Effect of varying soybean planting density on.maize (zea mays) and soybean (glycine max) yields in a maizesoybean Intercrop system in western Kenya
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A study to determine the effect of varying soybean planting density on maize and soybean yields in maize-soybean intercrops was carried out in Butere and Mumias districts of western Kenya during the short and long rain growing seasons of 2011 and 2012 respectively. To identify the predominant crops and understand the associated production practices in the smallholder fanning system a survey was carried out. Field experiments were laid out in a Randomized Complete Block Design (RCBD) with a factorial arrangement replicated three times. Three maize: soybean planting densities 1:1(Dl), 1:2 (D2) and 1:3(D3), representing rows of maize and soybean respectively, were tested. Three soybean varieties: NAMSOY 4m, SC SQUIRE and TGx 1987- 18F intercropped with imidazolinone- resistant (IR) maize were grown with sole soybean (D4) and sole maize (SM) included as controls. At harvest of the intercrops, soil pH, total Nand available P were assessed followed by maize rotation trial. Above and below ground biomass of soybean, nitrogen fixation, grain and dry matter yields of maize and soybean and land equivalent ratio were also measured. At completion of the study analyses of the economic analyses, net benefit, incremental net benefit and benefit cost ratio of the intercrop and rotation trials were carried out. The survey revealed low planting densities of legumes in the maize-legume intercrop system (1: 1 and 1:2) which is the predominant practice in Butere and Mumias districts of western Kenya. Fanners also adopted legumes cereal rotation but the population of legumes in the preceding season was low, hence its benefits; on subsequent maize crop were not fully exploited. Besides beans, fanners were growint:s.P'Ybeans due to their high nutritive value and selling price. Higher above and below ground biomass and N accumulation in the intercrops were recorded in D3 for soybean variety TGx 1987-18F. The D3 for variety SC SQUIRE recorded high soybean grain and dry matter yields. Maize grain and dry matter yields were significantly different across planting densities with .01 recoding higher yields. Land equivalent ratio of the intercrops in the short rains season was higher in D2 for soybean variety SC SQUIRE and in the long rains season in D3 for same variety. No significant differences were detected on soil pH, total N and available P before the experimental setup and after harvest of the intercrops. Maize succeeding D3 for soybean variety TGx 1?87 -18F recorded the highest grain and dry matter yields. The D3 for soybean variety SC SQUIRE succeeded by maize recorded the highest net benefit and benefit cost ratio. From the results of the study it was concluded that maizesoybean intercrops with high soybean population should be planted during the short rains season and rotated with maize in the long rains season. Planting density D3 for soybean variety SC SQUIRE is the recommended technology to improve soybean biomass production and N accumulation, increase yield of subsequent maize and provide high cash incomes to the smallholder fanners. However, D2 for soybean variety SC SQUIRE is also an attractive option as it provides high yields of both maize and soybean. A similar study involving control plots of sole maize and soybean in same plant populations as in the different intercrops is recommended for better comparison and evaluation of the effectiveness of the-intercrops. Key words: Intercropping, rotation, planting densities, biomass· and N accumulation, land equivalent ratio, residues incorporation, economic returns.