RESPONSE OF FOUR COWPEA VARIETIES TO BRADYRHIZOBIAL INOCULATION IN SOME SOILS OF NORTHERN GUINEA SAVANNA ZONE OF NIGERIA

Abstract

Yield of cowpea in savanna soils of sub-sahara Africa has remained low as a result of less effective native rhizobia, necessitating the use of elite inoculant strains to improve Biological Nitrogen Fixation. The objective of this study was to observe the response of four varieties of cowpea to inoculation by two elite strains of bradyrhizobia in some soils of Northern Guinea Savanna (NGS) zone of Nigeria. Two greenhouse experiments were carried out at the Teaching and Research Farm of the Federal University of Technology Minna. To determine (1) the size of the native rhizobial population in the soils using the Most Propable Number (MPN) technique and (2) The Need to Inoculate cowpea in the soils under study. The treatments for the MPN experiment consisted of (i) soils in area of close proximity to homestead (< 50 m), and those far from homestead (> 250 m) and (ii) cowpea varieties (Kanannado, IT93K-452-1, IT97K-499-35, and IT90K-277-2). The experiment was laid out in a 2 x 4 factorial in a completely randomised design (CRD) replicated four times. Treatment for the need to inoculate experiment consisted of (i) five locations (Rijau, Kontagora, Magama, Mashegu, Mariga,), in the NGS zone of Nigeria (ii) four nitrogen sources (plants inoculated with Bradyrhizobium sp. strain BR 3262, Bradyrhizobium sp. strain BR 3267, control (neither N nor inoculant rhizobia applied), and plants treated with N in form of urea at the rate of 100 kg N ha-1, (iii) four cowpea varieties, (iv) two levels of proximities to homestead. Proximity to homestead and cowpea varieties were the same as in the MPN experiment. The experiment was laid out in a 5 x 4 x 4 x 2 factorial fitted to a completely Randomised block design (CRD) replicated four times. In general, the MPN value ranged from 4.61 x 106 to 7.19 x 107 cells g-1 of soil. MPN value for IT90K-277-2 variety (9.27 x 106 cells g-1) was significantly lower than that estimated by the other varieties Kanannado (3.31 x 107 cells g-1), IT93K-452-1 (1.76 x1 07 cells g-1), and IT97K-499-35 (1.42 x 107 cells g-1). In spite of the high population of the native rhizobia in the soils, the cowpea varieties responded to inoculation suggesting the effectiveness of the elite bradyrhizobia strains used. Proximity of sampling site close to homestead had significantly (P < 005) higher Shoot dry weight than those away from homestead. The results from this experiment showed that the main effects of Nitrogen sources and varieties had significant effect on shoot dry weight of cowpea in the all locations. The shoot dry weight of plants that received 100 kg N ha-1 was marginally higher than those of the inoculated plants, indicating that elite rhizobia strain only partially met the N need of the cowpea crops. Further studies are suggested to evaluate larger number of bradyrhizobial on this environment.