[3]

HOW HERBICIDES ARE USED

Herbicides are used either on foliage or
in the soil.

Foliage applications

These treatments are made to leaves of
growing plants, usually as liquid sprays.
They kill plants by two methods — contact
or translocation.

By contact. This treatment kills only
the plant parts actually contacted by the
herbicide. However, the noncontacted
parts (i.e. roots) may die because they are
deprived of the essential contacted organs
(i.e. leaves). Adequate distribution of the
herbicide over the foliage is essential.
Selectivity may depend upon arrangement
and angle of leaves, differential wetting,
location of growing points, or upon spray
placement. Contact herbicides are most
useful to control seedlings.

By translocation. This treatment kills
the entire plant because the herbicide
moves within the plant. For example,
when applied to the leaves the herbicide
is translocated to the roots. It may also
move from older leaves to young growing
points. Therefore, herbicides of this type

are used on perennial plants as well as
annuals. Selectivity depends primarily on
physiological or biochemical differences
between plants.

Soil applications

These treatments are usually applied to
the surface of the soil but may also be
incorporated into the soil by cultivation,
or injected below the soil surface.

Timing of the application in relation
to the growth stage of the weeds and crop
is important. The application may be
made preplant, preemergence, or post-
emergence as related to the growth stage
of the crop plant.

Surface moisture must follow surface
treatments for most soil-applied herbicides
to be effective; you will obtain best results
when these herbicides are carried into the
soil by rainfall, overhead irrigation, or
flood irrigation. A physical incorporation
of the surface applied herbicide into the
top 2 inches of the soil followed by furrow
irrigation is usually as effective as rainfall
or overhead irrigation.

PRINCIPLES OF SELECTIVITY

A selective herbicide is one which signif-
icantly retards growth of an unwanted
plant species (the weed) without signif-
icantly damaging the desirable plant spe-
cies (the crop). Ideally, the weed is killed
but sometimes it is necessary only to re-
tard its growth long enough for the crop
to become dominant. A herbicide is selec-
tive to a particular crop only within cer-

tain limits; these limits are determined by
various factors because in any area there is
a complex interaction between plants,
their environment, and the herbicide.
Because of these variables, and because
of varying environmental conditions and
methods of herbicide application, selec-
tive herbicide control of weeds is relative,
not absolute.

THE ROLE OF THE PLANT

The factors involving plant (both weeds
and crops) response to control are: genetic
inheritance, age, growth rate, morphology
and physiology, and chemical and bio-
physical processes. Bearing these in mind
will help growers use herbicides more
effectively.

Genetic inheritance

The genetic complement of a plant deter-

mines the extent to which it responds to
its environment. These responses vary
from genus to genus, but within a genus
plant reactions to a given herbicide are
usually similar (strawberries are a notable
exception — they even vary from variety to
variety in their responses to herbicides).
Research is in progress to breed resistance
to specific herbicides into certain crops.

[4]

Crop recovers

Annual weed is killed

Growth habits. Crop roots 1, 3, 5 are deep, extensive and therefore recover from surface
sprays. Weed roots 2 and 4 are close to surface, easily killed by sprays.

Age

The age of a plant often determines its
response to a particular herbicide, and
young plants are generally easier to kill
than are older ones. Pre-emergence herbi-
cides which kill germinating weed seeds or
seedlings commonly have but little effect
on established weeds.

Growth rate

The growth rate of plants has a pro-
nounced effect on their reaction to some
herbicides. In general, rapidly-growing
plants are more susceptible to treatment
than are slower-growing ones.

Morphology

The type of morphology of a plant can
determine whether it is killed by a specific
herbicide. Morphological differences are
in root systems, location of growing
points, and leaf properties.

Root systems. Annual weeds in a field
of perennial crops can be controlled be-
cause perennial crops (e.g., alfalfa) have
deep, extensive root systems. Such systems
will recover from moderate injury to parts
above ground whereas annual weeds, hav-
ing shallow roots, will be killed. To re-
duce injury of the alfalfa to a minimum,
apply herbicides in winter (dormant pe-
riod) or immediately after cutting. The

type of herbicide used in this method of
control is a general-contact spray.

