Can Residual Effects of Green Manure Cover Crops (GMCC) Solve N Fertilizer Challenges in Conservation Agriculture (CA) Systems of Southern Africa?.

Declining fertility, excessive erosion, and low and declining crop yields are common throughout Southern Africa and current fertilizer use is as low as 8 kg ha-1 year -1on farmers’ fields leading to soil mining and extractive agriculture systems. Challenges in access to fertilizer can be attributed to many factors including an overall fertilizer shortage in Africa, high fertilizer prices, dysfunctional markets for agriculture produce, and limited farmer resources. Alternative fertilization strategies need to be explored and adapted to the different farming systems to increase production on farmers' field and improve the levels of biomass available for surface retention and animal feed. This is particular relevant in CA systems, where the system's performance depends on adequate groundcover. Green manure cover crops (GMCC) offer an opportunity to increase N fertilization for small-scale farmers independent from external inputs and, at the same time suppress weeds, pests and diseases. From 2008 to 2012 CA trials were carried out at the University Farm (UZ) and Domboshava Training Center (DTC) in Zimbabwe to assess the residual effects of seven GMCCs and two grain legumes on a subsequent maize crop. Highest biomass yields of up to 10.6 t ha-1 and 10.2 t ha-1 were recorded for Crotalaria ochroleuca (G. Don) and Crotalaria grahamiana (Wight & Arn.), respectively in DTC during 2010/11. Residual effects on maize were not always significant, however results show that maize grown after velvet bean (Mucuna pruriens L.) performs best compared to continuous maize. At UZ in 2009/10, 2010/11 and 2011/12 yields of maize after velvet bean were 3.1 t ha-1 (90%), 2.4 t ha-1 (54%) and 2.7 t ha-1 (56%) higher than the control with maize after maize. While these results show the potential of GMCCs as an alternative fertilizer source some challenges still remain including the economic sustainability of rotating maize with a “non productive” GMCC; its consequences on food security at the household level; the lack of seed of different GMCC species, and the adaptability of GMCCs to contrasting soil types and climates. While GMCCs may not work in all situations, incorporating GMCCs as relay- or intercrops may be a possible intervention strategy into existing farming systems with positive impacts on food security and subsequent crops.