Effects of cropping system and Cowpea variety (Vignaunguiculata L. Walp) on the diversity of native cowpea bradyrhizobia and millet yield in the Sudano Saheli anzone of Mali

Cowpea is an important food grain legume in the Sudano Sahelian zone of Mali. So far, an attention has not been addressed to the diversity of bradyrhizobia nodulating cowpea in this area. In order to fill this gap, effects of 3 cowpea varieties (IT89KD-374, CZ1-94-23-1 and CZ11-94-5C) on the diversity of native bradyrhizobia has been studied in 3 cropping systems from 2007 to 2008 at Cinzana Agronomic Research Station. The objective of the study was to identify effective bradyrhizobia strains nodulating cowpea and their impact on the intercropped or subsequent millet yield in the main cropping systems. Experimental design was a Randomized Complete Block Design using a factorial combination of 3 cowpea varieties and 3 cropping systems in 4 replications. PCR-RFLP and sequencing of the 16S - 23S of rDNA IGS were used to identify bradyrhizobia strains associated with cowpea varieties. Six IGS types have been identified. IGS type I (strain Bradyrhizobium sp.ORS 3642) was the most competitive regardless of cropping system and cowpea variety. The greater diversity averaged bradyrhizobia was found in variety IT89KD-374 (H' means = 0.39). Best millet grain yield in intercropped system (1520 kg ha-1) was obtained with cowpea variety IT89KD-374. The best above ground dry biomass production of subsequent millet (6 210 kg ha-1) has been obtained under CZ11- 94-5C - millet rotation. Types IGS IV and VI (ORS 3637 and STM 3378 strains respectively) appeared to be responsible for the performance of the Cowpea-millet rotation and cowpea variety CZ11-94-5C.