Cotton

Weed Management

Introduction

• In India manual and mechanical method of weed control continue to be the major methods of weed control. These methods are cumbersome, though they are more effective in controlling weeds. Usually weeding is done 3-4 times manually due to long period of crop growth. Hence, labour required for weeding is high and labour wages are increasing every year resulting in increased cost of cultivation. The introduction of improved varieties and hybrids, adoption of wider row spacings, slow growth in early stages, coupled with frequent irrigations, application of higher doses of fertilizer and intensive cropping systems pre-dispose the cotton crop to heavy infestation of weeds as these provide very congenial conditions for weed growth. Moreover, with the advent of monsoon rains, the manual and mechanical methods of weed control are hindered by wet field conditions due to frequent rains and thus the crop suffers due to weed competition.
• In Western countries, use of herbicides has become very popular and herbicides are extensively used in cotton to control weeds. In India, not much emphasis has been given on use of herbicides and weeds are removed manually since labour is cheap. Chemical weed control when followed with proper precision, is more efficient, economical in controlling weeds without toxicity to the crop. The herbicides are applied as pre-planting, pre-emergence and post-emergence in cotton from 10 days prior to planting to 40-50 days after planting. The use of pre-emergence herbicides require sufficient soil moisture to control the weeds. The use of postemergent herbicides in cotton, though not safe, can help to overcome the weed problems at later stage of crop growth and it requires precise application techniques. So, the integrated weed management involving herbicides and cultural methods (hand weeding/interculturing) is more effective and economical in controlling weeds in cotton. Use of herbicides alone will not eliminate the weeds completely.

Weed species associated with cotton

• The weed flora in cotton fields differ widely according to varied soil and climatic conditions. The most prevalent weed species in cotton field in irrigated tract of Karnataka are, Abutilon indicum, Panicum isachne, Bracharia romosa, Bracharia cruciformis, Euphorbia geniculata, Legasca mollis, Tridax procumbens, Flavaria australasica and Demodium duffusum, Cyperus rotundus, Desmostachya bipinnata, Panicum repens, Setaria sp. Digitaria marginata, Dinebra retroflexa, Lursia hexandra, Bracharia mutica, Commelina benghalensis, Euphorbia prostrata, Euphorbia hirta, Amarathus sp., Trianthema portulacastrum, Acanthosperium hispidum, Phyllanthus niruri, Cynotis sp.
• Many weeds are alternate hosts of insect pests. Many malvacious weeds especially – Hibiscus spp. Abutilon aspp. And Side spp are alternate hosts of cotton pests-jassids (Amarasca bigettula) aphids (Aphis gossypii) and many other cotton pests. Apart from these weeds some more are also alternate hosts, which have been listed.

Nature of crop weed competition

• In cotton fields moisture removed by weeds was more than that of cotton crop and at the top 15 cm soil layer weedy plots tended to be more drier than weed free plots. Smooth pig weed (Amaranthus hybridus) competition with cotton reduced soil moisture content from depths of 122 to 183 cm to a greater extent than cotton alone. When weeds were allowed to compete with cotton after first and second irrigations, reduction in yields of cotton to the extent of 20 per cent have been observed.
• The weeds deplete enormous amount of plant nutrients from the soil. For every kilogram of nitrogen removed by weeds, there was a yield reduction of 12 kg of seed cotton per ha. Leaving the field unweeded resulted in the uptake of nutrients in the order of 20, 16 and 13 kg/ha of N, P and K respectively, at 45 days after sowing of cotton. Weeds remove 5-6 times N, 5-12 times P2O5 and 2-5 times K2O than cotton crop at early stages and then reduce the cotton yields by 54-85 per cent.
• All weeds compete for space and light and reduce crop yields. When the moisture and nutrients are adequate, then the competition is mostly for light. Interception of light by canopies of cotton occur more slowly than canopies of weed and require 6-12 weeks for 80 per cent interception of light by cotton. Weeds take 8 weeks to reach maximum cover while cotton takes 16 weeks to reach 90 per cent cover.

