7.chemical control of weeds By Allah Dad Khan

CHEMICAL WEED CONTROL
Chemicals that are used for killing weeds or suppress the
plant growth are called herbicides.The practice of
killing the undersirable vegetation (that is weeds) with
herbicide is called chemical weed control.
Herbicides are chemicals that kill or alter the normal
growth of weeds.They can be divided into two main
groups: selective and nonselective. Selective herbicides
are those that control the target weed(s) without
damaging desirable turfgrass species. Nonselective
herbicides kill all vegetation (including turfgrasses) and
are used in lawn renovation or on weeds not controlled
by selective herbicides

History of herbicides/chemical weed control
1. The use of chemical weed control started with
inorganic copper salts e.g CuSO4 for broadleaf
weed control in cereals in Europe in 1896.
2. Other inorganic salts that were tested between
1900-1930 included nitrates and borates.
3. In 1912, sulphuric acid (H2SO4) was used for
selective weed control in onions and cereals. In
1932, the first organic herbicide, Dinitro-ortho
Cresol (DNOC) was introduced.
4. In the 1950s triazine was introduced. In 1974,
Glyphosate , frequently sold under brand name
Roundup for non-selective weed control was
introduced.

Chemical weed control
(contd.)
Agriculture witnessed tremendous changes
through the production of organic herbicides,
which came at a time when field workers were
reducing, high cost labour and productive cost
of production.Thus, farmers in advance
countries almost depended on herbicide
because it met their production challenges in
agriculture and relatively ignored other
methods of weed control.

(contd.)
There are various factors that made chemical weed control
popular than manual and mechanical weeding.
a) Less drudgery in chemical control than in cultural method of
weed control.
b) Preemergence application of herbicides protects crops from
the adverse effects of early weed competition
c) Field labour demand is lower than in manual and mechanical
control.
d) Faster than manual and cultural weed control
e) More effective against perennial weeds than other methods
of weed control.
f) Less likely to be adversely affected by erratic weather
condition than other methods of weeding.

(contd.)
Limitations of chemical weeds control
1. Weeds become resistant due to prolonged and constant use of a
given herbicide .
2. Sudden dry spell may cause failure of preemergence herbicides
3. Crop injury as a result of poor sprayer calibration or wrong dosage
calculation, faulty equipment or failure to follow label directions
4. there could be side effect on the applicator
5. Special skills are needed for effective herbicide use.
6. Herbicide use is limited under multiple cropping
7. Chemical weed control require special equipment which may not
be useful for other operations on the farm.

Herbicide classification
Herbicides are classified based on the following:
1. Based on time of application (when applied)
2. Based on point of application (where applied)
3. Based on Herbicide movements in plants (how
they move in plants) (Site of primary action)
4. Based on type of plants killed (Selectivity)
5. Based on chemical structure (Chemistry)
6. Based on Physiological action

HERBICIDES
Herbicide use in weed control has been the most
important in world agriculture because it destroys
weeds on a large scale either before or at emergence of
crop without disturbing the crop or soil and farmers
don’t depend heavily on human labour. Weed killers
consist of inorganic, organic, and biological herbicides.
Types of Inorganic herbicides
1. Ammonium sulfate
2. Ammonium thiocyanate
3. Sodium borate
4. Sulfuric acid
5. Sodium chlorate

Advantages and Limitation of Herbicides or
Chemical Method of Weed Control
Advantages of Herbicides or Chemical Method ofWeed
Control
 1)The use of herbicides as pre-plant and pre-emergence
treatment can control weeds, before their emergence from
the soil so that crop can germinate and grow in weed free
environment or with minimum competition during their
tender and seedling stage.This is not possible with other
methods of weed control.
 2) In broadcast sown and narrow spaced crops hebicides
prove very effective in reaching every weed. Mechanical
methods are not so effective in such crops.
 3) In wide spaced crops mechanical methods are effective
for controlling weeds in rows but it is leaves the intra-row
weeds. Herbicides reach to all places and control the weeds
i.e inter row and intra-row weeds.

Advantages and Limitation of Herbicides
or Chemical Method of Weed Control
 4)Weeds with similar morphological characters like crop
are escaped from mechanical method. But now herbicides
are available which can kill such weeds without damaging
the crop.
 5) Herbicides withhold the weeds for considerable period
after their application. In mechanical methods weeds tend
to grow back soon.
 6) Deep rooted, vegetatively propagated weeds can be
controlled by using translocated herbicides.The
mechanical methods like weeding or hoeing are not so
effective for their control. Sometimes the suitable
combination of mechanical methods ( deep ploughing or
digging) and chemical methods is more effective for
controlling such weeds.

