Field studies were conducted for four consecutive years to determine if repeated applications of trifluralin (preplant incorporated), metolachlor (preemergence), and fluazifop (postemergence) herbicides alone or combined could be utilized in soybean to effectively control grass weed populations and maintain optimum yield in an irrigated environment where metribuzin plus dinoseb (preemergence) and 2,4-DB plus linuron (postemergence) were used as a standard weed control system. After repeated applications each year, all treatments maintained similar yields that were no different from yields obtained from the standard system. Grass weed infestation did not increase significantly in the treatment that received only the standard weed control herbicides over the duration of the study.

A field study was conducted over 2 yr to compare efficacy and economics of glyphosate-resistant, sulfonylurea-tolerant, and conventional soybean (Glycine max) weed control programs. Herbicide programs in the three soybean systems provided at least 90% control of browntop millet (Brachiaria ramosa), prickly sida (Sida spinosa), yellow nutsedge (Cyperus esculentus), pitted morningglory (Ipomoea lacunosa), and hemp sesbania (Sesbania exaltata) in most cases and postemergence (POST)-only programs were as effective as preemergence (PRE) followed by POST programs. Control of hyssop spurge (Euphorbia hyssopifolia) ranged from 93 to 100% in glyphosate-resistant soybean and from 88 to 100% in conventional soybean, but control was 60 to 100% in sulfonylurea-tolerant soybean. Sicklepod (Senna obtusifolia) control was at least 91% in glyphosate-resistant and sulfonylurea-tolerant soybean but was 81% for the standard SAN 582 plus imazaquin PRE and acifluorfen plus bentazon early POST treatment in conventional soybean. In glyphosate-resistant soybean, glyphosate applied sequentially resulted in an average yield of 3,020 kg/ha with a net return of $407/ha. In sulfonylurea-tolerant soybean, chlorimuron applied sequentially yielded 2,500 kg/ha with a net return of $271/ha. Conventional soybean yield with the standard herbicide program was 2,770 kg/ha with a net return of $317/ha. Yields for the cultivars were equivalent when the same standard herbicide program was used. When weed control is satisfactory and herbicide costs relatively comparable, yield potential of the cultivar and seed cost, including any technology fee, would be key factors in selecting a weed management system.

Field studies were conducted in 1997 and 1998 to evaluate weed control and soybean (Glycine max) yield with cloransulam and diclosulam. Cloransulam at 35 g/ha applied preplant incorporated (PPI) and preemergence (PRE) controlled at least 91% of hyssop spurge (Euphorbia hyssopifolia) and prickly sida (Sida spinosa) and at least 82% of entireleaf morningglory (Ipomoea hederacea var. integriuscula) and pitted morningglory (Ipomoea lacunosa) at 7 wk after planting. Control of these species with cloransulam was not affected by method of soil application. Control of prickly sida with cloransulam PPI or PRE was better than with cloransulam postemergence (POST) at 18 g/ha. Soybean yields were similar with cloransulam PPI (2,880 kg/ha) or PRE (3,110 kg/ha) and were comparable to imazaquin PRE at 140 g/ha (3,080 kg/ha). Weed control and soybean yields with cloransulam POST were similar to that with chlorimuron POST at 11 g/ha. SAN 582 plus cloransulam PRE followed by cloransulam POST provided the highest soybean yield (3,450 kg/ha). Diclosulam PPI or PRE provided similar levels of control of hyssop spurge, Ipomoea morningglories, and prickly sida. Overall, control of these species with diclosulam PPI or PRE was similar to that obtained with sulfentrazone plus chlorimuron PRE. Soybean yields (≥ 3,290 kg/ha) were similar regardless of method of diclosulam application, and yields were equivalent to that of sulfentrazone plus chlorimuron PRE (3,270 kg/ha).