Mineral Nutrition and Integration of Forage Legumes into Smallholder Farming Systems, with Emphasis on Velvet Bean [Mucuna pruriens (L) DC var. utilis].
The major objective of this work was to evaluate and compare forage supply and soil fertility effects of velvet bean (Mucuna pruriens var. utilis) integrated into smallholder cropping systems as ley, intercrop and green manure for farmers engaged in semi-commercial offtake from livestock. The results also present information about the alleviation of mineral nutrient constraints in order to raise the productivity and probability for adoption of velvet bean as a forage and green manure crop in Zimbabwe, through a series of experiments conducted on farm with farmers. Socio – economic assessments of the suitability of these interventions were also incorporated into the work. Experiments were carried out in Zana resettlement and Dendenyore communal areas of Hwedza District, Zimbabwe (18o41'S latitude; 31o42’ E longitude; 1400 m asl) to compare different systems of integrating forage legumes in terms of forage production and effect on a subsequent maize crop. The systems included velvet bean, cowpea and archer (mixed with Katambora Rhodes grass) grown as leys, intercrops or green manure, with or without SSP or lime. A weed fallow and sole maize were also included as controls. A maize cropping phase followed most of the treatments in year 2 to determine the rotational benefits of the legume systems. The trials were conducted at nine farmers’ fields and the soils were generally acidic (pH<5 – calcium chloride method), inherently infertile (low available P, Ca, Mg and mineral N and total C% < 1.5%) sands, loamy sands or light clays. The biomass production in season 1 for velvet bean with inorganic P fertilizer applied at planting (16.4 kg ha-1) ranged from 6.5 to 17 t DM ha-1 (range was 3.7 to 11.8 t DM ha-1 without P fertilizer). There was a 15% increase in velvet bean dry matter yield with lime application. Velvet bean and cowpea showed a greater response to P fertilizer application (30.9% and 29.4%, respectively), compared with Archer and Katambora Rhodes grass (14.5%). Season 2 results showed a significant (P<0.05) residual effect of velvet bean on the maize stover and grain yield. They also pointed to a positive P residual effect of velvet bean, and revealed that biomass production in perennial leys would be greater in the second season than in the first, even when the leys are grazed during the dry season. Overall, there was an increase of more than 300 kg in maize grain yield for every tonne of green manure incorporated. This was a nitrogen use efficiency of 11 kg grain/kg N applied. These results show that the application of P and lime can substantially raise productivity of velvet bean and allow it to input large amounts of N that can benefit following maize crops. A further experiment on the effect of liming on velvet bean showed that the response of velvet bean to liming in season 1 was due to a combination of increase in pH and supply of Ca and Mg. In terms of overall N and P partial nutrient balances in the systems, the green manure velvet bean (with 16.4 kg-1 P) followed by maize (with 30 kg N/ha) system proved to be the most sustainable (overall partial N balance of -88.6 kg/ha and partial P balance of 10 kg/ha), while the maize (with 30 kg N/ha) following limed velvet bean (with P) grown for hay and the maize (with 30 kg N/ha) after weed fallow systems were the least sustainable for N (-389 kg/ha) and P (-9.2 kg/ha) balances, respectively, when comparing these three systems. The results of a gross margin analysis indicated a strong residual effect of P on maize yield and that farmers growing maize after velvet bean benefit in terms of a higher maize yield and reduced inorganic fertilizer costs. A survey revealed the high potential of velvet bean to improve the existing farming systems. Velvet bean appeared particularly beneficial and attractive to commercial oriented livestock smallholder farmers. For this, it would be grown as a ley crop and hay or pods and residue fed to cattle. It appeared less attractive to non-livestock smallholder farmers, where its use would be more as a green manure to feed the soil (and then subsequently maize and humans).