Nitrogen fixation by annual forage legumes and its contribution to succeeding wheat in the Ethiopian highlands

Soil fertility is declining in most agro-ecosystems in sub-Saharan Africa, and incorporation of forage legumes into production systems to utilize the nitrogen fixed by the legumes could alleviate the problems, if efficient nitrogen-fixing legumes are used. The amounts of nitrogen fixed by Lablab, Medicago, Trifolium, and Vicia species and their contribution to the following wheat crop were estimated in field experiments on an Alfisol at Debre Zeit in the Ethiopian highlands. The amounts of nitrogen (N) fixed ranged from 40 kg N ha-1 for T. steudneri to 215 kg N ha-1 for L. purpureus. The increase in grain yields of wheat following the legumes ranged from 16% for T. steudneri to 71% for M. tranculata where no N fertilizer was applied to the wheat. Additional N fertilizer applied to wheat at 60 kg N ha-1 had no significant effects on wheat grain or straw DM and N yields. In another experiment, eight lablab treatments consisting of factorial combinations of two cultivars (Rongai and Highworth), two Rhizobium inoculation treatments (inoculated and uninoculated) and two times of harvest (for hay at 50% flowering and for seed at seed maturity), were compared on lablab forage production and N yield, and residual effects on two succeeding wheat crops. Inoculation had no significant effects on nodulation, shoot DM or N yields. Rongai had significantly higher shoot DM and N yields than Highworth. Lablab harvested at flowering had significantly higher shoot DM and N yields than lablab harvested at seed maturity. Grain yields of the first wheat crop following the various lablab crops were 93-125% higher than grain yields of the wheat following wheat (continuous wheat) where no N fertilizer was applied. Therefore, lablab is a potential forage crop for incorporation into cereal production systems to improve feed quality and to reduce dependence on N fertilizers for cereal production.