Effects of soil incorporated faba bean and soybean biomass on yields of subsequent maize in Western Ethiopia

The decline of soil fertility is severe in western Ethiopia due to inappropriate cropping practices (monocropping, unbalanced nutrient application, continuous cultivation, removal of crop residues from the fields and suboptimal of fertilizer application). Hence, maize (Zea mays) productivity is low. The low productivity of maize can be reversed by use of biomass of faba beans and soybeans, popularly grown in the areas, in addition to use of nitrogen fertilizers. However, information on the effects of these practices farmers’ fields in western Ethiopia is sparse. The overall goal of this research was to investigate the contribution of biologically fixed N2 from precursor faba bean and soybean biomass together with nitrogen fertilizer on yields a subsequent maize crop in western Oromia. Field experiments were conducted on farmers’ fields and on station in Oromia National Regional State, western Ethiopia. The on-farm trials were conducted on the Ultisols of Toke Kutaye district, representing the highland ecology. The on-station trials were conducted at Bako Agricultural Research Centre, which also represent mid altitude sub-humid agro-ecosystems. Soil characterization was conducted on four maize farmers’ fields in Bako Tibe and Toke Kutaye districts of Western Ethiopia. The soybean (Glycine max) and faba bean (Vicia faba) precursor crops, without and with rhizobia inoculation, were grown at three sites in the 2013 cropping season. The soybean variety Didessa (medium maturity) and faba bean variety (Moti) were used. Rhizobia strains SB-12 and FB-1035 for soybean and faba bean, respectively, were used, at the rate of 10 g per kg of seed, and then pelleted with sugar to insure attachment of the inoculants with the seeds. In 2014, the faba bean and soybean precursor crops were incorporated into their respective soils, plots demarcated and experiments were laid out in randomized complete block design (RCBD) in a 2x2x3 factorial arrangement, with three replications. Two faba bean farmers’ fields and one soybean field were used. The factors were “without and with rhizobia inoculation”, “two maize varieties”, and “three N levels (0, 55 and 110 kg N ha-1)”. The two maize varieties were Jibat and Wenchi for highland areas in Toke Kutaye cropped to faba bean. For the mid-altitude areas cropped to soybean, the site used was the Bako Agricultural Research Centre, and the maize varieties were BH-543 and BH-661. Soil types of the experimental sites were classified as Ultisols. The soil nutrient status was differed among the four maize farmers’ fields indicating the importance of site and soil test based fertilizer recommendation for sustainable maize production. The pH of soil reaction is acidic. The mean grain yields and harvest index were significantly (P<0.05) higher for maize planted following faba bean precursor crop with the application of 55 and 110 kg N ha-1. Significantly (P<0.05) higher mean grain yield maize was obtained from application half recommended nitrogen fertilizer following faba bean precursor crop. Higher agronomic efficiency, fertilizer N (recovery) use efficiency and nitrogen use efficiency of maize were obtained from 55 kg N ha-1 application as compared to 110 kg N ha-1, which matched with higher grain yields of maize. Agronomic studies confirmed increased yields of maize following faba bean precursor crop without and with rhizobia inoculation and applying half recommended rate of nitrogen fertilizer (55 kg N ha-1) in high altitude areas of western Ethiopia. The grain yields of maize were significantly (P<0.05) higher with the application of half (55 kg N ha-1) and full (110 kg N ha-1) recommended rate of nitrogen fertilizer following soybean precursor crop. Planting of maize following soybean precursor crop biomass with half-recommended nitrogen is recommended for increasing maize yields. Higher agronomic efficiency and fertilizer N (recovery) use efficiency were obtained with 55 kg N ha-1 fertilizer application. Production of BH-661 and BH-543 maize varieties following soybean precursor crop with half recommended (55 kg N ha-1) rate improved mean grain yield and is recommended for maize in mid altitude areas of western Ethiopia. Therefore, fertilizer management practices following legumes precursor crop biomass incorporation that increase nitrogen use efficiency and improve yield of maize will likely be more effective and desirable options for the area. The results from this series of studies suggest possibilities for further research work on optimum nitrogen rate, interaction of soybean and faba bean precursor crop biomass incorporation with nitrogen rates in the production of maize in other areas of western Ethiopia.