INTERFERENCE BETWEEN HEMP SESBANIA (SESBANIA-EXALTATA) AND SOYBEAN (GLYCINE-MAX) IN RESPONSE TO IRRIGATION AND NITROGEN

Soybean yield loss from weed interference depends upon weed density an d competitiveness of crop and weed species in response to environment. Soil water availability and nitrogen fertility were evaluated for the ir effect on competitiveness of individual species in field experiment s. Early-season temperatures in 1995, which were cool compared to 1994 , slowed hemp sesbania growth without affecting soybean growth. This r esulted in negligible competition with soybean by hemp sesbania at den sities of 3 or 6 plants m(-2). In 1994, hemp sesbania grew above the s oybean canopy, decreasing soybean light interception 29 to 68%, and re ducing soybean yield 30 to 48%. Fertilizer nitrogen increased soybean competitiveness, as indicated by biomass production, only in irrigated plots with hemp sesbania at 3 m(-2), but did not affect soybean yield . Apparently, competition for light is a primary cause of soybean yiel d loss from hemp sesbania infestations. In growth chamber studies, sim ulating temperatures from the field, hemp sesbania growth was stimulat ed more by warm temperatures than was soybean. Hemp sesbania and soybe an dry weights increased 4.4- and 2.7-fold, respectively, at 30/20 C d ay/night temperatures compared to 25/15 C.