Limitation of Herbicides
(Chemical Method) :
 1)The use of herbicides requires technical know how
regarding choice of particular herbicide, time of application
safe dose method of application etc in the particular crop.
 2) Over and under-dose of herbicides can make a market
difference between the success or failure of weed control.
3) Certain herbicides because of their long residual effect
limit the choice of next crop in the crop rotation.
 4) Herbicides drifits may harm the neighbouring crops. E.g
Ester form of 2,4,-D may harm the neighbouring crop of
cotton, soybean, okra, etc.
 5) Herbicides use may cause environment pollution.

Types of organic herbicides
1. Over 200 organic herbicides are in use in the world agriculture today. Some of
the herbicides are either selective or non- selective while some are also
contact or systemic in their actions.
2. Oil: the petroleum oils used in agriculture consists of phytotoxic and
phytobland (non-phytotoxic ) oils.
3. Phytotoxic oils: kill plant by solubilizing cell walls, thus causing cells to
disintegrate. Phytotoxic oils can be selective or non selective.They have high
content of unsaturated fatty acids. Example of selective phytotoxic oils
include: diesel oils, while non-selective phytotoxic oils include Stoddard
solvent.
4. Phytobland / Non pyhtotoxic oils: these are light non herbicidal oils which are
added to herbicide to enhance their activity. They are used both as toxicant and
adjuvants. Examples of nonphytotoxic oils include sun 11 or corn oils.

Types of organic herbicides
ii. Organic arsenicals or methane arsonate herbicides eg. Cacodylic acid, MSMA,
DSMA.
iiiAliphatic acids e.gTCA, Dalapon
iv. Nitrophenols or substituted Phenol herbicides e.g dinoseb, DNOC and PCP
v.Phenoxycarboxylic acid derivative
(a) Phenoxyaceticacid herbicide; 2,4-D, MCPA.
(b) Phenoxypropionic acid herbicide; dichlorprop, mecopropane, fenoprop.
(c) Phenoxybutyric acid herbicide; 2,4-DB, MCPB.
(d) Phenoxy-Phenoxypropionic acid; dichlofop-methyl

vi. Amide derivatives
Chloroacetamide herbicides; acetochlor, alachlor,CDA
A (callidochlor), butachlor, diphenamid metolachlor,
propachlor.
Carboxyanilide herbicides; propanil
vii. Benzonitriles; Bromoxynil, dichlobenil and ioxynil.
viii. Carbamic acid derivates (carbamates)
Carbanilic acid derivatives; asalam, chlorpropham.
Thiocarbamate herbicides; butylate, EPTC, molinate,
thiobencarb.
Dithiocarbamate herbicides; CDEC, metham.

ix. Dinitroaniline herbicide; Benefin, (benfluralin), dinitramine, pendimethalin,
trifluralin, isopropalin.
x. Diphenyl ethers: acifluorfen, bifenox, lactofen, oxyfluofen.
xi. Substituted benzoic acids e.g chloramben, dicamba, DCPA
xii. Symmetrical triazines:
Chlorodiamino-s-triazine: atrazine, cyanazine, propazine, simazine
Methoxydiamino-s-triazine: atraton ana prometon
Methythiomino-s-triazines: ametryn, prometryne and terbutryn
xiii.Triazinones: e.g. hezazinone, metribuzin
ix. Substituted ureas e.g chlorbromuron,chloroxuron, diuron, linuron,
metobromuron, monuron
x. Sulfonylurea herbicides: chlorsulfuron, sulfometuron-methyl, classic, lindax
xi. Uracils herbicides: e.g bromacil and terbacil

xii. Miscellaneous herbicides:
1. Amitrole
2. Bentazone
3. Bipyridilium herbicides e.g difenzoquat, diquat and
paraquat
4. Cinethylin
5. Fosamine
6. Glufosinate- ammonium
7. Glyphosate
8. The imidazolinone herbicides
9. e.g. Buthidazole, Imazaquin, arsenal, Imazapyr
10. The picolinic acid derivatives: Picloram,Triclopyr
11. Oxadiazon
12. Sethoxydim (Akobundu, 1987)

Effectiveness of herbicide can be
modified by:
Environment, stage of maturity of target plant, type
of plant, plant part sprayed, how herbicide moves
within the plant, concentration of herbicides,
method of application and tissue of application.
Herbicides are named in three major ways:
Common name
Trade name
Chemical name of the active ingredient (chemical
formulae)
Structural formulae (Chemical Structure)

DEFINITION OF TERMS
ADJUVANTS:This is any substance in herbicide
formulation or added to spray tank or improve
herbicide activities or application
characteristics.
A CARRIER is a substance (gas, liquid or solid)
used to dilute or suspend a herbicide during its
application..
SURFACTANTS: this is a material which improves
the emulsifying, dispersing, spreading, wetting
or other surface modifying properties of liquid.

