Grain legumes in pearl millet systems in northern Namibia: An assessment of potential nitrogen contributions

A nutrient-balance model was used to investigate the nitrogen contributions of cowpea (Vigna unguiculata) to pearl millet (Pennisetum glaucum) intercropping systems in semi-arid northern Namibia. Data on nitrogen fixation, production, crop nitrogen off-take and competition effects came from two seasons of fieldwork. Supplementary data were taken from secondary sources. The model was used as a tool to attempt to identify grain legume management options with the potential to make significant contributions to soil fertility. The crop parameters pearl millet grain yield, nitrogen fixation rates, nitrogen harvest index and biomass production were found to be critical in determining system nitrogen inputs and outputs as was the form of residue management. The model indicated that it is extremely difficult to manage grain legumes in dryland environments in ways that lead to consistent increases in pearl millet grain yields, measurable against season-to-season variation due to other factors. Several of the options for improved legume management conflict strongly with farmers' risk-avoidance strategies and their tendency to invest preferentially in off farm activities in an environment where livelihoods have diversified considerably away from agriculture. Potential routes for increasing grain legume contributions to soil fertility in dryland areas are discussed.