Redroot pigweed (Amaranthus retroflexus L.) and robust foxtail [Setaria viridis (L.) Beauv. var. robusta-alba Schreiber (RWF) or Setaria viridis var. robusta-purpurea Schreiber (RPF)] were investigated regarding their ability to interfere with soybean [Glycine max (L.) Merr. ‘Amsoy 71′] at different weed densities and soybean row spacing throughout two growing seasons. Final weed densities for each species tended to reach a common value because of intraspecific interference regardless of the initial density. With cultivation, a narrow soybean row spacing (38 cm) resulted in less weed growth than did a wide row spacing (76 cm) but with no cultivation, the trend was reversed. Soybeans provided less interference to foxtail than to pigweed during both growing seasons. Interference from foxtail adversely affected soybean yield components and soybean seed yield more than did pigweed interference. Water-stress conditions in 1976 increased the intensity of weed interference and reduced soybean seed yield more severely than in 1975 when moisture was adequate throughout the growing season.
