SICKLEPOD (SENNA-OBTUSIFOLIA) CONTROL IN SOYBEAN (GLYCINE-MAX) WITH SINGLE AND SEQUENTIAL HERBICIDE APPLICATIONS

Field studies were conducted in 1991 to 1993 to determine the effects of PRE and POST herbicides alone and in factorial combinations on sick lepod control, weed biomass, and soybean seed yields. Herbicides evalu ated were metribuzin PRE, metribuzin + chlorimuron PRE, imazaquin PRE and POST, chlorimuron POST, and AC 263,222 POST. POST herbicides alone provided better sicklepod control than PRE herbicides alone. However, sequential (SEQ) treatments [PRE followed by (fb) POST] provided bett er sicklepod control than single herbicide treatments. Metribuzin and metribuzin + chlorimuron alone or used in a SEQ program provided simil ar PRE control of sicklepod, and both controlled sicklepod better than imazaquin in 1991 and 1992. Chlorimuron alone or in a SEQ program pro vided better POST control than AC 263,222 or imazaquin alone when soil moisture levels at planting favored weed seed germination. When soil moisture levels at planting were unfavorable for weed seed germination , the residual activity of imazaquin provided the best season-long con trol. SEQ herbicide applications allowed soybean to attain higher seed yields than single (PRE or POST) herbicide treatments all 3 yr. Howev er, only metribuzin fb chlorimuron in 1991 and 1992 and imazaquin fb i mazaquin and imazaquin fb chlorimuron in 1993 allowed soybean to achie ve seed yields similar to weed-free soybean. Soybean seed yields were greater with POST than with PRE herbicides 2 of 3 yr, but no single he rbicide treatment provided soybean seed yields similar to weed-free so ybean.