Competition of a Binary Weed System with Cotton (Gossypium hirsutum)

Cotton (Gossypium hirsutumL. ‘Stoneville 213’) was grown on Lucedale fine sandy loam with sicklepod (Cassia obtusifoliaL. ♯ CASOB) and a complex of redroot pigweed (Amaranthus retroflexusL. ♯ AMARE) and smooth pigweed (A. hybridusL. ♯ AMACH) in all possible combinations of 0, 1, 2, 4, 8, and 16 weeds of each species per 7.5 m of row. Seed cotton yields decreased as a quadratic function of increasing weed density. One pigweed and one sicklepod plant per 7.5 m of row reduced yields by 9 and 9.7% in 1979 and 1980, respectively. At low levels of infestation (≤4 weeds/7.5 m of row), the competitive effect of pigweed and sicklepod was additive; however, at the high densities, the competitive effect was not additive. Mechanical harvesting efficiency and cotton maturity were not decreased by any weed density. Sicklepod was more competitive than pigweed in both years.