Effect of legume-managed fallow on weeds and soil nitrogen in following maize (Zea mays L.) and wheat (Triticum aestivum L.) crops in the Rift Valley highlands of Kenya

The use of legume crops as alternatives to a fallow in wheat-maize rotations in the Rift Valley of Kenya may suppress weeds and add nitrogen to the system. A field study was set up in the Njoro and Rongai regions to evaluate the effect of managing the traditional fallow with chickpea (Cicer arietinum L.), field bean (Phaseolus vulgaris L.), soybean [Glycine max (L.) Merril], field pea (Pisum sativum L.) and dolichos [Lablab purpureus (L) Sweet] on the following wheat and maize crops. The objectives of this study were to assess the impact of improved fallow on weed control, soil nitrogen and performance of the following cereals in a bid to improve their yields. The legumes, along with wheat and maize, were grown as fallow-management treatments during the short-rains season, and their residues were incorporated into the soil before sowing the test cereal in the long-rains season. Weed biomass and species diversity reduced by 35-92% and 25-58% at Njoro, and 10-100% and 83-100% at Rongai, respectively, in the succeeding cereal against preceding traditional fallow. The perennial weeds, which included Digitaria scalarum, Pennisetum clandestinum and Conyza sp., were substantially reduced in the managed fallow. Legume-managed fallow significantly improved soil nitrogen status and the yield of the following cereals. Grain yield in managed fallow increased on average by about 39 and 17% over locations in maize and wheat, respectively. Despite yielding less soil-incorporated residue, the impact of legumes was greater than that of the traditional fallow yielding large amount of organic material. Among the legume species, dolichos showed outstanding positive effect on succeeding cereal crop, probably due to increased soil nitrate levels. Field bean and chickpea have a potential seed advantage, although the latter is susceptible to 100% loss by Helicoverpa armigera. Hence, managing the traditional short-rains fallow with legumes could reduce weed infestation, in addition to improving soil nitrogen for subsequent crops.