Weed effects on growth and yield of cowpeas (Vigna unguiculata L. Walp.)

An investigation was carried out at Kawanda Agricultural Research Institute (KARI) to determine the effect of weed management regimes and weed population density on growth and yield of cowpeas (vigna unguiculata L. Walp). Weeds in a cowpea crop were controlled using either a hand-hoe or glyphosate applied once at 30 days after planting (DAP), twice at 15 and 30 DAP and from planting to harvest. The unweeded cowpea was the control treatment. Cowpea was also subjected to different population densities of black jack (bidens pilosa L.), ranging from 0, 8, 16, 24, 32 and 40 plants per m2. This was to monitor the effect of interference by the weed on crop growth and yield. Results showed that the intensity of weeding influenced weed biomass. Higher weed count and infestation was observed in plots which were less frequently weeded. Cowpea growth, measured as leaf area index (LAI) and dry matter accumulation (TDM) per plant, was significantly reduced when the crop was left unweeded for more than two weeks after planting. Plants in unweeded plots, due to severe competition with weeds, were etiolated compared to other treatments. Maintaining weed free conditions throughout and weeding twice at 15 and 30 DAP resulted in better (P<0.05) cowpea growth and grain yield. The highest grain yield was obtained when weed-free conditions were maintained throughout the life of the crop (1280kg/ha) and the lowest in plots where weeding was done only once with glyphosate (271kg/ha). No yield was recorded in the unweeded plots. plants in this treatment were smothered by the weeds. The investigation further revealed that bidens pilosa at a higher population density adversely affected cow peas. Leaf area index, dry matter and grain yield were significantly reduced when blackjack population increased beyond 8 plants/m2. The weed density should not exceed 8 plants/m2 if a crop yield of not less than 600kg/ha is to be expected. The study revealed that frequent weeding of cowpea was uneconomical. Weeding once or twice with a hoe or maintaining weed-free conditions using a herbicide gave the best benefit to cost ratio.