Collard-cowpea intercrop response to nitrogen fertilization, redroot pigweed density, and collard harvest frequency

Field experiments were conducted in 1992 and 1993 to determine the effect of N fertility, cropping system, redroot pigweed (Amaranthus retroflexus L.) density, and harvesting frequency on collard (Brassica oleracea var. acephala D.C) and cowpea [Vigna unguiculata (L.) Walp.] growth. The N fertilization regimes were 0, 80, 160, and 240 kg · ha^-1, applied as urea in a split application. Four weeks after crop planting, redroot pigweed was seeded at 0, 300, and 1200 seeds/m². Between weeks 6 and 12, collard leaves were harvested at 1- to 3-week intervals. Year, N fertility, and cropping system interacted to determine collard leaf number and mass. For example, in 1992, with N at 160 kg · ha^-1, collards intercropped had more total leaf mass than those monocropped. Pigweed density had no effect on collard yields, which were greatest from the 3-week harvest frequency. Cropping system and pigweed density interacted to determine cowpea vine length, shoot dry mass, and branching. The high density of pigweed caused a 56% reduction of cowpea dry mass in 1992.