Bridging the grain legume yield gap through agronomy

Robert Abaidoo, r.abaidoo@cgiar.org, Steve Boahen, Anne Turner, and Mahamadi Dianda

Researcher inspecting cowpea pods. Photo by IITA

Researcher inspecting cowpea pods. Photo by IITA

IITA and its partners have made significant progress in breeding grain legumes that are high yielding and drought tolerant, and have better disease and pest resistance as well as consumer-preferred traits, such as seed size, texture, and color. The use of these new improved varieties has contributed to increases in productivity on farmers’ fields across sub-Saharan Africa.

While crop genetics is very important, the key to bridging the yield gap is to capitalize on the yield potential of a particular genotype and know how to manage it to maximize productivity in challenging environments. This is where the role of an agronomist becomes apparent: to design an integrated management system that reduces the effect of the biotic and abiotic stress factors limiting the productivity of a selected genotype in a given agroecology.Streaming and download Doctor Strange (2016)

Approach
Several collaborative projects, including N2Africa funded by the Bill & Melinda Gates Foundation through Wageningen University, are developing improved management options to enhance system productivity. The N2Africa project is being implemented in eight countries: DR Congo, Ghana, Kenya, Malawi, Mozambique, Nigeria, Rwanda, and Zimbabwe. It is a research-and-development partnership program that aims to develop, disseminate, and promote appropriate N2-fixation technologies for smallholder farmers, focusing on the major grain legumes. Although atmospheric air contains 78% N2, nitrogen (N) remains the most limiting nutrient for plant growth and also the most limited nutrient in degraded soils.

The good news is that legumes have the unique ability to fix atmospheric N through symbiotic association with root nodule bacteria. The opportunity exists through biological nitrogen fixation (BNF) to improve the yields of legumes in sub-Saharan Africa since current yields are only a small fraction of their potential. The integration of legumes in cropping systems can benefit associated cereal crops through N-sparing effects, N transfer, and non-N rotation effects. However, the process of BNF can be limited by several biotic and abiotic factors.

Enhancing biological N through bradyrhizobium inoculation and phosphorus application. Ino = inoculum, TSP = total super phosphate

Enhancing biological N through bradyrhizobium inoculation and phosphorus application. Ino = inoculum, TSP = total super phosphate

Evidence abounds that successful BNF depends on the interaction of environment (climate, rainfall day length, etc.), soil factors (acidity, aluminum toxicity, limiting nutrients), management (use of mineral fertilizers, planting dates and density, weed competition), legume species and variety, and rhizobium species and effectiveness. The current low crop productivity reported in legume-based systems can be attributed in part to the prevalence of these factors that limit BNF. In applying the study to legume-based systems, the N2Africa project expects that the identification of a combination of factors (see photos below), when appropriately managed, will optimize BNF and nutrient cycling in maize-based systems. This ability makes legumes a vital component of smallholder farming systems where the input of N fertilizer is almost negligible. Successful increases in legume productivity will lead to (1) increased availability of major sources of protein for direct consumption by rural households; (2) improved soil health through BNF and a reduced need for inorganic N fertilizers; (3) the breaking of pest and disease cycles of other crops when in rotation with legumes; and (4) improved income and health for the rural poor.

Preliminary results
In collaboration with the national agricultural research and extension systems (NARES) in the eight countries, the project has isolated several indigenous rhizobia strains, notably in Kenya, Nigeria, Rwanda, and DR Congo, from local farmlands to identify and characterize superior strains for enhanced BNF. The goal is to develop inoculum production capacity using superior native rhizobial strains through collaboration with private sector partners. In addition, several commercial inoculant strains are being evaluated to identify improved varieties with enhanced BNF for integration into specific farming systems. Results of the project have shown that the inoculation of improved soybean varieties resulted in higher yields in several project sites.

However, grain yields may be constrained in P-deficient soils, hence the combined use of P fertilizers and inoculum consistently produced higher yields (Fig. 1). Note from the same figure that responses to inoculants and P fertilizer are highly variable with yield in amended plots ranging from 0 to over 3 t/ha under on-farm conditions. This further stresses the need for local adaptation (see Vanlauwe6) and the need to observe the main factors determining such variability.

Figure 1. Range of responses to bradyrhizobium inoculation and phosphorus application.

Figure 1. Range of responses to bradyrhizobium inoculation and phosphorus application.

Within the N2Africa project, having detailed monitoring and evaluation (M&E) tools within large-scale adaptation and dissemination field campaigns is an important component of the ‘Research in Development’ concept, at the core of its learning objectives. Where soil pH and levels of P are not too low, an application of 20 kg P/ha is adequate for the proper growth of soybean, cowpea, and groundnut but in soils deficient in P or with low pH,40 kg/ha is optimum.

Related interventions
The project is also identifying high-yielding legume varieties with varying maturity durations for specific environments to provide farmers with options that will enable them to match varieties to the length of the growing season. For example, when the rain is delayed in a particular year or for some reason farmers delay planting, they can select short-maturing varieties that can fit into the remaining growing period.
A major emphasis is being placed on determining the best time to plant various legumes in several agroecologies in combination with appropriate row spacing and plant population. Planting at the right time enhances yield in many ways: (1) the growing period coincides with good rainfall despite its variability in some years; (2) the crop is exposed to optimum temperature regimes; (3) growth coincides with the optimum solar radiation and daylength that regulate vegetative and reproductive growth phases in legumes due to their photosensitivity; and (4) plants escape the major pests and diseases that limit yield.

Partnership
With project partners which include the national agricultural research and extension systems, nongovernmental organizations, community-based organizations, and farmers’ associations, these technologies have been developed into recommended packages and are being demonstrated on-farm. The demonstration plots are established with the direct participation of farmers who are responsible for the day-to-day maintenance to encourage hands-on learning. Field days are also organized during the growing season for individuals and farmers’ groups to create awareness about the technologies. The project encourages women’s participation as well. Other dissemination activities involve the distribution of inputs to project participants including improved seeds, inoculants, and P fertilizer and lime at agreed prices. The project has developed training programs to improve the skills of extension agents, farmers, and other stakeholders to ensure sustainability of the results after the project ends.

Outlook
It is expected that these agronomic interventions should lead to increased diversification of N2-fixing legume species in smallholder farming systems in sub-Saharan Africa, expansion in the cultivation of grain, greater productivity in legume-based farming systems, and enhanced family incomes and nutrition. In collaboration with microbiologists, plant breeders, and the private sector, the selection and dissemination of efficient rhizobial inoculant strains and improved varieties of grain legumes with enhanced BNF capacities adapted to various environmental stresses will improve the prospects of increasing legume components in cropping systems as well as enhancing the production of expanded ecosystem services.