An experiment was conducted on four smallholder farms and on-station in the sub-humid zone of northeast Zimbabwe for 12 years to assess how the rotation of groundnut with NPK-fertilized and unfertilized maize affected the productivity and sustainability of this common smallholder cropping system. At the sandy-soil Domboshava station, maize grain yield declined over 12 years of continuous maize cropping, and the rate of decline was larger when fertilizer was used. Maize grain yield was already low (around 0.7 t ha-1 without fertilizer) on the smallholder farm fields when the experiment began, and there was little evidence of further decline. A 3-year groundnut plus maize plus maize rotation raised maize grain yield on station both when fertilizer was used on maize and when not, with some benefits persisting into the second year of maize after groundnut. Three cycles of the rotation with fertilizer at Domboshava increased maize yields by 0.21, 2.92 and 2.26 t ha-1 in the first year after groundnut. With unfertilized maize, grain yields rose by 2.15, 1.52 and 3.61 t ha-1, which was double or more than double those from continuous maize plots. Accumulated over three rotation cycles (nine years) on station, the rotation gave 3.54 t ha-1 (13.2%) more maize grain than continuous maize with fertilizer and 5.33 t ha-1 (42.2%) more when fertilizer was not used, as well as almost 1 t ha-1 of groundnut grain. On the farms, overall yields of groundnut and maize were much smaller, because soils and management were poorer, and inputs few. Groundnut crops averaged less than 0.13 t ha-1 grains. The rotation raised maize grain yield only when no fertilizer was used on maize, where three cycles of the rotation raised maize grain yields by 0.21, 0.38 and 0.32 t ha-1. Accumulated over three rotation cycles, the rotation without fertilizer gave 0.51 t ha-1 (15.1%) more maize grain than continuous maize on the farms, and 0.4 t ha-1 of groundnut grain. It was concluded that the rotation of groundnut with maize can sustain the productivity of smallholder maize systems in sub-humid northeastern Zimbabwe in moderately fertile station conditions and can contribute on nutrient-depleted smallholder fields even when the crops are grown without fertilizer and with few management practices.
Key words: Groundnut, maize, crop rotation, soil infertility, longer-term experiment, on-farm research, sustainability, sub-humid zone, Zimbabwe.
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