Location of growing points. Growing
points of cereals are located at the base
of the plant and are protected from con-
tact herbicides by the surrounding leaves.
In some cases they are actually below the
soil surface. Therefore, any contact spray
remaining on cereals may injure the leaves
but will not contact the growing points.

Broadleaf plants have exposed grow-
ing points at the tips of the shoots and
in leaf axils. Because of this, the contact
herbicide actually strikes the growing
point.

Leaf properties. Certain leaf properties
protect crops treated by selective herbi-
cides. Liquid spray droplets can adhere to
only a small portion of the surface of nar-
row, upright leaves (as in cereals and
onions), or waxy leaf surfaces, or leaves
that are corrugated or formed of small
ridges. Therefore, when sprays hit such
leaves they bounce off in droplets or wet
the surfaces only in small spots, thus re-
ducing the effect of the herbicide.

Broadleaf plants have wide smooth
leaf surfaces, extending horizontally from
the plant stem. Such leaves intercept more
of the spray, which spreads over them and
does not bounce off. Therefore when
broadleaf weeds such as lambsquarters,

[5]

Crop remains healthy

Weed is killed

Location of growing points. Left: growing points
(arrows) are protected from sprays. Right: growing points
are exposed.

Crop remains healthy

Leaf properties. On narrow upright leaves (left)
spray bounces off, does not affect plant. On wide hori-
zontal leaves (right) sprays stick and kill plant.

»»» G

[6]

Crop remains healthy

Weed is killed

Leaf properties. On waxy leaves (left) sprays bounce off or forms discrete droplets.

?s. Un waxy leaves uettj sprays bounce orr or rorms
Right: On nonwaxy leaves, sprays stick and spread.

HERBICIDE

Leaf properties. On corrugated leaves (left) sprays bounce off and form discrete droplets.
On noncorrugated leaves, sprays stick and spread.

wild radish, pigweed, or wild mustard are
sprayed with contact herbicides, the spray
solution tends to spread as a thin film or
to remain as many small droplets that wet
a large portion of the leaf, thus surface-
killing the weed. The same sprays on cere-
als or onions tend to bounce off and leave
the plants uninjured.

Physiology

The physiology of the plant determines
how much herbicide is taken up by the
plant (absorption) and the extent it
moves within the plant (translocation).
The absorption and translocation of a

herbicide varies between different species,
and those which absorb and translocate
the most herbicide will be killed.

Absorption. Those plants which have
thin cuticle or large stomata (minute
pores on leaf surfaces) absorb more herbi-
cide. Wetting agents in herbicide formu-
lation are primarily used to increase kill-
ing properties by increasing absorption of
the herbicide. Suitable wetting agents
mainly increase absorption through the
somata and, probably, to some extent
through the cuticle.

Translocation. Once a herbicide has
entered a plant it must move from this

[7]

Crop remains healthy

Weed is killed

CUTICLE

HERBICIDE

Absorption. Thick cuticle (left) prevents absorption of herbicide.
Thin cuticle (right) permits good absorption.

S TOM ATA

Absorption. Few and small stomata (left) prevent absorption of herbicide,
Many large stomata (right) allow good absorption.

Absorption. No wetting agent (left) keeps absorption low.
Wetting agent favors good stomata absorption (right).

point of absorption to other parts of the
plant to exert its maximum effect. This
movement (translocation) occurs both up-
ward from the root to plant parts above
the soil, and downward from the leaves

to the underground parts of the plant.
Translocation rate and the amount of
herbicide translocated vary with different
herbicides, and among plant species; they
even vary within a given species under

[8]

Crop remains healthy

Point of application
Weed is killed

Translocation. Poor translocation of 2,4-D in grasses make them resistant to weed killer.
Good translocation of 2,4-D in broad-leaved weeds make them susceptible to weed killer.

different environmental conditions. Rate
and amount of translocation of 2,4-D, for
example, are usually greater in susceptible
species than in resistant species.

