The shrub Tephrosia vogelii can grow very quickly, up to 4 metres high, fixes nitrogen and can be used as green manure. In Maraba, Southwest Rwanda, coffee productivity is constrained by poor soil fertility and lack of organic mulch.
A 2-year study on 8 smallholder coffee farms trialled the effect of intercropping Tephrosia and coffee. The mulch produced from Tephrosia was also used on the coffee plots. In the first year, Tephrosia intercropped with coffee produced 1.4–1.9 tonnes per hectare of biomass and added 42kg–57kg of Nitrogen per hectare. This treatment increased coffee yields by 400kg–500kg per hectare, compared to traditional management methods. In the second year, Tephrosia produced between 2.5–3.8 tonnes per hectare of biomass and added 103kg-150kg of Nitrogen per hectare. This increased yields of coffee by 400kg per hectare.
Over the 2-year study, coffee yields increased between 23% and 36%. Tephrosia mulch was 87% as efficient as inorganic fertiliser used under similar conditions, and represented a saving of 30 days of labour hours per hectare compared to current farmer management through reduced labour required for weeding. Together the labour savings and the improved yields translated into the farmers producing 5kg of coffee per labour-day, compared to 3.4kg per labour-day under traditional management.