Date Published: July 01, 2018
Author(s): Joost van Heerwaarden, Frederick Baijukya, Stephen Kyei-Boahen, Samuel Adjei-Nsiah, Peter Ebanyat, Nkeki Kamai, Endalkachew Wolde-meskel, Fred Kanampiu, Bernard Vanlauwe, Ken Giller.
•The effect of inoculation was evaluated in 2082 on-farm soyabean trials across Africa.•Significant but moderate responses were observed.•Variability was high and largely unexplained by considered environmental factors.•Promiscuous varieties had similar yields but lower responses than specific types.•Strong responses coincided with better uninoculated yields of promiscuous varieties.
Increasing production and productivity of grain legumes are widely recognized as important components of sustainable intensification strategies for sub-Saharan Africa (SSA) (Vanlauwe et al., 2014). The ability of legumes to use atmospheric nitrogen fixed by symbiotic rhizobial bacteria, offers the potential for improving yield without nitrogen fertilizer. In soyabean, nitrogen fixation can only occur in the presence of compatible bacterial strains, typically of the genus Bradyrhizobium. Soyabean is an Asian crop with a relatively brief history of cultivation in Africa (Mpepereki et al., 2000). Many improved varieties developed for SSA derive from North American stock and were thought to establish symbiosis with only one Bradyrhizobium species, B. japonicum, that was assumed not to occur natively in African soils (Kueneman et al., 1984). Field tests performed in the 1970s and 80 s (Chowdhury, 1977, Nangju, 1980, Pulver et al., 1982, Pulver et al., 1985, Kueneman et al., 1984) suggested that the absence of compatible B. japonicum in African soils may limit nitrogen fixation and productivity of such varieties.
Improving rhizobial nitrogen fixation in grain legumes is expected to offer tangible benefits to smallholder farmers in terms of increased production and enhanced residual soil fertility for subsequent crops (Giller and Cadisch, 1995). In soyabean, using productive varieties that can establish effective symbiosis and inoculating with elite rhizobium strains are possibly the most cost-effective ways of achieving such improvement. Our results confirm that on average, inoculation has a positive effect on grain yield, both for specific and promiscuous varieties, although average responses were moderate compared to earlier estimates from SSA (Pulver et al., 1982, Ronner et al., 2016) and the benefit of inoculation was minimal in some specific cases.