Suitable Legumes for Cover Cropping in the Irrigated Arid Southwest.

A study was conducted in Las Cruces, NM, to evaluate the performance of selected green manure legumes for production of biomass and addition of nitrogen for the benefit of succeeding cash crops. Legumes that were evaluated included adzuki bean (Vigna angularis), pigeon pea (Cajanus cajan), mung bean (Vigna radiata), lima bean (Phaseolus lunatus), bush bean (Phaseolus vulgaris), green bean (Phaseolus vulgaris L.), tepary bean (Phaseolus acutifolius), moth bean (Vigna aconitifolia), cowpea (Vigna unguiculata), [var. iron & clay, var. CC36 and var. Café Afrique (CA)], lablab (Lablab purpureus), Guar (Cyamopsis tetragonoloba) [var. Durga and var. Evergreen] and sesbania (Sesbania exaltalta). Replicated trials were conducted over two seasons to document the performance of the various legume species. Results over two years of trials showed that sesbania produced the highest biomass of 18 Mg ha^-1 followed by cowpea var. iron & clay (9 Mg ha^-1) and lablab (8 Mg ha^-1). Analysis of tissue samples collected at flowering showed significant difference in the total nitrogen content of the biomass of the legumes.  Pigeon peas and bush beans had the highest nitrogen contents (4.1% and 3.9% respectively), while mung bean and sesbania had the lowest tissue nitrogen contents (1.9% and 1.7% respectively). However, when the total N that could potentially be returned to the soil by the legume biomass was calculated, sesbania would deliver the highest amount of N (278 kg N ha^-1) after soil incorporation of cover crop, followed by cowpea cultivar iron & clay (248 kg N ha^-1) and lablab (215 kg N ha^-1). This study shows that legumes can add significant improvement to the irrigated soils of the arid southwest.