Pre-plant weed control, optimum N rate and plant densities increase butternut (Cucurbita moschata) yield under smallholder irrigated conditions in the Eastern Cape Province of South Africa

Weed management is the most important limiting factor in butternut production by smallholder irrigation farmers in South Africa. Post-emergence chemical weed control options for butternut farms are very limited and often ineffective since most of the registered selective herbicides control annual grasses but not the broadleaf weeds. An on-farm experiment was therefore conducted to investigate the effects of pre-plant weed control (application or non-application of glyphosate to kill the first flush of weeds before planting), nitrogen (N) rate (60, 90, 120, 150 and 180 kg ha-1) and plant density (10, 000, 20, 000 and 30, 000 plants ha-1) on weed biomass and butternut yield. Pre-plant weed control resulted in a six-fold decrease in weed biomass, while increasing plant density from 10,000 to 30,000 plants ha-1 decreased weed biomass by 47%. No marketable fruits were obtained when planting was done without prior weed control. Yield increased significantly with increase in plant density and the optimum density was estimated to be 25,000 plants ha-1. Yield increased with N rate and the rate giving the highest marginal rate of return was 120 kg N ha-1, which gave a yield of 26.7 t ha-1. These findings suggest that pre-plant weed control, and use of optimum N rate and plant density are pre-requisites to successful butternut production.