Horseweed (marestail) continues to be the #1 weed escape in Michigan soybean fields. Resistance to ALS-inhibitors (Group 2) and/or glyphosate (Group 9) leaves few options for postemergence horseweed control. In addition to multiple herbicide-resistance, prolonged emergence, changes in tillage practices and capabilities of long distance seed dispersal have also contributed to the increased prevalence of horseweed in many of Michigan’s soybean fields. In the past, we have conducted research that shows that fall-seeded cereal cover crops provide early-season horseweed suppression compared with no cover. Additionally, delaying cover crop termination by ‘Planting Green’ produced higher cover biomass, reduced horseweed biomass, and improved soybean yields compared with early terminated covers. Through other observations, we have noticed that soybean canopy may have a significant impact on horseweed growth. Narrowing soybean row widths in addition to cover crops and effective herbicide programs may be the best integrated approach to managing horseweed. In 2020 with funding provided by the Michigan Soybean Promotion Committee, we were able to expand on our previous research and examine the following objectives. 1) Evaluate the effect of a fall planted cereal rye terminated one week before soybean planting and one week after soybean planting for the suppression of glyphosate-resistant horseweed in no-till soybean. 2) Determine the contribution of soybean row width on horseweed suppression by comparing soybean planted in 7.5-, 15-, and 30-inch rows. 3) Compare the integrated approaches of cover crop and soybean row width with and without a POST herbicide application on horseweed management. 4) Provide this new and additional information on the management of herbicide-resistant horseweed to Michigan soybean producers. We are proposing to continue with these objectives during the 2021 growing season. This research will be important to provide effective information to soybean growers to help manage this troublesome economically significant weed problem.

1) Evaluate the effect of a fall planted cereal rye terminated one week before soybean planting and one week after soybean planting for the suppression of glyphosate-resistant horseweed in no-till soybean.
2) Determine the contribution of soybean row width on glyphosate-resistant horseweed suppression by comparing soybean planted in 7.5-, 15-, and 30-inch rows.
3) Compare the integrated approaches of cover crop and soybean row width with and without a POST herbicide application on horseweed management.
4) Provide this new and additional information on the management of herbicide-resistant horseweed to Michigan soybean producers.

The continued spread of glyphosate/multiple-resistant horseweed will have a detrimental effect on the economics of Michigan crop production. This research will be important to provide effective information to soybean growers on management of this economically significant weed problem. It will also provide options in addition to herbicides to aid in the management of a weed that can develop resistance to multiple-herbicide sites of action.