Biophysical

Biophysical differences between plants
such as adsorption and membrane stability
may determine whether or not a plant is
killed.

Adsorption. Adsorption of herbicides
by plant cells inactivates herbicidal ma-
terial, probably by a physical rather than
a biochemical process. Radioactive tracer
studies have shown that the movements of
herbicides are slowed down by the sur-
rounding plant tissues. In extreme cases
the herbicides may be so tightly bound to
some plant constituent that it is not
readily translocated from the point of

application to the site of action, or it may
even be so tightly held that it is unavail-
able for herbicidal action.

Membrane stability. Oil toxicity in car-
rots and other crops of the carrot family
is one of the oldest examples of biophys-
ical selectivity. The selective oils used for
weed control in carrots kill weeds by dam-
aging the cellular membranes and allow-
ing the cell sap to flow into the intercel-
lular spaces; this causes death of the cells
and subsequent drying-out of tissues. Be-
cause their cellular membranes are resis-
tant to this damage the carrots are not
killed.

Biochemical

Biochemical reactions in various plants
protect them from injury by certain herbi-
cides. Their reactions include enzyme in-

[9]

ADSORPTION

Crop remains healthy

Weed is killed

_JA lie

CELL WALL

PROTOPLASM
VACUOLE

NUCLEUS

Biophysical. Left: herbicide is adsorbed by cell wall, and is prevented from
reaching protoplasm. Right: herbicide not adsorbed by cell wall, reaches protoplasm.

SELECTIVE OIL TOXICITY

CELL MEMBRANE

CELL SAP

CELL SAP

Biophysical. Carrot cell membranes (left) are resistant to selective oil. Membranes stay
intact, keeping cell sap inside. Weed cell membranes (right) are damaged by selective oil,
allowing cell sap to leak into intercellular spaces.

BLOCKAGE OF NORMAL ENZYMATIC REACTION
Crop remains healthy Weed is killed

SUBSTRATE

ENZYME

Biochemical. Herbicide (left) does not interfere with enzyme reaction and metabolism.
Herbicide (right) alters structure and attachment of enzyme and upsets metabolic processes.

activation and herbicide activation or
inactivation.

Enzyme inactivation. Many herbicides
reduce enzyme activity in one plant spe-
cies but not in another, thereby selectively
interfering with one or more of the plants
metabolic processes. This phenomenon
can kill certain plants and leave others
unharmed.

Herbicide activation. Activation of a
harmless chemical into a plant killer
sometimes can be used in selective weed
control. For example, the relatively harm-
less compound 2,4-DB is changed in some
sensitive plants into the weedkiller 2,4-D,
while in resistant plants (e.g. alfalfa) this
reaction takes place very slowly.

Herbicide inactivation. Some plants

[10]

Activation of herbicide

Biochemical. In resistant crop (such as alfalfa) harmless 2,4-DB is not
converted to plant killer 2,4-D. In susceptible weed (right) 2,4-DB is converted to 2,4-D.

Inactivation of herbicide

JHERBICIDE

Biochemical. Left: herbicide simazine is decomposed by corn, liberating C02 gas and
eliminating most of the plant killer. Right: simazine is taken up by weed and remains in plant.

can cause a herbicide to decompose into

a harmless compound. For example, corn

> decomposes simazine and carbon dioxide

gas is then given off by the plant; this pro-
tects the corn from accumulating lethal
amounts of herbicide.

ROLE OF THE HERBICIDE

Molecular configuration

Variation in molecular configuration of a
herbicide changes its properties, which in
turn modifies its effects on plants. This is
illustrated by the drawing at top of next
page, which shows the herbicides triflura-

lin and benefin. The only difference is
that a (-CH2-) group is moved from one
side of the molecule to the other. Tri-
fluralin kills lettuce even when applied at
a minimal herbicidal rate, but benefin will
control weeds without harming lettuce.

