MSU Extension

In 2007, Michigan State University’s Diagnostic Services confirmed the first glyphosate-resistant weed biotype found in Michigan. This glyphosate-resistant weed was a horseweed (also known as marestail) population that was collected from a Christmas tree plantation in Mason County, Michigan. Since this discovery, MSU, with funding provided by Michigan Soybean Promotion Committee, has been actively screening horseweed and other weed populations that growers have suspected to be resistant to glyphosate. In 2010, two horseweed samples suspected of being resistant to glyphosate were collected from a no-till soybean field and a stale seed-bed sugarbeet field in Ionia and Gratiot counties were confirmed resistant to glyphosate. In 2011, six additional horseweed samples from Lapeer, Clinton, and Washtenaw counties were confirmed resistant to glyphosate and some of these samples are multiple resistant to glyphosate and ALS-inhibitors (FirstRate and Classic), which makes this weed extremely difficult to control in soybean.

Horseweed

Management of glyphosate-resistant horseweed

Many of the strategies that are used to reduce the development of glyphosate-resistance can also be used to manage glyphosate-resistant weeds. However, unlike other weeds, even if you follow the strategies to reduce the development of glyphosate-resistant weeds, you may find glyphosate-resistant horseweed in your fields. The main reason for this is that horseweed seed is wind-blown and can blow in from other sources. In fact, a paper in Weed Science strongly suggests that horseweed seeds can enter the planetary boundary layer, where long-range transport is possible, which makes containing these populations nearly impossible.

It is extremely important to take a proactive approach to managing horseweed, whether it is glyphosate-resistant, ALS-resistant, or contains multiple resistances to both classes of herbicides. Control of glyphosate- or ALS-resistant horseweed can be extremely difficult postemergence in soybean, since these are the only effective POST herbicide options available that can be effectively used in Roundup Ready soybean.

Key principles to horseweed management:

To effectively manage horseweed it is important to control horseweed prior to planting.

2,4-D ester (1 pt/A) or Sharpen (1 fl oz/A) should be included in glyphosate burndown applications prior to planting soybean.

• Remember a minimum of seveb days is needed between the application of 2,4-D ester (1 pt/A) and soybean planting.
• Methylated seed oil at 1 percent v/v must be included with Sharpen + glyphosate tank-mixes. Remember Sharpen or Sharpen the containing products (OpTill or Verdict) cannot be tank-mixed with Valor (flumioxazin) or Authority (sulfentrazone) containing products. There is no wait prior to planting soybean for Sharpen.

Liberty (22 fl oz/A) + Sharpen (1 fl oz/A) + MSO (1 percent v/v) + AMS or Gramoxone Inteon (3 pt) + Metribuzin (8 fl oz) + 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 to 6 inches in height.

Spring burndown applications with residuals will help prevent new emergence of horseweed.

• In soybean, the herbicides that provide good residual activity of glyphosate-resistant horseweed are the ALS-inhibitors, chlorimuron and cloransulam (Classic and FirstRate containing products) if the population is not ALS-resistant; Valor; and metribuzin-containing products. The most effective of these residual herbicides are the ones that contain two different modes of action. In soybean, the premixes Valor XLT, Envive, Sonic, Authority First, Authority XL, Gangster, Canopy, and Authority MTZ. Authority (Spartan) alone only provides fair control of horseweed. In many of the metribuzin containing premixes the rate of metribuzin maybe too low to provide adequate residual activity, according to research conducted at Ohio State University. Remember, many of these products have pH restrictions and long rotation restrictions to sugarbeet, dry bean, and other specialty crops.

Management of horseweed in corn is generally not an issue. There are several effective pre- andpost-herbicides that can be used to control glyphosate-resistant horseweed in corn.

For more information and specific herbicide recommendations, there is a fact sheet on Controlling Horseweed on page 171 in E-434, “2012 Weed Control Guide for Field Crops.” Also visit the Glyphosate, Weeds, and Crops website to view a regional bulletin on the Biology and Management of Horseweed.