Legume hosts differ in the range of partners with which they form symbioses. At one end of the spectrum are legumes such as chickpea (Cicer arietinum) which nodulate with a restricted number of rhizobial strains or species and are thus considered as specific in their rhizobia requirement. At the other end is cowpea (Vigna unguiculata) which is considered as the most promiscuous (or non-specific) of the grain legumes, nodulating with a wide range of both fast- and slow-growing rhizobia. Similar relationships exist among rhizobia although rhizobia that were isolated from tropical soils had previously been grouped as members of the ‘cowpea miscellany’ because cowpea is a test plant nodulated by many poorly characterized slow-growing rhizobial strains (Giller, 2001). It is now clear that rhizobia in the tropics constitute a highly diverse group of both fast and slow- growing types with a wide range of symbiotic specificities. Legumes that have a specific requirement for rhizobia (e.g. soybean, chickpea) need inoculation. This can be achieved using commercially available rhizobial inoculants already tested in African conditions. However, inoculant use in smallholder farming systems of sub-Saharan Africa is generally low due to the poor presence of inoculant infrastructure and expertise in the region.
The global market for rhizobial inoculants is dominated by soybean, due to its strong requirement for inoculation. The specific nodulation requirement of most commercial varieties of soybean arose because they were bred largely in North America where indigenous rhizobia compatible with soybean were absent. This has led to inoculation with soyabean being the standard practice in North America. The North American breeding program was highly successful in increasing potential grain yield of soybean such that materials from North America have been used in many other countries. Promiscuous, multipurpose varieties of soybean have been selected in Nigeria by IITA by crossing high-yielding varieties from North America with more promiscuous materials from Asia (Giller, 2001). In southern Africa, highly promiscuous varieties were identified from early introductions of soybean that originated from China (Mpepereki et al., 2000). In areas where inoculants are not readily available the promiscuous varieties have a particular niche for smallholder farmers.
Bean is relatively promiscuous in its rhizobial requirement but often forms nodules with ineffective strains and thus may respond to inoculation. Earlier studies showed consistent small responses to inoculation that gave a mean yield increase of around 8% across 30 trials in the highlands of East Africa (Amijee et al., 1998; Giller et al., 1998). With improved quality inoculants and more effective strains that can readily be identified (Figure 2) we are confident it is worthwhile to invest in research to develop inoculant technologies for beans.
Figure 2. Yield response of soybean varieties to inoculation with rhizobium in a smallholder farmers’ field in Zimbabwe. Sonata and Solitaire are newer, specific varieties from Seed Co, Roan and Nyala are older, specific Seed Co varieties. Magoye and Local are older, promiscuous varieties (Kasasa, 1999).
The other grain legumes (cowpea and groundnut) are promiscuous and unlikely to respond to inoculation unless inoculated with selected strains applied in high concentrations. In these cases increasing BNF inputs in smallholders’ fields will be achieved by legume selection, agronomic approaches and increasing the area cultivated. Potential for selection of better strains that may result in inoculation responses will be explored through research.