Associated impacts of common weeds on Bacillus thuringiensis (Rt) cotton: a case study of Mwea, Kenya

Abstract

Cotton (Gossypium hirstum L.) is an important cash crop in Kenya but its yields are low due to various constraints including weeds, pests and diseases. Cotton is especially sensitive to weed competition because it grows relatively slowly in the early stages, and does not reach full ground cover until eight or more weeks after germination. The effects of weeds on the cotton crop can be caused by competition for light, water and nutrients, and will depend on the type and density of weed growth. Weeds also act as alternative host for insect pests that attract the cotton plants. The African bollworm, if not properly controlled, is an important pest responsible for close to 100% yield loss. Although Bt cotton is known to affect insect pests, not much is known about its competition with weeds. It is however postulated that unlike conventional cotton, Bt cotton may out compete weeds as it is believed that it has a competitive advantage than conventional cotton over weeds because of the fact that it would not be attacked by pests and would have a head start in the early growth stages. The main objective of this study was to establish the effect of weeds on cotton growth and to evaluate the competitive ability of Bt cotton against common weeds in Kenya. This study therefore sought to find whether and when this competitive advantage was achieved under field conditions, and the role of weeds in harbouring pests of cotton. The experiment used a completely randomized block sampling regime using weeded and non-weeded plots. Growth parameters ( namely plant heights, number of bolls, number of leaves and number of attacked leaves) of Bt and conventional cotton plants were assessed under both weedy and weed free conditions during the growing season. There was no significant (p>O.OS) difference in number of leaves, bolls and pest damage between Bt and non-Bt cotton (HART89M) varieties. HART89M had higher (P<O.OS) final height compared to Bt cotton variety. The difference in plant growth parameters between weeded and non-weeded plots was found to be significant (P<O.OS). The common weeds found were Commelina benghalensis (mean density = SO.041±1.79), Waltheria indica (mean density = 49.416±1.34) and Cleome monophylla (mean density = 49.2S±0.8S). Generally it was concluded that Bt cotton had no competitive advantage over weeds compared to conventional cotton. This conclusion has implications in cotton stewardship programmes in that farmers need to understand that Bt cotton, thuogh is not attacked by bollworms, requires the necessary number of weedings in order to avoid losses caused by weeds