By Christy Sprague, Extension Weed Specialist 

Glyphosate- or multiple-resistant horseweed (marestail) is fairly common in many of Michigan’s no-till soybean fields (see map of Michigan below). In fact, of the 18 horseweed samples tested for resistance this winter by Michigan State University Diagnostic Services, 100 percent of them were resistant to glyphosate (Group 9) and 78 percent were resistant to both glyphosate (Group 9) and the Group 2 (ALS) herbicides.

Because of resistance problems and weather conditions that favored horseweed emergence last year in 2015, horseweed control failures were common in soybeans. Many of these control failures could have been avoided with an appropriate burndown herbicide program. To control horseweed, whether it is glyphosate-resistant, ALS-resistant or contains multiple resistances to both classes of herbicides, it is extremely important to take a proactive approach.

Effective Burndown Options

To effectively manage horseweed, it is important to control horseweed prior to planting. 2,4-D ester (1 pint per acre) orSharpen (1 fluid ounce per acre) should be included in glyphosate burndown applications prior to planting soybean.

Remember, a minimum of seven days is needed between the application of 2,4-D ester (1 pint per acre) and soybean planting. Methylated seed oil at 1% v/v must be included with Sharpen + glyphosate tank-mixes. There is no wait prior to planting soybean for Sharpen, unless Sharpen or Sharpen the containing products (Optill, Optill Pro or Verdict) are tank-mixed with Valor (flumioxazin) or Authority (sulfentrazone) containing products, then there needs to be 14 days between applications of these products and planting soybean.

Counties where glyphosate- (Group 9) or multiple- (Group 2 and 9) resistant horseweed (marestail) have been confirmed in Michigan.

Liberty (29-36 fluid ounces per acre) + Sharpen (1 fluid ounce per acre) + MSO (1% v/v) + AMS or Gramoxone (3 pints) + Metribuzin (8 fluid ounces) + crop oil concentrate (1% v/v) are two other burndown treatments that were extremely effective at controlling glyphosate-resistant horseweed.

Horseweed is most susceptible in the rosette stage (less than 2 inches in height). Herbicides should be applied before plants are 4 to 6 inches in height. Spring burndown applications with residuals will help prevent new emergence of horseweed.

In soybeans, the herbicides that provide good residual activity of glyphosate-resistant horseweed are the ALS-inhibitors (Group 2), chlorimuron and cloransulam (Classic and FirstRate containing products) if the population is not ALS-resistant. However, many of our populations are resistant to the ALS-inhibiting herbicides so our better residual herbicide choices are Valor, Spartan (Authority) (Group 14) or Metribuzin (Group 5) containing products.

The most effective of these residual herbicides are the ones that contain two different herbicide Sites of Action groups (Groups 14 and 5). In many of the metribuzin containing premixes, the rate of metribuzin may be too low to provide adequate residual activity, in many cases it is best to have 6 to 8 ounces per acre of metribuzin for residual control.

Remember many of these products have pH restrictions and long rotation restrictions to sugarbeet, dry bean and other specialty crops. Additionally, there needs to be 14 days between herbicide application and soybean planting if Valor or Spartan (Authority) containing products are tank-mixed with Sharpen or Sharpen containing products.

For more information and specific herbicide recommendations, there is a fact sheet on Controlling Horseweed on page 199 in E0434, “2016 Weed Control Guide for Field Crops.” Also, visit TakeActionOnWeeds.com to view a regional bulletin on the “Management of Herbicide-Resistant Horseweed in No-till Soybeans.”