Herbicide systems consisting of PRE, early POST, and late POST options arranged factorially were compared for control of sicklepod in narrow-row soybean with modified acetolactate synthase (ALS) (E.C.4.1.3.18). Other weeds present included common cocklebur and mixed infestations of entireleaf, ivyleaf, pitted, and tall morningglories. PRE options were alachlor or alachlor plus metribuzin plus chlorimuron. Early POST options included chlorimuron, chlorimuron plus thifensulfuron, and no herbicide applied 3 wk after planting. Late POST options were chlorimuron and no herbicide applied 5 wk after planting. POST herbicides were more effective than PRE herbicides on all weeds. Chlorimuron and chlorimuron plus thifensulfuron applied early POST were equally effective on these weeds and usually more effective than chlorimuron applied late POST. There was no advantage of two POST applications compared with a single early POST application. Greatest net returns were obtained in systems using only early POST herbicides. There was no economic advantage from using metribuzin plus chlorimuron PRE in systems that included an early POST herbicide.

Systems consisting of flumetsulam, metribuzin plus chlorimuron, or imazaquin applied PPI with trifluralin or PRE with metolachlor were compared with and without chlorimuron POST for control of sicklepod and entireleaf morningglory in narrow-row soybean at four locations. Control of sicklepod and entireleaf morningglory by soil-applied herbicides was generally inadequate. Control of sicklepod by flumetsulam exceeded control by metribuzin plus chlorimuron or imazaquin at one location. Entireleaf morningglory control by flumetsulam was similar to or less than control by metribuzin plus chlorimuron or imazaquin. Chlorimuron POST was a more important component of management systems for these weeds than was flumetsulam, metribuzin plus chlorimuron, or imazaquin PPI or PRE. Pooled over soil-applied herbicides, chlorimuron POST increased late-season control of sicklepod and entireleaf morningglory 25 to 61% and 22 to 54%, respectively; increased soybean yield 20 to 55%; decreased foreign matter contamination 5 to 13%; and increased net returns $34 to $185/ha. When used in conjunction with chlorimuron POST, flumetsulam, metribuzin plus chlorimuron, and imazaquin applied PPI with trifluralin or PRE with metolachlor increased late-season control of sicklepod and entireleaf morningglory only when control by trifluralin or metolachlor followed by chlorimuron POST was less than 66 and 77%, respectively.