DEFINITION
EMULSIFYING AGENTS (EMULSIFIERS)
These are chemicals that improve the suspension of particles of
one liquid in another liquid.They are also referred to as
emulsifiers.
WETTING AGENTS
Wetting agents are surface active agents that reduce the
interfacial tension as well as improving the contact between
a liquid and surface on which it is applied.
STICKERS: These are spreaders which also reduce the surface
tension of other liquid and decrease the possibility of
aqueous solution to form discreet droplets.
DETERGENTS:They are cleansing chemicals used mainly for
cleaning equipment/sprayers.

HERBICIDE FORMULATION
• This is a process by which pure chemicals (e.g.)
the active ingredient of a herbicide is prepared
and made available for use in a form that will
improve handling, storage, application, efficacy
and safety.
• In order to produce a good commercial
herbicide, the formulation chemist must try to
maintain a good chemical additives such as
emulsifiers, wetting agents and inert materials
to make a new herbicide formulation.

Reasons why herbicides are
formulated:
 To reduce the concentration of the active ingredient through dilution in
appropriate solvent.
 To make the pure chemical available in a form that will permit uniform
distribution of target.
 To reduce the level of contamination and hazard during handling and
application.
 To improve the efficacy of the herbicide through slow release of the
active ingredient.
 Better protection from degradation.
 Greater uptake by the weed.
 To reduce cost of weed control with that particular herbicide. For
example, the choice of wettable powder over emulsifable concentrate
and vice-versa may be, based to a large extent on which of the
formulation is easy to produce and market

Types of herbicide formulation
1. Water soluble (WSC, SL)
2. Emulsifiable concentrate (EC)
3. Wettable powder (WP)
4. Flowable formulation (FW, F)
5. Granular Formulations (G)
6. Water Dispersible Granules (EDG, SG, DG)
7. Salts
8. Pellets
9. Microencapsulation

CLASSIFICATION OF HERBICIDES
Herbicides are classified according to their
selectivity, mode of action and the timing of
application.
SELECTIVITY
A non-selective herbicide kills or damages all plant
life in a treated area (e.g., Roundup).
A selective herbicide will kill weeds in a
germinating or growing crop without harming
the crop beyond the point of recovery (e.g., 2,4-
D used to control broadleaved weeds in a grass
pasture; Avadex to control wild oats in cereals.)

Nature of the Weed
Perhaps the most important consideration in
selectivity is the nature of the weed itself. A
listing of characteristics associated with 2,4-D
susceptibility and resistance will serve as an
example:

Weed Susceptibility to 2,4-D
Susceptible Tolerant or Resistant
Plant form erect spreading, prostrate
Leaf shape broad, large
narrow, short or linear, eg.,
grasses
Leaf position
horizontal, rosettes
formed
upright or pendulous,
rosettes lacking
Leaf surface rough, hairy
waxy, very hairy,
corrugated
Growing point terminal, axillary protected (sheath, crown)
Root depth shallow deep
Condition growing actively dormant
Age young mature

Biochemical Selectivity
 Some plants can tolerate a given amount of
herbicide within their tissues, whereas others will
succumb due to the differing chemical makeup of
the plants concerned. It is thought that the resistant
plant is able to break down the herbicide to nontoxic
components, whereas the susceptible plant cannot.
 Comparing corn, which is resistant to atrazine, with
oats, which is susceptible, the greater amounts of
atrazine which accumulate in the oat leaves and
eventually kills the plant are due to three factors:
oats absorb more atrazine, translocate it more
effectively, and cannot break it down as readily.