[ii]

H3C-H2C-H2C-N-CH2-CH2-CH

/ \

HC

\ /

CH

H3C-H2C-N-CH2-CH2-CH2CH

02N-C

/ v

C-NOs

HC

\ /

CH

C F3
trifluralin

Formulation

The formulation of a herbicide is of
vital importance in determining whether
it is selective or not to a given species. Per-
haps the most extreme type of formula-
tion to induce selectivity into a herbicide
is the solid granular formulation which
causes the herbicide to "bounce off" the

C F3
benefm

crop and fall to soil without sticking to
the foliage of the crop. Other substances
known as adjuvants and surface-active
agents (surfactants) are often added to
improve the application properties of a
liquid formulation — these may increase or
decrease toxicity. The addition of non-
phytotoxic oils or surfactants to atrazine

Selective placement. Granular formulation of herbicide allows herbicide to bounce off
crop and fall on ground to control germinating weeds.

[12]

UJ

Selective placement. Shielded sprays protect crops from being sprayed with herbicide by
having spray confined in shields (left) or crop covered by shields (right).

Selective placement. Directed sprays are directed toward base of crop plant, favoring
minimum coverage of crop and maximum coverage of weeds.

or diuron induces foliar contact activity in
these normally soil-active herbicides; these
herbicides have little if any foliar activity
in their usual formulation.

How the herbicide is used

A herbicide can be applied so that little
of it contacts the crop and much of it
contacts the weed. This can be accom-
plished by using shielded or directed
sprays. Shielded and directed spray meth-
ods are normally used with herbicides
applied to the leaves.

Shielded spray. In the shielded spray
method, shields prevent the herbicidal
spray from touching the crop while the
weeds are covered by the spray. This is
accomplished by placing the spray nozzles
under a hood, or by covering the crop
with a shield.

Directed spray. The directed spray
method is usually used where the crop is
higher than the weeds. Drop nozzles are
used to spray weeds between the crop
rows, and very little herbicide contacts
the crop.

ROLE OF THE ENVIRONMENT

The dominant factors of environment
which affects selectivity are soil type, rain-
fall or overhead irrigation, and temper-
ature. In general, soil type in combination
with the amount of rainfall determines
the actual location of a specific herbicide

in the soil, and temperature controls the
rate of plant processes and growth.

Some herbicides not inherently selective
may be made to function selectively be-
cause of their location in the soil. Such
selectivity depends upon different rooting

[13]

Crop remains healthy

Weed is killed

Position of herbicide in soil. Deep-rooted crop (left) is not affected by herbicide which
remains near soil surface. Shallow-rooted weed is killed by herbicide which stay near surface.

Position of herbicide in soil. Shallow-rooted crop (left) remains alive if herbicide moves
beyond its rooting zone. Deep-rooted weed is killed when herbicide is leached into the deeper
zones of the soil.

habits of crop and weed. If you want to
remove deep-rooted weeds while leaving
the shallow-rooted crops intact you must
use a herbicide which readily moves be-
yond the rooting zone of the crop into the
rooting zone of the weed. On the other
hand, if you want to remove shallow-
rooted weeds from a deep-rooted crop, you
must choose a herbicide which remains
near the soil surface.

Among the factors affecting the move-
ment of herbicides in soil are water solu-
bility of the herbicide, amount of rainfall,
and soil type. In general, high water solu-
bility of the herbicide, high amounts of
rainfall, and light soil types favor a deeper
penetration of the herbicide, while low
water solubility of the herbicide, less rain-
fall, and heavy soil types favor a shallower
penetration of the herbicide.

[14]

HOW TO APPLY SELECTIVE HERBICIDES

As selectivity is relative, it is possible to
injure the crop or fail to obtain good weed
control if the selective herbicide is not
used properly. The following suggestions
will minimize your risk.

Read the label. The manufacturers of
herbicides have gone to considerable ex-
pense and effort to prepare a precise label
indicating on what crops the herbicide
may safely and legally be used, what weeds
it will control, what rates you should use,
and what special techniques may be re-
quired. Read the label and follow its
suggestions.

Find rate of application. You must cali-
brate the sprayer to obtain the proper
rate of application. The rate of applica-
tion depends upon nozzle spacing on the
boom, nozzle pressure, nozzle size, and
tractor speed. To determine the volume
of spray, run a test using the following
procedure:

1. Make sure all nozzles are delivering the
same amount of spray and producing a
uniform pattern; replace those that are
not.

