Abstract:

Dry beans (JPhaseolus vulgaris L.) are major legume crops and an important source of protein in Uganda. Their production is constrained by low soil fertility and weed competition. Field experiment was carried out at Thompson Farm (Freeville Farm), Cornell University in 1995 and 1996, and at Namulonge Agricultural and Animal Production Research Institute (NAARI) in Uganda in the 1997A and B seasons. The goal was to determine the effect of competition between beans and weeds (Galinsoga parviflora Cav., Bidens pilosa L. and Solanum nigrum L.) on bean and weed nutrient uptake and bean yield under low soil fertility. Low levels of nutrients (NPK, OPK, NOK, NPO and OOO) were broadcast applied at 50, 50 and 60 kg N, P and K ha1, respectively. These were combined with bean-weed treatments in a split-plot design with nutrients as the main plots and bean-weed combinations as the sub-plots. Results indicate that high bean dry matter was recorded under full nutrients and under P and N deficit conditions. High weed dry matter was observed in the presence of N and P. Beans are better accumulators of N and the weeds are better accumulators of P, K, Ca and Mg. Bean yield was high under P deficit condition resulting in high total revenue and net profit. Galinsoga parviflora caused 33-42%; S. nigrum 29% and B. pilosa 46-50% yield reduction in beans. Beans are likely to be competitive with weeds under P deficit condition and weeds are likely to be very competitive with beans in the presence of N and P. A screenhouse experiment under NPK, NPO and NOK nutrients indicated that high shoot and root relative growth rates, and high root elongation and growth rates of the weeds contribute to their competitiveness especially with adequate N and P. Beans are more competitive with weeds under P deficit conditions due to early emergence, erect growth for a longer time, and more root volume compared to the weeds. This is likely to contribute to better growth and yield of beans under P deficit condition. A survey was also carried out in Uganda on farmers’ fields to determine the relationship between weeds and soil chemical and physical characteristics and cropping system-related factors. Both soil factors and cropping systems affected weed abundance. It was noted that B. pilosa abundance was positively related to soil P and N while G. parviflora was positively associated with soil Ca and pH. Both weeds were widely distributed and were reported by farmers to cause yield losses to beans of over 30%.