SYMBIOTIC EFFECTIVENESS OF BRAZILIAN RHIZOBIAL STRAINS IN IMPROVING N-FIXATION AND PRODUCTIVITY OF COWPEA IN MINNA, SOUTHERN GUINEA SAVANNA OF NIGERIA

In a bid to evaluate the effectiveness of two Brazilian rhizobial strains in improving nitrogen fixation and productivity of cowpea (Vigna unguiculata (L.) Walp) in the southern Guinea savanna of Nigeria, field trials were conducted in 2016 and 2017. Four rhizobial inoculation levels (control, inoculation with BR 3262, BR 3267 and application of 90 Kg N ha) and three varieties (IT93K-452-1, IT99K573-1-1 and TVx 3236) were arranged in randomized complete block design. Inoculation with BR 3267 significantly increased nodule number by 27% over the control in 2017 and there was significant variation in the response of the varieties to inoculation in the two years in respect of nodule weight. Plants fertilized with 90 Kg N ha consistently had the least number of nodules, weight and percentage of effective nodules in the two years. N-uptake and shoot biomass yield was however significantly higher in the N-fertilized and uninoculated plants than plants inoculated with the BR strains. Plants inoculated with both strains fixed significantly lower nitrogen than the uninoculated plants. However, the inoculated plants partitioned greater nitrogen to their seeds having significantly higher % protein in their seeds than the uninoculated plants. Plants fertilized with 90 kg Nha had the highest grain yield (1761.2 kg ha) which was at par with the value recorded in the uninoculated plants (1479.60 Kg ha) but significantly higher than the values recorded in the inoculated plants (1320.1-1338.0 Kg N ha). It could therefore be concluded that the BR strains tested in this study are not more effective than the indigenous strain in improving N-fixation and productivity of cowpea in the study area.