Production of large biomass yields and weed suppression from cover crops have been major constraints affecting success and uptake of conservation agriculture technologies by smallholder irrigation farmers. A field study was undertaken to evaluate biomass accumulation and N uptake by oats (Avena sativa), grazing vetch (Vicia dasycarpa), faba bean (Vicia faba), forage peas (Pisum sativum) and Lupin (Lupinus angustifolius) and their winter weed suppression efficacy in the 2007 and 2008 winter seasons. Cover crops were grown at two fertiliser levels: no fertiliser and fertilized. Control plots were included where no cover crop was grown. At the end of each winter season, glyphosate was applied to kill the cover crops and maize planted. Oats, grazing vetch and forage pea’s cover crops produced mean dry weights of 13873, 8945.5 and 11073 kg ha-1 respectively while lupin had the lowest dry weight of 1226 kg ha-1. Oats responded to fertilisation while, there was little or no response from the other cover crops. Oats and grazing vetch also reduced weed density by 90 and 80% respectively while lupin only reduced weed density by 23% compared with the control plots. Grazing vetch fixed a mean of 112 kg N ha-1. The results suggest that legumes such as grazing vetch and forage peas may be grown to maximise biomass yields with minimal fertilizer inputs. Amount of biomass produced was a major factor in controlling winter weeds, while there was a progressive decline in the winter weed burden from the first to the second season. The low C: N ratio of grazing vetch (<15) and its high N content may make it attractive for resource-limited farmers in warm-temperate climates.
Key words: Biomass yield, conservation agriculture, cover crops, weeds dynamics.
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