Contributions of shading, soybean row-width, and planting green on horseweed management compared with soil-applied residual herbicides

Glyphosate-resistant (GR) horseweed is a problematic weed for Michigan soybean growers. Additionally, rosette- and upright- horseweed growth types have been observed co-emerging during mid- to late-summer in several Michigan fields. In the greenhouse, shade levels from 35 to 92% reduced rosette and upright horseweed biomass 31 to 99% compared with the upright growth type grown under 0% shade. Greater reductions in biomass occurred under 69 and 92% shade. Thus, increased shading by planting in narrow rows and/or planting green into cereal rye may improve horseweed suppression. A field experiment conducted over three site-years compared the effect of fall-planted cereal rye terminated with glyphosate 1 wk after planting (planting green) with a preemergence (PRE) residual herbicide program (glyphosate + 2,4-D + flumioxazin + metribuzin) on horseweed control in soybean planted in three row widths (19-, 38-, and 76-cm). Planting green or applying a residual herbicide program across all row widths reduced horseweed biomass 86 to 91% and 95 to 99%, respectively, compared with soybean planted with no cover in 76 cm rows, 4 to 6 wk after planting (WAP). At soybean harvest, when a noneffective postemergence (POST) herbicide (glyphosate) was applied horseweed biomass was 42 and 81% lower by planting green or applying a residual herbicide program compared with no cover, respectively. Similarly, planting soybean in 19 cm rows reduced horseweed biomass compared with 38- and 76-cm rows. When an effective POST program was applied, similar horseweed biomass reductions were observed by planting green or applying a residual herbicide across all row widths. Additionally, soybean yield and economic returns were similar between planting green and applying a residual herbicide in 1 of 2 site-years. Integrating planting green and an effective POST herbicide program offers an alternative horseweed management strategy to applying a residual preemergence herbicide program.