2. Fill tank with water to a predetermined
level.

3. Measure off a convenient test distance,
such as 500 feet.

4. Select a medium tractor speed and noz-
zle pressure which can be increased or
decreased if necessary.

5. Start the tratcor some distance before
the starting marker so that the entire
measured distance is traversed at the
operating speed.

6. Open the boom valve as you pass the
starting marker, and close it at the end-
ing marker.

7. Measure the width of the spray pattern.

8. Refill the tank with water to the prede-
termined level and measure the number
of gallons used.

Use the information gained in this test
run in the following formula to calculate
how many gallons of the spray you will
use per acre:

gallons per acre =

square feet per acre (43,560) x gallons used

test distance in feet x width of spray
pattern in feet

Example:

The sprayer delivered 15 gallons of
water over the 500 feet test distance and
the spray pattern width was 20 feet wide:
gallons per acre —

43,560 x 15

= 65.3 gallons per acre

500 x 20

If this rate of delivery is not suitable,
change nozzle pressure, tractor speed, or
both, and recalibrate. Increasing the pres-
sure will increase the rate of delivery,
while increasing the tractor speed will de-
crease the rate. You can also change the
rate by changing nozzle size or nozzle spac-
ing on the boom.

Add the recommended amount of herbi-
cide to this volume of spray solution to
obtain the desired rate of application.
When you use liquid formulation, correct
the determined volume for the volume of
liquid added in the herbicide formulation.

Watch application rate, particularly
with granular application. The use of
granular formulations of herbicides is rela-
tively new and equipment for such appli-
cations is still being perfected. Be ex-
tremely careful to obtain the recom-
mended application rate within safe
ranges of selectivity.

Avoid drift hazards. The drift of hor-
monal types of herbicide onto sensitive
crops has long been a problem. You can
minimize the hazard by using shielded
booms and low-pressure nozzles.

Avoid application under extreme cli-
matic conditions. Certain environmental
conditions such as rainfall, soil type, tem-
perature, wind, and humidity may strongly
affect your weed-control results. Apply
herbicides under extreme environmental
conditions only if published research re-
sults or personal experience indicate that
such use is a suitable practice.

Get necessary permits. To insure safe
use, permits are required for purchase of
certain selective herbicides such as 2,4-D.
You may get these permits from your local
County Agricultural Commissioner.

Go slowly. When you first try a selective
chemical weed control practice, do it on a
limited scale until you have obtained per-
sonal experience with it.

[15]

TERMS USED IN SELECTIVE WEED CONTROL

Absorption — The process by which herbicides are taken into plants, by roots or foliage

(stomata, cuticle, etc.).
Adsorption — The binding of substances on the surfaces of solids.
Annual — A plant that completes its life cycle in one year and then dies. Commonly

classified as summer annuals and winter annuals.
Aromatics — Compounds derived from the hydrocarbon benzene (C6H6).
Band application — An application of spray or dust to a continuous restricted area, such

as in or along a crop row rather than over the entire field area.
Basal treatment — An application of herbicides to the stems of plants at and just above

the ground line.
Biennial — A plant that completes its growth in 2 years. The first year it produces leaves

and stores food; the second year it produces fruits and seeds.
Broadcast (blanket) application — An application of spray or dust over an entire area

rather than only on rows, beds, or middles.
Carrier — The liquid or solid material added to a chemical compound to facilitate its

storage, shipment, or use in the field.
Compatible — Quality of two compounds that permits them to be mixed without effect

on the properties of either.
Concentration — The amount of active material in a given volume of diluent. Recom-
mendations and specifications for concentration of herbicides should be on the basis

of pounds per unit volume of diluent.
Contact herbicide — A herbicide that kills primarily by contact with plant tissue rather

than as a result of translocation.
Cotyledon leaves — The first leaf, or pair of leaves, of the embryo of seed plants.
Crown — The point where stem and root join in a seed plant.
Directed spray — An application made to minimize the amount of herbicide applied to

