Influence of Cotton (Gossypium hirsutum) and Soybean (Glycine max) Planting Date on Weed Interference

Separate field experiments were conducted for cotton and soybean in 1990 and 1991 to determine the influence of planting date on yield loss due to interspecific interference from entireleaf morningglory and sicklepod and to determine the relative competitiveness of each weed species. Percent soybean yield loss due to weed interference increased as planting date was delayed from early May to early June. Averaged over weed species, yield losses from 1.7 weeds m−1row were 10, 18, and 20% for soybeans planted in early May, mid-May, and early June, respectively. Yield loss from 6.7 weeds m−1row were 17, 31, and 35% at the early May, mid-May, and early June planting dates, respectively. Percent seed cotton yield losses averaged over weed species in 1990 were 33 and 28% for the early May and early June planting dates, respectively, at 1.7 weeds m−1and 50% for both planting dates at weed densities of 6.7 plants m−1. The only experimental factor that significantly affected seed cotton yield in 1991 was weed density. Unlike soybeans, planting date had little effect on weed interference in cotton. Entireleaf morningglory was more competitive than sicklepod in both crops. Results suggest that selection of optimum soy bean planting dates may be a viable means of reducing losses due to weed interference.