Effects of some preemergence herbicides on weed control in cottton (gossypium hirsutum L.)
Some promising preemergence herbicides were evaluated for efficacy and longevity of weed control in cotton at two sites (Tebere and Kibos) within the cotton growing areas of Kenya. None of the herbicides studied showed any injury to the cotton seedlings nor was there any apparent reduction in crop vigour as a result of the application of the herbicides. Loss in crop vigour was attributed to weed competition depending on the degree of weed control and the residual effect of the herbicides tested. The herbicides mixtures provided broader spectrum of weed control than the single herbicide treatments. Mixture of 2-ethyl- 6-methyl-N-(2-methoxy-l-methyl-ethyl)-N-chloroacetanilide (metolachlor) plus 3-(3,4-dichlorophenyl)-1,1-dimethyl urea(diuron) at 1.0 kg active ingredient (a.i.) each per hectare effected excellent control of the grasses and most broadleaf weed species. However, it showed weakness under heavy rains which flooded the fields. The herbicide mixture maintained an effective control of weeds for about eight weeks after herbicide application. Similarly impressive performance were also shown by metolachlor plus 2,4- bisisopropylamino-6-methylthio-l,3,5 triazine (prometryn) at 2.0 kg a.i. per hectare; metolachlor plus 3-(3,4-dichlorophenyl)-1-methoxy- 1-methylurea(linuron) at 1.0 kg a.i. each per hectare, and a mixture of 2-chloro-2',6'-diethyl-N-(methoxymethyl) acetanilide(alachlor) and linuron at 1.0 kg a.i. per hectare. (ii) Seed cotton yield responses were commensurate with the longevity and spectrum of weed control by the herbicides. Mixtures of metolachlor/prometryn at 2.0 kg a.i. per hectare, metolachlor/diuron and metolachlor/linuron at 1.0 kg a.i. each per hectare had seed cotton yield responses of 1807.4, 1423.5 and 1532.7 kg per hectare respectively at Tebere. None of the herbicides remained phytocidally effective beyond eight weeks after application. The highest seed cotton yields (2449.9 kg/ha) were, however, received from cultivated check treatments. At Kibos the performance of the herbicides were confounded by flooding early in the trial. None of the herbicides showed complete control of all weed species found at the experimental sites. There were apparent differences in the competitive abilities of the uncontrolled weed species. This was reflected in the varied yield responses under similar levels of weed pressure.