Weed management in semi-arid agriculture: application of a soil moisture competition model

Weed water uptake is a key competitive factor in semi-arid, low-input farming. Smallholder farmers in Zimbabwe have long recognized the need for good weed control, both during the cropping and the dry season, as timely tillage restricts weed growth and transpiration losses. However, access to draught animal power (DAP) determines a farmer's ability to winter plough and to carry out timely planting and weeding operations. Unfortunately, computer simulation studies to date have often failed to relate crop responses to management restrictions and weed competition. A weed management and water competition routine was developed, to simulate the effect of weed management on crop performance. This routine was incorporated in the PARCHED-THIRST crop growth model and calibrated with results from a 2-year weeding trial. The effects of weed competition on crop performance were assessed over a 30-year simulation period, for four levels of access to DAP and labour. The results demonstrate how sub-optimal crop management affects weed competition and crop production. The timely access to sufficient DAP and labour is particularly crucial during planting operations, as a delay in planting can severely reduce crop performance.