Cowpeas as a summer cover crop for forage rye

Cowpeas [Vigna unguiculate (L) Walp] as a summer cover crop have the potential to provide N to a cool-season crop. The objectives of this study were to evaluate two cowpea cultivars grown as a cover crop to supply N for forage rye (Secale cereale L.) and to document cowpea N and C contributions to the soil. The 2-yr study was conducted at the Texas A&M AgriLife Research Center at Overton on a Darco loamy fine sand (loamy, siliceous, semiactive, thermic Grossarenic Paleudult) during 2014-2016. Treatments were arranged in a split-plot design with four replicates with cover crop as the main plot and N rate as subplot. Cover crop treatments included 'Combine' (COM) and 'Iron and Clay' (IAC) cowpeas and summer fallow. In late summer, cowpeas were incorporated as green manure, and four rates of fertilizer N were applied to rye. Total 2-yr cowpea biomass incorporated as green manure was similar at 4,596 kg ha(-1) and 5,058 kg ha(-1) for IAC and COM, respectively. Cowpea was not effective as a green manure crop at providing N for rye biomass. Total soil N showed no difference (P >= .34) between cowpea entries or N rate within a soil depth. Each N rate increased rye biomass (P < .01) linearly (P < .001) in each year. Soil organic C from cowpeas increased about 6-8% over 2 yr. The extent of decomposition of cowpeas on the deep, sandy Darco soil limited nutrient transfer from cowpeas during this 2-yr period.