7.chemical control of weeds By Allah Dad Khan

CHEMICAL WEED

CONTROL

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.

Chemical 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 controle) 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.

Chemical weed control (contd.)Limitations of chemical weeds control1. 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 applicator5. Special skills are needed for effective herbicide use.6. Herbicide use is limited under multiple cropping7. 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 sulfate2. Ammonium thiocyanate3. Sodium borate4. Sulfuric acid5. Sodium chlorate

Advantages and Limitation of Herbicides or Chemical Method of Weed Control

Advantages of Herbicides or Chemical Method of Weed 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.

iii Aliphatic acids e.g TCA, 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 derivativesChloroacetamide herbicides; acetochlor, alachlor,CDA A (callidochlor), butachlor, diphenamid metolachlor, propachlor.Carboxyanilide herbicides; propanilvii. 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. Amitrole2. Bentazone3. Bipyridilium herbicides e.g difenzoquat, diquat

and paraquat4. Cinethylin5. Fosamine6. Glufosinate- ammonium7. Glyphosate8. The imidazolinone herbicides9. e.g. Buthidazole, Imazaquin, arsenal, Imazapyr10. The picolinic acid derivatives: Picloram,

Triclopyr11. Oxadiazon12. 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 nameChemical 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 AGENTSWetting 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. Salts8. Pellets9. Microencapsulation

CLASSIFICATION OF HERBICIDES

Herbicides are classified according to their selectivity, mode of action and the timing of application.

SELECTIVITYA 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 as Torch (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 and Tordon 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:4. Perennial weeds are generally not controlled.5. Less effective under dry or cold soil conditions.6. Residue may restrict crop rotation the following year.7. 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:7) Specific stage of application required on both crop and weed

variable emergence may be a problem.

8) Flush of weeds after spraying generally not controlled.9) 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, and7. plant uptake.

PERSISTENCE OF HERBICIDES IN THE SOIL

Micro-Organism DecompositionThe 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 Decompositiondecomposition 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 Soil1. 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:

LeachingLeaching 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 and Treflan 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.

PhotodecompositionUltraviolet light from the sun decomposes many herbicides applied to the

soil surface. Some herbicides such as Treflan are recommended for soil incorporation as they break down readily when exposed to sunlight.

Plant UptakeThe uptake of herbicides by plant roots results in their removal from the

environment; hence, reduced concentrations in the soil.

FACTORS AFFECTING HERBICIDE EFFECTIVENESS

The Weed LeafFor 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.

ClimateUnder 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 DayThis 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 of Weed1. 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.

NutritionHerbicide 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.


CultivationChemical 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 MatterSoils 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.

Soil TextureHigher herbicide rates are needed on silts and clays because they provide more

surface area for chemical binding. Sandy soils require the lowest rates.Soil MoistureFor 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.

Chemical CompatibilitySome 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 Resistance1. 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 wildlife7. 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.

8. 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

CONTROLBy

Allah Dad Khan

7.chemical control of weeds By Allah Dad Khan

Science

Transcript of 7.chemical control of weeds By Allah Dad Khan