the crop. This is usually accomplished by setting nozzles low with spray patterns inter-
secting at the base of the plants just above the soil line.
Emergence — Appearance of the first part of the crop plant through the ground.
Emulsifying agent — A material which facilitates the suspending of one liquid in another.
Emulsion — A mixture in which one liquid is suspended in minute globules in another

liquid; for example oil in water.
Growth stages — (1) Tillering stage — when a plant produces additional shoots from a

single crown, as in wheat. (2) Jointing stage — when the internodes of the stem are

elongating. (3) Boot stage — when the seed head of a plant begins to emerge from the

sheath — usually applied to the grain crops.
Herbicide — A chemical used for killing plants.
Perennial — A plant that lives from year to year. In many cases (in cold climates) the

stem dies down but the root persists.
Post-emergence treatment — Treatment made after the crop plants emerge.
Pre-emergence treatment — Treatment made after a crop is planted but before it

emerges.
Pre-plant treatment — Treatment made before the crop is planted.
Rate and dosage — These terms are synonymous, but "rate" is preferred. Usually refers

to the amount of active ingredient material (such as 2,4-D acid equivalent) applied

to a unit area (such as 1 acre) regardless of percentage of chemical in the carrier.
Rhizome — Underground stem capable of sending out roots and leafy shoots.
Selective herbicide — A compound more toxic to weeds than to the crop in the field.

Helps control weeds without damaging the crop.
Soil sterilant — A material which renders the soil incapable of supporting plant growth.

Sterilization may be temporary or practically permanent.

[16]

Spray drift — The movement of airborne spray particles from the spray nozzle to beyond

the intended contact area.
Stolon — Runners or stems that develop roots and shoots at the tip or nodes, as in the

strawberry plant.
Stool — To produce crown shoots; to tiller.

Surfactant — A material which in pesticide formulation imparts emulsifiability, spread-
ing, wetting, dispersability, or other surface-modifying properties.
Suspension — A liquid or gas in which minute solid particles are dispersed but not

dissolved.
Systemic herbicide — A compound which is translocated within the plant and has an

effect throughout the entire plant system.
Translocation — Transfer of food or other materials such as herbicides from one plant

part to another.
Volatile — Quality which makes a compound evaporate or vaporize (change from a liquid

to a gas) at ordinary temperature on exposure to air.
Wetting agent — A compound which when added to a spray solution causes it to spread

over wet plant surfaces more thoroughly.
For current recommendations on the use of selective herbicides consult the latest Uni-
versity of California WEED CONTROL RECOMMENDATIONS— these are revised
annually and are available from your local Farm Advisor.

Because basic factors of selectivity interact with one another and because they are so
complex, the results are not always predictable; therefore, an expected selectivity may
be lost under an unusual set of conditions. Public agencies and herbicide manufacturers
have gone to considerable expense to prepare recommendations and/or labels indicating
on what crops a given herbicide may safely and legally be used, what weeds it will con-
trol, what rates should be used, and what special techniques may be required. Thus it is
only logical to read these recommendations and labels carefully and to follow their sug-
gestions equally carefully.

To simplify the information, it is sometimes necessary to use trade names of products or
equipment. No endorsement of named products is intended nor is criticism implied of similar
products not mentioned.

Co-operative Extension work in Agriculture and Home Economics, College of Agriculture,

University of California, and United States Department of Agriculture co-operating.

Distributed in furtherance of the Acts of Congress of May 8, and June 30, 1914.

George B. Alcorn, Director, California Agricultural Extension Service.

25m-9,'71 (P5182L) VL

his publication is one of many that are written, pro-
duced, and distributed by the University of California
Division of Agricultural Sciences.

hey cover many subjects, from agronomy to zoology.
They cover many crops, from alfalfa to zucchini.
Some report new research findings . . . some tell "how to
do it."

Most are free ... for some there is a charge.
All are listed in a catalog that is issued annually.

o get a copy of the catalog, visit the office of your local
University of California Farm Advisor, or write to:

Agricultural Publications

University Hall

University of California

Berkeley, Calif. 94720