It is important to note that biochemical selectivity can be
lost if excessive herbicide rates are applied.
 Depth of Rooting: Most germinating weeds arise from seeds in the top one-
quarter to one-half inch of soil. A soil applied herbicide will be taken up by weed
roots in this zone, whereas crop seeds planted deeper than this will germinate in
a herbicide free zone.
 Influence of Herbicide on Selectivity: The herbicide itself can influence
selective action due to:
 a. Nature of the chemical molecule;
b. the concentration applied;
c. the formulation used;
d. surface tension and spreading qualities.
 Selective action is observed for example where MCPB (a 4 carbon chemical) is
used for weed control in clover or peas.The weeds are able to change the MCPB
to MCPA (a 2 carbon chemical) which is much more toxic than the original
herbicide.The crop cannot do this, so in essence the weeds commit suicide.
 Exceeding recommended use rates can kill or injure normally tolerant non-target
plants. Use of oil soluble ester formulations of 2,4-D are often more effective on
many weeds than water soluble amine formulations as they can more easily
penetrate the waxy coating on plant leaves.

MODE OF ACTION(or how the herbicide works to
kill a weed)
1. Contact herbicides - kills plant parts covered by the herbicide and are directly
toxic to living cells.There is little or no translocation or movement of the
material through the plant. Contact herbicides are effective against annual
weeds but they only "burn off" the tops of perennial weeds - chemically
mowing them. Contact herbicides may be selective, such asTorch (bromoxynil)
which kills broadleaved weeds in cereals without damaging the crop, or
nonselective, such as Gramoxone (paraquat) which kills any green plant
material.
2. Systemic herbicides - absorbed by either the roots or above ground parts of
plants, these herbicides move or are translocated in the plant.They exhibit a
chronic effect; that is, the full effects may not show for a week or more after
treatment.An overdose on the leaves may kill the leaf cells more quickly, thus
preventing translocation to the site of action in a plant.The old axiom "If a little
is good, more will be better" does not apply to herbicide use rates. Systemic
herbicides can be selective, as in the case of 2,4-D, MCPA, Banvel andTordon or
non-selective, as with Roundup.
3. Soil Sterilants - chemicals which prevent growth of plant life when present in
the soil.These products will prevent plant growth for periods of a few months
to a number of years. Examples include bromacil, tebuthiuron and atrazine at
high rates.

TIMING OF APPLICATION
a. Pre-Plant Soil Incorporated - these herbicides
are applied to the soil before the crop is sown.
They are incorporated in the soil to prevent loss
due to vapourization and breakdown by
sunlight.Trifluralin and EPTC are examples.
b. Pre-Emergence - applied to soil prior to
seeding or after the crop is sown but before
crop and weeds emerge. In most cases, the
weeds germinate in treated soil while the crop
germinates below the herbicide zone.
c. Post-Emergence - sprayed directly on the
weeds after they are up and growing.

HERBICIDE TYPE: PRE-PLANT INCORPORATED ( Soil
Applied)
Advantages:
1. Early control of weeds, minimizing competition.
2. Weeds are controlled where wet or windy weather after
emergence can delay spraying.
3. Work load is distributed.
Disadvantages:
1. Perennial weeds are generally not controlled.
2. Less effective under dry or cold soil conditions.
3. Residue may restrict crop rotation the following year.
4. Soil erosion may be a problem, as additional tillage for
incorporation is required.

HERBICIDE TYPE: POST-EMERGENCE (Foliar
applied)
Advantages:
1) Type and density of weed can be seen before herbicide application.
2) Oil texture does not directly affect herbicide choice or performance.
3) Oil moisture has little influence on level of control.
4) Few post emergent herbicides leave a soil residue which will restrict
subsequent cropping rotation.
5) Incorporation tillage is not required.
6) Top growth control of several perennial weeds is possible.
Disadvantages:
1) Specific stage of application required on both crop and weed variable
emergence
may be a problem.
2) Flush of weeds after spraying generally not controlled.
3) Wet or windy weather can delay application.

PERSISTENCE OF HERBICIDES IN THE SOIL
The length of time that a herbicide remains active or persists in the
soil is extremely important as it relates to the length of time that
weed control can be expected. Also, residual activity is important
as it relates to phytotoxic after-effects that may prove injurious
to succeeding crops or plantings.
 Seven factors affect the persistence of a herbicide in the soil;
1. micro-organism decomposition,
2. chemical decomposition,
3. adsorption on the soil colloids,
4. leaching,
5. volatility,
6. photodecomposition, and
7. plant uptake.

