Declining of soil fertility, especially deficiency of nitrogen is one of the major factors adversely affecting crop productionin Ethiopia. Though the problem can be addressed through application of inorganic fertilizers, the unprecedented increase in the cost of fertilizer is increasingly limiting the use of this input. Increasing N-availability through Bradyrhizobium-legume symbiotic N-fixation process is a viable alternative for improving N-nutrition of crops. However, there is a need for identifying effective Bradyrhizobium strains compatible with specific varieties of leguminous crops. Therefore, this study was conducted to investigate the effect of differentBradyrhizobium strains on growth, yield and yield components of Cowpea (Viginaunguiculata(L.)Walp)varieties at Hawassa. A factorial combinations of four treatments [three Bradyrhizobium strains GN100, GN102 and MB140) and a positive control], and five Cowpea varieties (Bole, Black eye bean, TVU, Assabot and Wonder) were laid out in RCBD with three replications. The plot size was a 2.2m x 3 m and cowpea seeds were planted with inter and intra row spacing of 50cm by 20cm respectively. Data on nodule number, nodule volume, nodule dry weight and agronomic parameters like shoot dry weight, leaf area index (LAI), plant height, number of branches, pods, seeds per pod, grain yield, biomass yield, harvest index (HI), 100 seed weight, total plant N and some soil chemical parameters(OC%, P, N&pH) were collected and subjected to ANOVA.Inoculaton with Bradyrhizobiumsignificantly increased the nodulation parameters, Agronomic performance, yield and yield components. Nodule parameters yield and yield components of the five Cowpea varieties inoculated with strain GN102 were significantly superior followed by GN 100 and MB 140. On Black eye bean and Assabot varieties, Strain GN102 increased the grain yield by 14 and 9%, respectively relative to the positive control. Cowpea varieties also varied significantly in their performance with respect to nodule parameters, yield components and total nitrogen content. Accordingly, Black eye bean variety performed best relative to the other four varieties. Bradyhizobia by varieties interaction effects were also significant in terms of yield and yield components. Compared to before planting,strain GN100, GN 102 and MB140 increased soil N by 51.6, 46 and 40%, respectivelyafter planting.Despite the commonly reported non responsiveness of Cowpea for inoculation, the result presented here indicated the potential that exists to increase the yield of the crop through selecting and inoculating with effective Bradirhizobial strain. Keywords: Biological N-fixation,Bradyrhizobium strainsCowpea, inoculation, Nitrogen, nodulation and Soil fertility declin
