Cultivar and Phosphorus Amendment Impacts on Organically Managed Forage Cowpea Yield and Composition

Abstract

Core Ideas Cowpea is well‐adapted for organic systems but cultivar differences are not well explored. Cultivars differed widely in biomass, stand density/seedling disease, and quality. Cowpea cultivars examined did not respond to P fertilizer in low soil P status soils. Cowpea (Vigna unguiculata Walp.) is a warm‐season legume with many traits that make it an attractive forage or cover crop for organic systems. Eight cowpea cultivars were evaluated under organic management at two locations for stand establishment, forage yield and quality, and weed biomass. The experiment was arranged in a strip‐plot design with two P fertilization rates, amended (45 kg P ha−1) and unamended, to evaluate responsiveness to P fertilization in low native soil P status (Mehlich‐1 P < 10 mg P kg−1). Cowpea was seeded at 209,000 seeds ha−1. Stand density at 4 wk indicated the highest plant populations from cultivar Iron & Clay (166,000 plants ha−1), intermediate populations from Speckled Purple Hull, IT82E‐18, and IT85‐867‐5F (143,000 to 138,000 plants ha−1), and lowest populations from IAR7/8‐5‐4‐1, Coronet, KV×396, and IT97K‐556‐4 (128,000 to 118,000 plants ha−1) primarily due to presence of seedling diseases caused by Fusarium spp. Speckled Purple Hull and Iron & Clay had the highest total yield (4922 and 4623 kg ha−1, respectively). Yield was lowest from IT82E‐18, Coronet and IAR7/8‐5‐4‐1 (1958–2585 kg ha−1), likely due to low plant populations (IAR7/8‐5‐4‐1, Coronet) and higher weed biomass than cowpea biomass (IAR7/8‐5‐4‐1, Coronet, IT82E‐18). There was no statistical difference in cowpea biomass between unamended (3422 kg ha−1) and P‐amended plots (3150 kg ha−1), or differences in cowpea tissue P concentration. High forage quality values of top‐performing cultivars suggest that they are well adapted to address low summer forage quality in applicable forage systems.