Field studies were conducted in 1988 and 1989 to examine the interrelations of cultivation and herbicide band width in controlling pitted morningglory and sicklepod in soybean. Alachlor + imazaquin PRE followed by imazaquin + surfactant POST were not applied or were applied on bands 15, 30, 45, 60, 75 cm wide or were broadcast. Plots were cultivated zero, one, two, or three times. Without cultivation, at least a 60-cm wide band was needed to achieve maximum soybean seed yields. A slight linear increase in soybean seed yield in response to increasing band width was observed with one cultivation. With two cultivations, soybean seed yields were similar with and without herbicides, but a 15-cm wide herbicide band was needed to achieve maximum production with three cultivations. Greatest gross returns exclusive of weed management costs were realized with two or three cultivations and a 15-cm wide band.

Glasshouse experiments were conducted to evaluate the effects of preemergence applications of metribuzin, pendimethalin, alachlor, and imazaquin on the competitive relationship between soybean and sicklepod, tall morningglory, or common cocklebur. An equal-ratio series design was employed where the proportion of species in a mixture remained constant and total plant density was varied. The ratio of soybean:sicklepod fresh weights was altered by alachlor and imazaquin while the ratio of soybean:tall morningglory fresh weights was altered only by imazaquin. The ratio of soybean:common cocklebur fresh weights was altered by metribuzin and imazaquin. Increasing herbicide rates resulted in higher soybean:weed fresh weight ratios and higher herbicide response coefficients.

Experiments were conducted to evaluate the effects of preemergence herbicides on the growth characteristics and competitiveness of sicklepod and common cocklebur to soybean. Alachlor was applied at 1.1 or 2.2 kg ai ha–1 and metribuzin at 0.2 or 0.4 kg ai ha–1 in glasshouse and field experiments. Alachlor at 2.2 kg ha–1 reduced sicklepod dry weight and leaf area 4 weeks after planting. Sicklepod water use was initially lowered by both rates of alachlor while soybean and common cocklebur water use was initially lowered by metribuzin at 0.4 kg ha–1. Sicklepod and common cocklebur fresh weights were reduced at soybean harvest by the high rates of alachlor and metribuzin. Soybean competing with sicklepod injured by 1.1 or 2.2 kg ha–1 of alachlor produced yields greater than soybean competing with untreated sicklepod. Soybean yields were not improved when metribuzin was used on common cocklebur.