PERSISTENCE OF HERBICIDES IN THE SOIL
Micro-Organism Decomposition
The principal micro-organisms in the soil are algae, fungi, actinomyces, and bacteria.They must have
food for energy and growth. Organic compounds of the soil provide this food supply, except for a
very small group of organisms that feed on inorganic sources. Micro-organisms use all types of
organic matter, including organic herbicides. Some chemicals are easily decomposed (easily
utilized by the microorganisms), whereas others resist decomposition.
Chemical Decomposition
decomposition destroys herbicides through interaction with the soil constituents of oxygen, hydrogen
or water. Hydrolysis (interaction with water), for example, is responsible for inactivating atrazine
in the soil.
Adsorption of Herbicide by the Soil
1. Herbicides tend to leave the soil solution and are adsorbed by clay and organic matter particles
making the herbicide unavailable for uptake by the weed.
2. Research has shown that:
3. Soils high in organic matter require relatively large amounts of pre-emergence and soil sterilant
herbicides for weed control.
4. Soils high in clay content require more herbicide than sandy soils for pre-emergence or soil
sterilant weed control.
5. Soils high in organic matter and clay content tend to hold the herbicides for a longer time than
sands.The adsorbed herbicide may be released so slowly that the chemical is not effective as a
herbicide.

The extent to which a herbicide is
leached is determined principally by:
Leaching
Leaching is the movement of a herbicide in solution through the soil.
Leaching occurs in any direction (downward, upward, sideways).
Solubility of the herbicide in water.
Amount of water passing through the soil.
Adsorptive relationships between the herbicide and the soil.
In general, those herbicides which are completely water-soluble are
most easily leached. Salts of 2,4D are water-soluble and leach
readily through porous, sandy soils whereas esters of 2,4-D are
low in solubility and do not leach easily.
Herbicides have been known to move upward in the soil. If water
evaporates from the soil surface, water may move slowly
upward.The water may carry with it soluble herbicides. As the
water evaporates, the herbicide is deposited on the soil surface.

Volatility
Herbicides may evaporate and be lost to the atmosphere as volatile gases.
The volatile gases may or may not be toxic to plants.The volatile gases
may drift to susceptible plants such as those from the ester forms of 2,4-
D causing injury to susceptible crops such as tomatoes or grapes.
Eptam, Eradicane andTreflan are examples of volatile herbicides which
must be incorporated into the soil immediately following application to
prevent loss of herbicide to the atmosphere.
Rain or irrigation water applied to a dry soil will usually leach the herbicide
into the soil, or aid in its adsorption by the soil particles. Once adsorbed
by the soil, the loss by volatility is usually reduced.
Photodecomposition
Ultraviolet light from the sun decomposes many herbicides applied to the
soil surface. Some herbicides such asTreflan are recommended for soil
incorporation as they break down readily when exposed to sunlight.
Plant Uptake
The uptake of herbicides by plant roots results in their removal from the
environment; hence, reduced concentrations in the soil.

FACTORS AFFECTING HERBICIDE EFFECTIVENESS
TheWeed Leaf
For foliar active herbicides, the chemical must be retained on and penetrate through the leaf surface.
Wax cuticle, hairs on the leaf and the angle of the leaf all determine retention and adsorption.
Surfactants or surface active ingredients can be added to herbicide formulations to increase their
wetting ability and penetration into the leaf.
Climate
Under dry or cool conditions, plants grow slowly and translocation of systemic herbicides is
slow. Cloudyconditions interfere with food production in the plant thereby slowing translocation
and reducing effect of systemic herbicides. High temperatures and low humidity
increase evaporation of the herbicide from the weed leaf.The weed leaf surface is also difficult to
penetrate under these conditions as the plants tend to develop a heavy wax cuticle. High
humidity often damages the leaf cuticle allowing for a high absorption of herbicide into the
weed. Warm temperatures and high humidity are generally favoured for best control with
systemic herbicides. Avoid spraying in wind as herbicide will be lost to drift. Rainfall during or
after application can wash the herbicide off the leaf, hence, reducing effectiveness; but rain or
irrigation is required for many soil applied herbicides such as Dual and Primextra. Always read the
herbicide label to determine under which environmental conditions the product will work best.

Time of Day
This factor relates both to temperature and light. Generally carbohydrate levels in the leaves are
lowest in the morning and highest in the late afternoon. Spraying early in the morning is more
effective.
Age ofWeed
1. As a rule, weeds in the seedling stage are usually most susceptible to herbicides.Young weeds
are growing actively and there is a great deal of movement of food and water throughout the
plant. Systemic herbicides, once inside the plant, will be moving with this material to the
growing points.
2. With established perennial weeds, such as Canada thistle, treatment at the bud or early flower
stage is usually more effective.This is because the root food reserves are expended and newly
produced food along with herbicide is being moved back down to the root system to again build
up reserves. With Canada thistle and other perennials, it is the root we must attack for control.
3. It is critical to follow herbicide label directions regarding stage of weed growth to obtain
optimal control.
Nutrition
Herbicide injury to weeds is greatest when nutrient levels are high and the weeds are growing
vigourously. Adequate fertility also improves weed control by increasing competition by the crop.

