• Weeds compete with crop plants for soil moisture, nutrients, light and space and reduce yields. They also harbour pests and diseases and serve as alternate hosts. The fibrous root system of weed interferes with harvesting of groundnut plants. Weed competition is one of the most important constraints in crop production. Hence, weed control is essential for getting higher yields in groundnut.
• Weed problem is severe in the early stages because of slow seedling emergence and slow initial growth of groundnut crop. The growth habit of groundnut plant makes weed removal difficult once weeds are established in the rows. Weed competition persists in later stages also, especially the inter-row weeds which escape in earlier weeding, nut sedges which regenerates from underground tubers and late season broad-leaved weeds. * Late season weeds appear in flushes in the crop and some of them grow even taller than the crop.
• Weeding can not be done once peg penetration and pod development of the crop commences.

• The loss in yield of groundnut pods due to competition by weeds ranged from 30 to 34% and sometimes up to 100%. The variation in yield loss depends upon weed intensity, influenced by seed-bank, longevity, dormancy and periodicity.
• The yield losses are higher in rainfed crop and also in Spanish bunch compared to an irrigated crop and Virginia runner. It is reported that the rainfed Spanish bunch groundnut crop removed 38.8, 9.2 and 23.5 kg N, P and K per hectare respectively from unweeded plot.
• The reduction in pod yield because of weed infestation was 52.3% in Spanish bunch and 18% in Virginia runner types. As the area under Spanish bunch is in increasing trend, it is of paramount importance to go for integrated weed management to exploit the maximum yield potential of improved bunch varieties.

• It begins when crop and weeds grow in close proximity and root system gets overlapped. The competition becomes further severe when weeds emerge earlier than or along with the crop.
• The critical period of weed competition in groundnut lies from 3 to 6 weeks after sowing depending on several environmental factors.

• Even though grassy weeds dominate, serious competition is exercised by broad leaved weeds also. Besides, a sedge, Cyperus, rotundus Linn. dominates when other weeds are controlled. It is usual to observe emergence of different flushes of weeds in different stages of crop growth.
• The predominant weeds associated with groundnut crop are given below.

Dactyloctenium aegyptium (Linn.) P.Beauv - Crow foot grass

Echinochloa colonum (Linn.) Link - Water grass

Panicum repens Linn. - Torpedo grass

Chloris barbata Sw. - Many spiked chloris

Commelina benghalensis Linn. - Tropical spider wort

Cynodon dactylon (Linn.) Pers. - Bermuda grass

Tragus biflorus Schult.

Eragrostis ciliaris (Linn.) R.Br. - Stink grass

Brachiaria ramosa (Linn.) Stapf - Signal grass

Saccharum spontaneum Linn. - Thatch grass

Sorghum halepense Linn. - Johnson grss

Cyperus rotundus Linn. - Nut grass

Tridax procumbens Linn.

Celosia argentea Linn.- Feather cocks comb

Celosia polygonoides Retz.

Euphorbia hirta Linn.- Pill-pod euphorbia

Phyllanthus maderaspatensis Linn.

Phyllanthus niruri Linn. - Flyroost leaf flower

Amaranthus viridis Linn. - Pig weed

Cleome viscosa Linn. - Spider flower

Trianthema portulacastrum Linn. - Dessert horse - purslane

Portulaca oleracea Linn. - Common purslane

Trichodesma indicum R.Br

Boerhaavia diffusa Linn. - Hog weed

Boerhaavia hispida K.Schum

Eclipta alba Hassk. - Yerbadetajo

Tribulus terrestris Linn. - Puncture vine

Euphorbia pulcherrima Linn.- Common Poinsettia

Acanthospermum hispidum DC - Star bur

Ageratum conyzoides Linn. - Tropical ageratum

Croton sparsiflorus Morung - Croton

Gomphrena celosiodes Jacr. - Globeamaranth

Coccinia grandis (Linn.) Voigt - India ivygourd

Cassia occidentalis Linn. - Coffee senna

Digera arvensis Forsk. - Amaranthus

Leucas cephalotes Spreng

Solanum nigrum Linn. - Black night shade

• Herbicides are applied either to the soil (soil application) or to the foliage (foliar application). Depending on the properties of herbicides, mode of action and selectivity, different methods are adopted.
• Environmental factors, convenience and cost are other factors that influence the choice of correct method of application. An improper method of application can result in poor weed control and/ or severe crop injury.

• Herbicides are sprayed on the soil surface to form a uniform herbicide layer. A total spray fluid of 900 lit/ha is required to have an effective weed control.
• The herbicide thus sprayed, due to their low solubility may penetrate only a few centimeters into the soil. Weeds germinating in the top layer are killed due to incidental absorption of herbicides.
• In certain cases, where water availability is a problem or where sprayers are not available or where it is not convenient to spray, sand is used as a carrier. Sand is mixed with herbicides @ 50 kg/ha and broadcasted for uniform spreading.

• Blanket application is the application of herbicides over the entire leaf area. For the control of Cyperus weeds, blanket application of glyphosate (15 days prior to sowing) on the foliage of weed population (3-4 leaf stage) is recommended for better absorption and killing of rhizomes in the soil.

• The chemical in commercial product that is directly responsible for the herbicidal activity is called active ingredient (a.i.). Active ingredient of each herbicide formulation is displayed by the manufacturer on the container. It is generally expressed as per cent by weight or by volume.

