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 eff ect of N fertility, cropping system, redroot pigweed (Amaranthus retro flexus L.) density, and harvesting frequency on collard (Brassica oler acea var, acephala D.C) and cowpea [Vigna unguiculata (L.) Walp.] grow th. The N fertilization regimes were 0, 80, 160, and 240 kg.ha(-1), ap plied as urea in a split application, Four weeks after crop planting, redroot pigweed was seeded at 0, 300, and 1200 seeds/m(2). Between wee ks 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 branc hing. The high density of pigweed caused a 56% reduction of coupes dry mass in 1992.