Date Published: July 01, 2018
Author(s): A.C. Franke, G.J. van den Brand, B. Vanlauwe, K.E. Giller.
•We retrieved 44 publications and 199 observations comparing continuous cereal with grain legume-cereal rotation in SSA.•Cereal after legume yielded on average 0.49 t grain ha−1 or 41% more than continuous cereal.•Sustained residual benefits of legumes with large N applications indicate the importance of non-N effects.•Relevant non-N effects include improved P availability, changes in SOM, and in pest, disease and striga pressure.
Diversification and intensification through inclusion of grain legumes in cereal, root or tuber based cropping systems represents a key technology in the drive towards the sustainable intensification of agriculture in sub-Saharan Africa (SSA) (Vanlauwe et al., 2014). Grain legumes fix atmospheric nitrogen gas (N2) that can contribute to the nitrogen (N) economy of fields, provide other rotational benefits to subsequent crops, produce in situ high-quality organic residues with a high N concentration and a low C to N ratio, and thereby contribute to integrated soil fertility management (ISFM) (Giller, 2001, Vanlauwe et al., 2010). Their protein-rich food and feed products have a good market demand in SSA where marketing channels are available (Chianu et al., 2011). The wide range of grain legume crops and varieties with different growth durations and other characteristics suggest that legumes have a potential niche in a wide range of farming systems in SSA. Legume production may be enhanced by replacing cereals or other non-legume crops, by intensifying crop production (instead of fallowing land or including legumes as an intercrop with cereals), or by expanding the area of farmland. Quantifying the rotational effect of grain legumes on subsequent crops is important for understanding the adoption potential of legume technologies as well as their impact on sustainability of production. Grain legumes often yield less and demand more labour than cereal crops due to labour-intensive manual harvesting, threshing, weeding and sowing practices (Franke et al., 2010, Ojiem et al., 2014). The rotational effects of legumes on cereal yields may nevertheless make legume-cereal rotations more attractive in terms of productivity and economic performance than continuous cereal cropping (Franke et al., 2014). However, the impact of legumes on subsequent cereals is highly variable, depending on soil fertility status, agro-ecological conditions, crop type and management, which in turn are affected by farmers’ diverse socio-economic conditions (Ojiem et al., 2006). Quantifying and understanding the variability in rotational benefits will help in the tailoring of legume technologies to environments in SSA where they work best.
Our review of the literature shows that rotational benefits of grain legumes on cereal yields are substantial and widespread. While there may be a positive bias in reporting, the consistent responses at multiple sites differing in climate or soil fertility provide strong support that cereal yields are enhanced after grain legumes. The residual effects of grain legumes should take a prominent place when evaluating the pros and cons of sustainable intensification with grain legumes. We did not consider some of the wider cropping and farming system effects of grain legumes in this review. For instance, grain legumes can make up an important component of an ISFM strategy increasing overall crop productivity and soil fertility beyond N benefits (Vanlauwe et al., 2010, Vanlauwe et al., 2015). While grain legumes on their own are unlikely to increase soil organic matter contents (see Section 3.2.4), increased residue inputs as a result of a successful ISFM strategy can do so. Residues of grain legumes also play an important role in integrated crop-livestock systems, which offer opportunities and challenges to optimise natural resource use and organic input applications to crops (Duncan et al., 2016). We did not consider grain legume-cereal intercropping systems which are common in parts of SSA (e.g. common bean-maize mixed cropping in East Africa and cowpea relay cropping in cereals in West Africa). While the N2-fixing capacities of grain legumes are maintained in an intercrop, though at a lower rate due to competition with the cereal, the benefits of grain legumes as a non-host of pests, diseases and striga in cereals could be strongly reduced.