Cowpea Biomass Productivity Under Organic Management in the Southeastern United States As Influenced by Cultivar and Soil Phosphorus

Cowpea (Vigna unguiculata) is a warm-season, multi-purpose legume that is well-adapted to the southeastern USA and has many traits that make it an attractive forage or cover crop for integration into organic production systems, including high rates of nitrogen fixation, phosphorus (P) use efficiency, regrowth ability, and high digestibility. Eight cowpea varieties were evaluated under organic management at two locations in summer 2014 for stand establishment, biomass productivity and weed biomass. The experiment was arranged in a strip-plot design with two soil phosphorus (P) levels, amended (45 kg P/ha) and unamended, to evaluate cultivar responsiveness to P fertilization in soils of low native soil P status (Mehlich-1 P < 10 mg P/kg). Cowpea was seeded at 222,300 seeds/ha, managed organically, and biomass harvested twice during the growing season. Stand counts four weeks after planting indicated the highest plant populations from ‘Iron & Clay’ (201,000 plants/ha), intermediate populations from  ‘Speckled Purplehull’, ‘IT85-867-5’ and ‘IT82E-18’ (168,000 to 183,000 plant/ha) and lowest populations from (‘Coronet’, ‘IAR7/8-5-4-1’, ‘KVx396’, and ‘IT97K-556-4’; 147,000 to 156,000 plants/ha), likely due to presence of seedling diseases caused by Fusarium spp. Biomass was significantly influenced by cultivar (p<0.001) and soil P (p=0.08), but not the interaction (p>0.1). ‘Speckled Purplehull’ and ‘Iron & Clay’ had the highest annual biomass (5349 and 5092 kg/ha, respectively), likely due to an indeterminate growth habit, rapid regrowth ability, and potentially greater resistance to seedling diseases. Biomass was least from ‘IAR7/8-5-4-1’, ‘Coronet’ and ‘IT82E-18’ (2137 to 2849 kg/ha), likely due to low plant populations (‘IAR7/8-5-4-1’, ‘Coronet’) and more determinate growth habits (‘Coronet’, ‘IT82E-18’).  Interestingly, soil P amendment only slightly affected cowpea biomass (p =0.08; 1998 kg/ha in P-amended vs. 2173 kg/ha in unamended treatments), likely due to slightly increased weed biomass (p<0.05; 2686 versus 2383 kg/ha) in P-amended treatments. Results suggest that cultivar choice is an important consideration given wide variability in cultivar biomass production and likely differences in seedling disease susceptibility.