Alternative strategies are needed for management of glyphosate-resistant (GR) horseweed in soybean. Integrating a cereal rye cover crop with soybean planted in narrow rows may improve control and reduce herbicide selection pressure for herbicide-resistant horseweed biotypes. Four site-years of experiments were conducted in Michigan to determine whether fall-planted cereal rye terminated with glyphosate 1 wk prior to (early termination) or 1 wk after (planting green) planting in combination with narrow-row soybean improved GR horseweed management. At postemergence (POST) herbicide application, horseweed biomass was reduced by 71% to 90% when soybean was planted into cereal rye, regardless of termination time, compared with no cover across all row widths. Planting green or narrow-row soybean suppressed horseweed through soybean harvest. When glyphosate was applied POST (noneffective), horseweed biomass was 36% to 46% lower when planting green compared with early terminated cereal rye and no cover. Similarly, planting soybean in 19- and 38-cm rows reduced horseweed biomass by 48% and 28%, respectively, compared with 76-cm rows. Cereal rye did not affect soybean yield pooled over 3 of 4 site-years; however, narrow row soybean yielded 11% to 18% higher than 76-cm rows. Soybean yield was 11% higher when an effective POST herbicide was applied. In conclusion, fall-seeded cereal rye or narrow-row soybean suppressed horseweed compared with no cover and 76-cm rows; however, the effects of early termination did not last throughout the growing season in most cases. Delaying cover crop termination by planting green reduced horseweed biomass and density through soybean harvest, but reduced yield in 1 site-year due to an increased incidence of white mold. These cultural practices have a positive influence on suppressing horseweed that should be part of an overall horseweed management strategy; however, the use of an effective POST herbicide is still needed for complete season-long horseweed management.

Integrated strategies for management of glyphosate-resistant (GR) horseweed are needed to reduce reliance on herbicides. Planting a cover crop after corn or soybean harvest in the Upper Midwest may reduce horseweed establishment and growth. Experiments were conducted in Michigan to determine if cereal rye and winter wheat, seeded at 67 or 135 kg ha−1, and terminated with glyphosate at 1.27 kg ae ha−1 1 wk before planting (early termination) or 1 wk after soybean planting (planting green) would suppress establishment and growth of GR horseweed. Cover-crop biomass was 212% to 272% higher when termination was delayed by planting green compared with early termination. At the time of termination, cover crops reduced GR horseweed biomass 41% to 89% compared with no cover. Planting green increased the C:N ratio of cover-crop residue, which improved residue persistence and GR horseweed suppression at the time of POST herbicide application, approximately 5 wk after planting. Planting green reduced GR horseweed biomass 46% to 93% compared with no cover at the time of POST herbicide application; early termination provided less consistent suppression. Cover crops alone did not suppress GR horseweed through soybean harvest. Soybean yield was 30% to 108% greater when planting green compared with early termination at 2 site-years. Cereal rye and winter wheat, seeded at 67 or 135 kg ha−1, provided early-season GR horseweed suppression. Results from this research indicate that the practice of planting green may improve GR horseweed suppression through the time of POST herbicide application.

Auxin herbicides are used in combinations to control glyphosate-resistant horseweed preplant burndown. Herbicide labels for 2,4-D–containing products require a 30-d rotation interval for planting cotton cultivars not resistant to 2,4-D. Dicamba labels require an accumulation of 2.5 cm of rain plus 21 d per 280 g ae ha–1 rotation interval for planting cotton cultivars not resistant to dicamba. Previous research has shown that cotton injury caused by dicamba applied 14 d before planting was transient with little effect on cotton yield, whereas 2,4-D has little effect on cotton when applied 7 d prior to planting. Injury caused by dicamba and 2,4-D is inversely related to rainfall received between herbicide application and cotton planting. Experiments were conducted to evaluate cotton tolerance to halauxifen-methyl, a new Group 4 herbicide, applied at intervals shorter than labeled requirements. Experiments were established near Painter and Suffolk, VA, and Belvidere, Clayton, Eure, Lewiston, and Rocky Mount, NC, during the 2017 and 2018 growing seasons. Herbicide treatments included halauxifen, dicamba, and 2,4-D applied 4, 3, 2, 1, and 0 wk before planting (WBP). Visible estimates of cotton growth reduction and total injury were collected 1, 2, and 4 wk after cotton emergence (WAE). Cotton stand and percentage of plants with distorted leaves were recorded 2 and 4 WAE. Cotton plant heights were recorded 4 and 8 WAE. Halauxifen was less injurious (9%) than dicamba (26%) or 2,4-D (21%) 2 WAE when herbicides were applied 0 WBP. Cotton stand reduction 2 WAE by halauxifen was less than 2,4-D and dicamba when applied 0 WBP. Injury observed from herbicides applied 1, 2, 3, and 4 WBP was minor, and no significant differences in cotton stand were observed. Early-season cotton injury was transient, and seed cotton yield was unaffected by any treatment.