Weed populations, weed control with imazethapyr, and Glycine max yields were affected by a 4-yr history of different weed control practices. A range of chemical and mechanical practices were applied in separate field experiments initiated under high and low weed densities. In the high-density experiment, plots kept weed free for 4 yr averaged 24 Setaria faberi plants m−2 compared with 200 to 600 plants m−2 with the various weed control treatments. In plots with a history of mechanical control, weeds not controlled by imazethapyr reduced G. max yield by 340 kg ha−1 compared with plots that were kept weed free during the same period. In the low-density experiment, weed control history had less effect on weed densities. For example, S. faberi densities ranged from 19 plants m−2 for the weed-free plots to 195 plants m−2 with mechanical control. Weed control history had little effect on weed control with imazethapyr or G. max yields in imazethapyr-treated plots. While weed-free conditions for 4 yr greatly reduced weed densities, imazethapyr application still increased G. max yields 22% in the low-density experiment and 51% in the high-density experiment. Differences in densities of individual annual broadleaf species also developed in response to weed control history in both experiments.

Combined effects of herbicides, herbicide rates, and cultivation for weed control in soybeans [Glycine max (L.) Merr. ‘Williams’] were studied using full and one-half rates of either chloramben (3-amino-2,5-dichlorobenzoic acid) or alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] + linuron [3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea] with no, one, and two cultivations. The full rate was 2.2 kg/ha for alachlor and chloramben and 0.70 kg/ha for linuron. Alachlor + linuron was better than chloramben, and the full rate was better than the one-half rate. One or two cultivations were better than no cultivation. Cultivations were more effective when used with alachlor + linuron than when used with chloramben or when used alone. Alachlor + linuron at the full rate with one or two cultivations produced the best soybean yields. A one-half rate of alachlor + linuron with one or two cultivations yielded the same as a full rate of alachlor + linuron alone or with one cultivation, chloramben at one-half rate with two cultivations, and chloramben at a full rate with two cultivations. Cultivations can be effective by increasing weed control and yields when herbicide rate or effectiveness has been reduced. Use of a cultivation in addition to the preemergence herbicides used in this study is necessary for improved weed control and yields.

Combinations of herbicides applied preemergence and postemergence, with and without cultivation, were evaluated for their effectiveness for controlling weeds in soybeans [Glycine max (L.) Merr. ‘Williams’]. Annual weeds were controlled best when a sequential treatment of a mixture of alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] + linuron [3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea] was applied preemergence, and glyphosate [N-(phosphonomethyl)glycine] was applied postemergence, followed by one cultivation. The largest soybean yields were obtained when alachlor + linuron applied preemergence was followed by a cultivation, bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] applied postemergence, bentazon applied postemergence and a cultivation, or glyphosate applied postemergence and a cultivation. Regression analysis of the soybean yield vs. weed control data for all treatments indicated that soybean yields were suppressed an average of 180 kg/ha when glyphosate was applied postemergence. A cultivation in addition to the preemergence and postemergence treatments improved both weed control and soybean yield.

Field studies were conducted from 1998 through 2000 to determine the influence of crop rotation and level of herbicide system for johnsongrass, entireleaf morningglory, and smellmelon control in glyphosate-resistant cotton and corn. Three different crop rotation schedules were used including cotton–cotton–cotton, cotton–corn–cotton, and corn–cotton–corn. Herbicide systems involving various degrees of input levels (low, medium, and high) were compared with a conventional standard program. In 1998, weed control ranged from 80 to 95% for all herbicide systems when the rotation was corn–cotton–corn. In 1999 and 2000, the low-input herbicide system controlled entireleaf morningglory 76 to 78% late in the season. Decreased smellmelon control (78%) was also observed with the conventional standard during this same period. In the cotton–corn–cotton rotation, late-season entireleaf morningglory control decreased each year in the low-input system, regardless of crop. In 2000, late-season evaluations indicated lower weed control of all three species with the conventional standard program compared with the other input systems. Yield data from 2000 suggested that corn and seed cotton yields were influenced by crop rotation.

Field experiments were conducted across the north-central United States to determine the benefits of various weed control strategies in corn. Weed control, corn yield, and economic return increased when a preemergence (PRE) broad-spectrum herbicide was followed by (fb) postemergence (POST) herbicides. Weed control decisions based on field scouting after a PRE broad-spectrum herbicide application increased weed control and economic return. Application of a PRE grass herbicide fb a POST herbicide based on field scouting resulted in less control of velvetleaf and morningglory species, corn yield, and economic return compared with a PRE broad-spectrum herbicide application fb scouting. Cultivation after a PRE broad-spectrum herbicide application increased weed control and corn yield compared with the herbicide applied alone, but economic return was not increased. An early-postemergence herbicide application fb cultivation resulted in the highest level of broadleaf weed control, the highest corn yield, and the greatest economic return compared with all other strategies. Weed control based on scouting proved to be useful in reducing the effect of weed escapes on corn yield and increased economic return compared with PRE herbicide application alone. However, economic return was not greater than the PRE fb planned POST or total POST strategies.