Soybean response to weed interference and defoliation

Field experiments were conducted in Louisiana over 2 yr to evaluate the inf luence of full-season interference from johnsongrass, common cocklebur, or hemp sesbania at densities of 2.5, 0.5, and 2.0 plants m(-1) of row, respec tively, and simulated insect defoliation of soybean on weed and soybean gro wth. Defoliation at R2 (full bloom) and R5 (beginning seed development) soy bean growth stages was accomplished by removal of zero, one, or two leaflet s per soybean trifoliate, which approximated 0, 33, and 66% defoliation, re spectively Height and dry weight of all weeds were nor affected by soybean defoliation level or defoliation stage. Soybean height 3 wk after defoliati on at R5 was not influenced by weed interference, soybean defoliation level , or defoliation stage in either year Averaged across soybean defoliation l evels and stages in 1994, johnsongrass, common cocklebur, and hemp sesbania reduced soybean yields 30, 15, and 14%, respectively. In 1995, johnsongras s reduced soybean yield 35%. As soybean defoliation level increased, a line ar decrease in soybean yield was observed. Averaged across weeds and soybea n defoliation stages, 33 and 66% defoliation reduced soybean yield G and 20 % in 1994 and la and 33% in 1995, respectively. Defoliation at R5 resulted in 10% lower yield than defoliation at R2 in one of two years. Yield reduct ion due to combinations of weeds and soybean defoliation was additive.