Effect of roundup® (glyphosate) on diversity and abundance of weeds in cotton fields in Mwea, Kenya

Abstract

Weeds interfere with cotton production by competing for important resources including light and nutrients. Several approaches are taken to control weeds in order to achieve maximum yields. Where genetically modified cotton exhibiting resistance to Roundup'" have been commercialised, Roundup'" has been extensively used In controlling weeds. There, are however, reports of weeds developing resistance to Roundup ® and interference with weed diversity where Roundup ® has been used for long periods. In this study we tried to mimic a glyphosate tolerant field by covering convectional cotton variety Hart 89M with polythene papers to protect cotton plants from herbicide effect during spraying. The objective of this study was to investigate the effect of glyphosate on weed diversity and abundance, the presence of glyphosate tolerant species and effect of weeds on growth of cotton. This study was important because the country is embarking on reviving the cotton industry by use of modern technology like Bt (Bacillus thuringiensis) cotton and Herbicide resistant cultivars that would reduce the amount of labour for weed control. Information on effect of Roundup'" on weed diversity is not available in Kenya so this study will help In determining if it is feasible to popularise the use of herbicides particularly Roundup'f for weed control. Weed diversity was sampled using three 0.5 x 0.5m quadrats before and after treatments in each subplot. The treatments included in this study were Roundup'" spray (21/ha), hand weeding untreated check and natural vegetation. The experimental design was a split plot design with the treatments as the main plots and the timing of treatment as the subplots. The R-program was used for data analysis and diversity was analysed using the Renyi diversity index. A total of 43 weed species was recorded before spraying but this reduced to 30 species after spraying. The most abundant species were Euphorbia geniculata, Spermacoce laevis, Digitaria velutina and Bidens pilosa while the others were trace. The diversity and abundance of weeds decreased significantly after spraying with Roundup'[ 9 weeks after germination. The results obtained show that Roundup'f is effective in controlling most of the weeds in Mwea where only Commelina benghalensis exhibited tolerance 21days after spraying. Early hand weeding and spraying with Roundup'[ reduced weed density which in turn resulted in taller cotton plants (108.14±O.687 and 104.39±O.950cm), more squares (8.19 and 6.43 per plant) and higher productive bolls at (8.26 and 6.13 per plant). From this study it is evident that early weed removal is necessary if a farmer is to realize maximum yields. Spraying with Roundup'f reduced the diversity and abundance of most weeds but the time of herbicide application is also important