Evaluating the effect of increased rates of rhizobial inoculation on grain legume productivity

Abstract

Intrinsic promiscuity in cowpea and bean enables plants to nodulate with native rhizobia, though sometimes ineffective rhizobia may occupy nodules, resulting in poor response to inoculation. Field trials were conducted from 2014 to 2017 in Marondera, Natural Region II, Zimbabwe, to determine the effect of increasing inoculation rates on legume growth parameters, nitrogen uptake and grain productivity. Treatments included an un-inoculated control and inoculant rates of ×1 (standard), ×2, ×3, ×4, ×5, ×7 and ×10 for both cowpea (rhizobia - inoculant-strain-MAR 1510) and bean (rhizobia-inoculant-strain-CIAT 899). Biomass productivity ranged from 2.05 (×2) - 2.94 t ha⁻¹ (×4) and 1.10 (×10) – 1.95 t ha⁻¹ (×4) for cowpea and bean, respectively. Nitrogen uptake increased with increasing inoculation rates reaching up to 57.56 kg N ha⁻¹ for bean (×4) and 100.20 kg N ha⁻¹ for cowpea (×3). The uninoculated control was not significantly different from the standard, {(×1); 1 g inoculant 500 g seed⁻¹} treatment, for cowpea nodule weight and grain productivity. The highest cowpea and bean nodule weights were recorded from the ×3 and ×4 treatments, respectively, in the first season. Cowpea grain yield significantly varied across treatments, ranging between 0.63 and 1.55 t ha⁻¹ with the ×3 recording the highest yield. The “×4” treatment recorded the highest bean grain productivity reaching up to 0.88 t ha⁻¹. It can be concluded that, increasing rhizobia cells concentration per unit seed up to ×3 (cowpea) and ×4 (bean) improves response to inoculation and grain productivity suggesting a need to change product formulation or increase inoculation rate.