Common waterhemp has become a problem weed species in Midwest soybean production. Determining the critical interference period after soybean and common waterhemp emergence is necessary for the implementation of weed control practices before soybean seed yield loss occurs. Field experiments were conducted during 1996, 1997, and 1998 to determine the influence of duration of common waterhemp interference on soybean seed yield. Removal of common waterhemp 2 wk after soybean unifoliolate leaf expansion resulted in soybean seed yield equivalent to a season-long weed-free control. Delaying common waterhemp removal until 4 wk after soybean unifoliolate leaf expansion resulted in decreased soybean seed yield. Allowing common waterhemp interference to persist 10 wk after soybean unifoliolate leaf expansion reduced soybean seed yield by an average of 43% over 3 yr. These results suggest that soybean producers should implement common waterhemp management strategies earlier than 4 wk after soybean unifoliolate leaf expansion in order to reduce the potential loss of soybean seed yield.

Nomenclature: Acifluorfen; sethoxydim; common waterhemp, Amaranthus rudis Sauer. AMATA; soybean, Glycine max (L.) Merr. ‘Pioneer 9362’, ‘Pioneer 9363’.