Influence of Cowpea and Nitrogen Rate in a Cropping System with Forage Rye.

Cowpea (Vigna unguiculata [L.] Walp) as a summer cover crop is beneficial both as a source of N or as a grain crop. By utilizing cowpea and rye (Secale cereale L.) in a double cropping system, cowpeas are multifunctional as an N source and a seed crop; whereas, forage rye can be used for winter pasture. The primary objective of this study was to evaluate a double cropping system to optimize rye forage and cowpea production concurrently while enhancing N use efficiency. In a separate greenhouse study, 37 cowpea plant introduction germplasms were evaluated for maturity and seed yield. This study was conducted at the Texas A&M AgriLife Research and Extension center at Overton, TX on a Darco soil. Three types of summer cover cropping (late cowpea, ‘Iron and Clay’; early cowpea, ‘Combine’; and fallow) and four N rates (0, 34, 67, 101 kg ha^-1) were evaluated in a factorial combination arranged in a split plot design. Cowpea biomass incorporated averaged 2557 kg ha^-1 for Iron and Clay and 3134 kg ha^-1 for Combine. ‘Elbon’ rye was mechanically harvested and dry matter yield (DMY) was calculated and analyzed using PROC MIXED and PROC REG in SAS. The summer crop treatment had no significant effect on DMY of rye; however, N rate effected (P < 0.0001) rye DMY. A positive linear relationship (r² =0.79) was evident for rye DMY and N fertilizer rate, which all differed from each other. The green manure cover crop of cowpea was not influential as an N input on forage rye yield during the first year of cropping with the unfavorable autumn climatic conditions. In the cowpea germplasm evaluation, line 7 produced more (P < 0.05) seed by weight than lines 35, 11 and 23, with other lines intermediate.