Cultivation
Chemical weed control is more effective in fields that are cultivated because more weed seeds are
brought to the surface to germinate.With perennial weeds, cultivation lowers the root reserves
making them more susceptible to herbicides.
Soil Organic Matter
Soils with high O.M. such as peats or mucks require higher rates of chemical than do sandy soils.This is
because more of the chemical is bound to the soil colloids in high O.M. soils.
SoilTexture
Higher herbicide rates are needed on silts and clays because they provide more surface area for
chemical binding. Sandy soils require the lowest rates.
Soil Moisture
For soil applied herbicides, even mechanically incorporated products, some moisture is required to
place them in the soil solution for weed uptake. Soil herbicides generally work best in a warm,
moist soil.
ChemicalCompatibility
Some chemicals, when present in combination with other chemicals, are more toxic to plants and
animals than when present alone. If the field has been sprayed with a herbicide previously,
spraying with another chemical may cause great crop damage. Read the label to determine
compatibility with other pesticide products.

SOME PROBLEMS ASSOCIATED WITH HERBICIDE
USE
 Injury to the crop.
Faulty applications and environmental factors can lead to crop damage
from herbicides.
Residues in the crop.
Herbicide residues in crops cannot exceed the safe and acceptable levels set
by the Federal government. In many cases you will see
recommendations telling you not to apply chemicals within a certain
number of days of harvest or excessive chemical levels in the food will
result.
Residues in soil.
Some chemicals persist in the soil in phytotoxic concentrations.The land
manager must be aware of this if susceptible crops are to be planted
subsequently.
Compatibility with other pesticides.
Two chemicals mixed together might cause crop injury where either one
would not when applied separately.

SOME PROBLEMS ASSOCIATED WITH HERBICIDE
USE
Occupational hazard.
Herbicides as a rule are quite safe to use, having a low toxicity to
man and animals.All precautions must be taken, however, to
protect the applicator and persons who may come in contact
with the chemical.
 Air pollution.
 a) Spray droplet drift.
b)Vapour drift (volatile gases)
Water Pollution.
Herbicides may enter water courses and groundwater either by
direct application through accident or indirectly via leaching and
runoff from sprayed areas.
Disposal of used containers.
Irresponsibility in disposal of used pesticide containers sometimes
leads to poisonings of animals and man

Resistant weeds
1. Weed populations contain a very small number of individual
plants that are naturally able to withstand a particular herbicide
(or herbicide group) which is registered to control the weed.
This naturally occurring resistance is not due to weather
conditions or application technique but is an inherent
characteristic of the genetic makeup of the resistant plants.
2. Because of their low frequency of occurrence, these resistant
plants go undetected.The continual use of the same herbicide
(or herbicide group) year after year allows them to set seed and
multiply. Futhermore, by removing the susceptible weeds,
there is more opportunity for the resistant strain to flourish.
3. It is important therefore to know the herbicide groups so that
herbicides with different mechanisms of action can be selected
as part of the strategy in preventing a build up of resistant
weeds. For information on herbicide groups refer to the Field
Crop Guide For Commercial Growers.

Resistance Types
Managing Resistance
1. rotate crops.This usually results in using a diversity of herbicides.
2. rotate herbicides with different modes of action.
3. use tank mixes that control the target weeds by different modes of action.
4. use short residual herbicides whenever possible. Use long term residual
herbicides wisely and not continuously on the same field.
5. integrate herbicide use with other weed control strategies.
6. Practice good sanitation practices to prevent movement of weed seed with
soil, machinery, crop residue, etc.
Effects on wildlife
1. Some animals depend on weeds as a food source. Altering the botanical flora
could have detrimental effects on some animals but in turn could lead to a
better environment for others.
2. Great care must be exercised when using herbicides in or adjacent to riparian
areas. Read labels or contact habitat management personnel for information
on setbacks from watercourses and riparian zones.

CHEMICAL
WEED
CONTROL
By
Allah Dad Khan

7.chemical control of weeds By Allah Dad Khan

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