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Early Management Horseweed Control in Soybeans is Important

Glyphosate- or multiple-resistant horseweed, or marestail, is fairly common in many of Michigan’s no-till soybean fields. In fact, of the 12 horseweed samples we tested for resistance during the 2014-15 winter, all showed some sort of resistance. Nine were multiple-resistant to glyphosate (Group 9) and ALS-inhibiting herbicides (Group 2), two were resistant to glyphosate only and one was resistant to the ALS-inhibitors only.

Because of resistance problems and weather conditions that favored horseweed emergence in 2014, horseweed control failures in soybeans were by far the most common phone call I received last summer. 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 for horseweed control
To effectively manage horseweed, it is important to control horseweed prior to planting. 2,4-D ester (1 pint per acre) or Sharpen (1 fluid ounce per acre) should be included in glyphosate burndown applications prior to planting soybeans. 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 percent v/v must be included with Sharpen plus glyphosate tank-mixes. There is no wait prior to planting soybeans for Sharpen, unless Sharpen or its 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.

Liberty (29-36 fluid ounces per acre) plus Sharpen (1 fluid ounce per acre) plus MSO (1 percent v/v) plus AMS, or Gramoxone (3 points) plus Metribuzin (8 fluid ounces) plus crop oil concentrate (1 percent 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 4-6 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 maybe too low to provide adequate residual activity. In many cases it is best to have 6-8 ounces per acre of metribuzin for residual control.

Remember, many of these products have pH restrictions and long rotation restrictions to sugarbeets, dry beans 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, see the Controlling Horseweed (Marestail) fact sheet on page 189 in “2015 Weed Control Guide for Field Crops,” Michigan State University Extension Bulletin E-434. Also, visit TakeActionOnWeeds.com to view a regional bulletin on the “Management of Herbicide-Resistant Horseweed in No-till Soybeans.”

Source: Christy Sprague, Michigan State University Extension

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