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Critical period of weed competition

Intercultivation and weed management

Critical period for crop growth is upto 60 days, there fore fields should be free from weed upto this period. Three to four intercultivation (hoeing) should be taken once in 15 days intervals after 30 days after sowing. It will helps in keeping the plot weed free as well as for maintenance of soil moisture. Under rainfed condition, hand weeding/ hoeing helps in keeping the plot free from weeds. Whereas, in transitional or coastal tract, weed control is done with the help of hand weeding, hoeing and application of weedicides like 1250 g of diuran 80% or 5 litre of pendimethalin 30%. The quantity of spray solution required per ha is 500-750 litre.

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Cultural method

• Timely weed control in cotton crop is one of the important agronomic practices for increased productivity. Hand weeding is still a practical and efficient method of elimination of annual and perennial weeds (Rao, 1983). Tillage is one of the most effective tool in the control of annual weeds. In many experiments three hand weeding/hoeing proved superior in terms of growth and yield of cotton over other methods i.e., application of herbicides alone and integration of chemical and cultural practices.
• Inter cropping of short duration legume for green manure as much in between wide spaced cotton crop can reduce the weed intensity besides increasing cotton yields.

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Chemical method

Effect of weed control methods on seed cotton and weed control efficiency

• Chemical methods of weed control consists of using herbicides as pre-planting, pre-emergence, post-emergence and in integration with cultural practices.

Pre-emergence/preplanting application

• Pendimethalin (a) 1 to 1.52 a.i ha is an effective pre-planting or pre-emergence herbicide to control nearly 75% of weeds including annual grasses.

Foot Note: Pre-planting application refers to applying herbicides 10-15 days prior to sowing and incorporation by blade harrow. Pre-emergence application refers to applying herbicides on the day of sowing or within 2-3 days after sowing.

• Diuron (a) 0.75 to 1.0 Kg ai/ha could also be used a pre-emergence heerbicide in case the field is infested with more broad leaved weeds than annual grasses.
• Pre-emergence application of Fluchloralin @.3.5 Kg/ha, Nitrofen @ 1.5 Kg/ha or alachlor @ 3.5 Kg ai/ha are also effective in controlling the weeds in most sensitive first 5-6 weeks of the crop.
• In general, pre-emergence/pre-planting application of herbicides is more effective than post emergence application – as the former can provide weed free condition in the initial vulnerable period of 5-6 weeks after sowing.

Post-emergence application

• Directed application of paraquat @ 0.60 to 0.75 l a.i. ha-1 results in excellent control of annual weeds . Application of diuron @ 1.0kg a.i. ha-1 + paraquat @ 0.25 l a.i. ha-1 recorded lower weed population, when applied at 0 days after sowing. Fluazifop butyl at 0.5 kg a.i. ha-1 control the weed competition effectively at the critical stages of crop growth and there by enhance the growth and yield of cotton. Paraquat @ 2 l/ha as post-emergence (POE) would give good control of weeds in later stages and thereby increasing the seed cotton yields.

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Integrated method of weed control

• Integrated weed management is the use of all feasible control methods including the use of herbicide (Hill, 1982).
• Application of pre-planting/pre-emergence herbicides are not very effective for a long period and there will be quick regeneration of weeds in later stages.
• One hand weeding/hoeing in combination with herbicides results in effective weed control. Application of diuron or Pendimethalin integrated with hand weeding at 40 days will have weed free condition and sizable increase in seed cotton yield.
• Diuron with one interculturing reduces the weed population and increases the yield by 82 per cent compared to unweeded check. So intercultivation/hand weeding along with pre emergence herbicide application is more effective than herbicide alone and give good yields and weed control on par with normal manual methods of weed control.

Effect of herbicides on crop growth and yield

• In case of diuron applied as pre-emergence, some abnormal symptoms like curling of leaves and chlorosis are noticed about 7 to 10 days after germination. Similar observations are made at ARS, Dharwar with higher dose of Pendimethalin, there will be thickening of cotyledon leaves, curling of leaves and elongation of petiole. These symptoms persisted for about 30 days of sowing. Further growth and development of plants is normal in both the cases.
• Application of paraquat as post-emergence causes temporary localized foliage chlorosis or burnings without yield reductions.

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