Abstract: Field studies were conducted from 1997 to 1999 to evaluate the influence of weed competition and herbicides on glyphosate-resistant soybean at the Belleville Research Center at Belleville, IL. Season-long weed competition did not delay maturity or reduce soybean population. However, season-long weed competition reduced soybean height by 10% and grain yield by 68%. Competition from weeds 30 cm in height or less did not increase days to maturity or reduce plant population, height, or grain yield. Chlorimuron plus thifensulfuron caused 6% height reduction at 21 d after treatment. Despite the injury from herbicides, none increased days to maturity or reduced plant height, population, or grain yield of soybean. The soil-applied herbicides alachlor, imazaquin, and pendimethalin and the postemergence herbicides acifluorfen, bentazon, chlorimuron, imazethapyr, and thifensulfuron did not reduce yield of glyphosate-resistant soybean.

Nomenclature: Acifluorfen; alachlor; bentazon; chlorimuron; glyphosate; imazaquin; imazethapyr; pendimethalin; thifensulfuron; soybean, Glycine max (L.) Merr. ‘Asgrow 4401 RR’.

Additional index words: Herbicide injury, soil-applied herbicides, preemergence, postemergence.

Abbreviations: DAT, days after treatment; POST, postemergence; PRE, preemergence; UAN, urea ammonium nitrate.