The amount of commercial product to be used = ------------------------ X 100

• Improper application even at recommended rates causes phytotoxicity to crops. It may be due to wrong method of application. Stage of the crop at the time of herbicide application is known to influence markedly the relative injury to plants. Application of herbicides into deeper layers causes damage to crops.
• When volatile herbicide like fluchloralin is incorporated into deeper layers, lateral roots of groundnut crop are affected.
• Giving heavy irrigation after the application of soluble herbicides also causes severe damage to crops.
• The herbicidal residues also affect the succeeding crop. The extent of damage by the herbicidal residue depends on dosage applied, nature of the herbicide and sensitivity of succeeding crop. This residual toxicity can be reduced by delay in sowing of the subsequent crop.
• Care should be taken to maintain sufficient soil moisture at the time of herbicide application.
• Avoid herbicidal spray when there is wind.
• Continuous application of same herbicide in the same field should not be done to avoid weed shift.
• Care should be taken to apply herbicide when there is susceptible crop in the neighbouring field.
• Herbicide selected for the intercropping system should have the property of selectivity to both crops. E.g. application of fluchloralin to groundnut + cotton or tapioca + groundnut intercropping system.
• Efficiency of herbicides (particularly the herbicide having photo-decomposition property eg. fluchloralin) can be increased if these are applied in the evening hours.

• Weed management practices can be grouped into cultural or preventive, physical or mechanical and chemical weed control.

• Cultural practices such as method of sowing (broad bed and furrow system), plant density (optimum plant population of 33 numbers per m²), crop varieties (spreading and semi-spreading varieties or bunch varieties with more branches and foliage), dose and method of fertilizer application (plough sole placement), time and method of irrigation (based on physiological growth phases adopting IW/CPE ratio) have pronounced effect on crop-weed interference.

• Mechanical control involves the removal of weeds by various tools and implements including hand weeding.
• Inter-cultivation has to be started as soon as the rows of groundnut seedlings are visible and repeated at regular intervals till 45 days after sowing or till the plants occupy the greater portion of the land. Shallow inter-cultivation is adequate.
• As soon as the soil is reasonably dry after a rain, the surface has to be stirred to create a dust mulch especially in vertisol to conserve moisture and also to facilitate infiltration of subsequent rains.
• If preparatory cultivation is done thoroughly year after year, weed population gets reduced and consequently the number of inter-cultivation can be reduced.
• The usual method of weed control in groundnut is hand weeding. Hand weeding or harrowing for weed control in groundnut crop is done for the first time about 25 DAS and this is repeated once or twice.
• Beyond 45 DAS, no weeding is done as it disturbs the elongating/penetrating pegs and developing pods.
• When groundnut is sown behind the country plough, weeding by hand hoes or manually operated star-weeder or peg type weeder are found useful for working between rows not only for weeding but also for creating soil mulch.
• When groundnut is sown as a row crop using a seed drill, inter-cultivation by using bullock-drawn harrows of different types up to peg penetration stage is easy and economical. This practice does not ensure weed free condition particularly within the crop row all through the crop period, as escapes appear again as late season weeds.
• Inter row cultivation in the form of hilling to bury weeds has not been found beneficial.

• Chemical weed control is preferred because of less labour involvement and no mechanical damage to the crop that happens during manual weeding. Moreover, the control is highly effective as the weeds even within the rows are killed, which invariably escape during mechanical control.
• Where labour is scarce and expensive, the use of herbicides either alone or in combination with cultural methods to control the weeds effectively and in time has become a necessity.
• The use of herbicides has some constraints. There are only a limited number of herbicides with selective action, which can be used in groundnut crop.
• Herbicides are expensive and the farmers are reluctant to invest on them for a rainfed groundnut crop because prolonged dry spells may occur during the crop period reducing the yield considerably.
• The herbicides used for weed control are classified according to their time of application.

• The herbicide is applied to the soil at least 10 days before sowing of groundnut and the most commonly used herbicide is fluchloralin (2.2 lit/ha) which is a selective herbicide.
• As this herbicide undergoes photo-decomposition, it has to be incorporated into the soil. If incorporation of herbicides into soil is not done, irrigation has to be given immediately after application.

• The herbicide is applied to the soil surface within 3 days of groundnut sowing. The effect of pre-emergence herbicides lasts 6-7 weeks after which fresh flushes of weeds emerge.
• The herbicides recommended are

pendimethalin (3.3 lit/ha);

metalachlor (2.0 lit/ha) and

fluchloralin (2.2 lit/ha). Pre-emergence application of any one of the above herbicides + one hand weeding on 30 DAS was reported to be effective in controlling weeds in groundnut.

• The herbicide is applied when the groundnut crop is 3-6 weeks old. There is no satisfactory herbicide for this purpose available in the market in India. However, alachlor and nitrofen are used to some extent, applied to the soil 20-25 days after sowing and immediately after an inter-cultivation
• The effectiveness of herbicides depends on the environmental conditions prevailing at the time of application and immediately afterwards, the physiological stage of the crop and weeds, soil type, moisture, organic matter, clay content, pH and atmospheric temperature and humidity.
• Sandy-loam soil with low organic matter content requires less quantity of herbicides than sandy-clay loam.
• Soil moisture has to be optimum for herbicides to get activated, otherwise irrigation has to be given. Excess soil moisture leaches the herbicides and may cause root injury especially in sandy soil.
• Herbicides like fluchloralin and trifluralin breakdown when exposed to sunlight and such herbicides have to be incorporated into the soil.
• It is generally not economical to use herbicides for the control of weeds in rainfed groundnut unless intercultivation implements cannot be used or labour is not available or very expensive.
• For the control of perennial (Cyperus) and annual weeds in groundnut a package of management practice is recommended.
• Pre-sowing (15 days prior to sowing of groundnut) blanket application of

glyphosate @ 10 ml,

ammonium sulphate @ 10 g and

teepol @ 1 ml/lit followed by

pre-emergence application of

metalachlor (2.0 lit/ha) +

one hand weeding on 30 days after sowing efficiently control both perennial and annual weeds in the crop.