Soyabean response to rhizobial inoculation under contrasting smallholder agro-ecological conditions in Zimbabwe

Abstract

Soyabean is grown widely with rhizobial inoculants on commercial farms in Zimbabwe, but its inoculation response in smallholder cropping environments is poorly characterized. Symbiotic interactions of six rhizobial strains (MAR 1305,1306,1326,1494, 1497 and 1515) from the Grasslands Rhizobium Collection were evaluated under contrasting soils and rainfall zones (natural region) in Zimbabwe. The rhizobial strains were evaluated on one specific and three promiscuous soyabean varieties (SC Saga and TGX (1740-2F/ 1987-628/ 1987-11E). The study was conducted over two rainfall seasons namely 2011-2012 and 2012-2013.  Field experiments were set up as a randomized complete block design with two factors (soyabean variety and rhizobial strain), a negative control without inoculation and a positive control inoculated with commercial inoculant strain MAR 1491. All treatments were replicated three times. Nodulation was significantly (P< .0001) influenced by rhizobial strain inoculation with nodulation ranging from 28 to37 nodules per plant. Over the two seasons, soyabean inoculated with MAR 1305, 1306, 1494 and 1515 consistently gave higher grain yield higher (average of 600kg/ha) compared to that inoculated with commercial inoculant strain MAR 1491 (average of 400kg/ha). Nodulation of both promiscuous and specific varieties decreased with annual rainfall amounts in the order: Natural Region (NR)II> NRIII> NRIV. Rhizobial strains MAR 1305, 1306 and 1494 were superior in NR II and NRIII while rhizobial strain MAR 1515 was superior in NR IV. Our results show that identification of superior strains best suited to smallholder cropping environments and inoculation response are crucial for increasing soyabean productivity in these environments.