Closing legume yield gaps

Blog post by Dr Ken Giller, N2Africa

“To grow beans you have to own a cow” – Marie Therese Nyiransekuye, a farmer in Gakenke district, Kivuruga sector, Cyintare cell/village, Rwanda

The breeders have produced some fantastic varieties of our major grain legumes. These have been improved in a wide range of traits, including increased grain yield, pest and disease resistances, taste and cookability, nutritional value through biofortification and environmental adaptation with particular focus on drought tolerance.

A major reason for success of grain legumes in smallholder farming systems of the tropics is their ability to capture nitrogen gas (N₂) from the air in symbiosis with rhizobia. I am excited that the Legumes CRP has symbiotic nitrogen fixation as one of the product lines – a topic that enthralled me for the past 30 years.

In the over-worked and depleted soils of Africa the grain legumes are unable to grow and yield anywhere near their full potential. In the project, N2Africa: Putting nitrogen fixation to work for smallholder farmers in Africa (www.N2Africa.org) we focus on closing legume yield gaps. We recognise the importance of a good legume genotype (G_L) in combination with the best rhizobial strains (G_R) in determining the yield potential. The (G_L × G_R) need to be selected for specific agroecological environments (E) in terms of climate and soils and their overall performance depends on good management (M) including good agronomy (planting date, spacing, weeding, basal fertilization and inoculation). So the expression of the crop phenotype in the field depends on the interactions between (G_L × G_R) × E × M.

Many legumes are able to nodulate readily with rhizobia bacteria that are found naturally in soils – they are said to be promiscuous. Some legumes require specific types of rhizobia that are not widely distributed – inoculation with rhizobia often stimulates growth and increased yields of these “specific” legumes.

Two examples stand out for me of particular success in breeding of legumes. The first is climbing beans in Central Africa. Introduction of climbing beans in the mid-1980s by the Rwandan Agriculture Board, supported by CIAT Bean Team has transformed the hillsides of Rwanda. The climbing bean varieties can fix at least 300 kg N ha^-1 in a single season, yielding up to 5 t ha^-1 of grain and more than doubling the yield of subsequent maize crops. Yet, as the above farmer’s quote shows, manure is needed to achieve the yield potential of these varieties. Indeed we are finding synergistic impacts of manure, small doses of phosphorus (P) and rhizobial inoculation on vigour and yield of beans.

The second example is promiscuous soyabeans in West Africa. Over the past 30 years, well-adapted, non-shattering, vigorous soyabean varieties have been bred by IITA and partners that are promiscuous and can nodulate with rhizobia present in the soil. These varieties were introduced into northern Ghana less than 20 years ago but now dominate the smallholder farms with total production of more than 40 thousand tonnes each year. In many fields they nodulate and fix well, yet as soil organic matter contents fall with continuous cultivation and lack of inputs, the soils become inhospitable for rhizobia to survive.

Over the past four years we have shown that rhizobial inoculation together with a small basal dose of P can sometimes treble yields on smallholder farms. Although P fertilizer is an expensive input, the additional yield gained by adding inoculant at sowing actually makes the P fertilizer economically attractive. Indeed, there is a growing body of evidence that many of the major grain legumes, including chickpea, cowpea and common bean can all benefit from rhizobial inoculation.

We look forward to collaborating with many scientists throughout the CGIAR Research Program on Grain Legumes to realize the full genetic potential of their new releases in farmers’ fields. Together we can fill the technology pipeline and provide tailored solutions for farmers to close legume yield gaps!

About the author

Ken Giller is Professor Plant Production Systems (http://www.pps.wur.nl), within WaCASA (the Wageningen Centre for Agroecology and Systems Analysis – http://www.wacasa.wur.nl) at Wageningen University. Ken’s research has focused on smallholder farming systems in sub-Saharan Africa, and in particular problems of soil fertility and the role of nitrogen fixation in tropical legumes, with emphasis on the temporal and spatial dynamics of resources within crop/livestock farming systems and their interactions. He is author of the standard text “Nitrogen Fixation in Tropical Cropping Systems” published in second edition in 2001. He leads a number of initiatives such as N2Africa (Putting Nitrogen Fixation to Work for Smallholder Farmers in Africa – http://www.n2africa.org/). Ken joined Wageningen University in 2001 after holding professorships at Wye College, University of London, and the University of Zimbabwe. He is currently on sabbatical leave in Arusha, Tanzania with IITA and the Nelson Mandela African Institution of Science and Technology.

From Lead point (Blogs), http://grainlegumes.cgiar.org/