MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.  

In the following article in this issue of the Field Crop CAT Alert, Steve Gower with MSU Diagnostic Services reports the first confirmation of glyphosate-resistant horseweed (or marestail) in Michigan. Steve has been actively screening weed samples for glyphosate-resistance over the last five years. We have been extremely lucky in Michigan, because of all of the samples that Steve had tested over the years none had been confirmed glyphosate-resistant until now.

Glyphosate-resistant weeds are not new to the United States. Currently, there are seven different weed species that are resistant to glyphosate. In fact, the first weed was a horseweed (marestail) population that was confirmed glyphosate-resistant in 2000 in Delaware. Since this first discovery, glyphosate-resistant horseweed has been found in 15 states with Michigan now being added as number 16. Our diverse cropping systems, I believe has helped us slow down the development of glyphosate-resistant weeds. However, now that we have resistance, there are several steps we need to do to manage resistant weeds and to further reduce the spread of glyphosate-resistance.

Earlier in Steve’s article, you learned that the horseweed population that was confirmed glyphosate-resistant was from a Christmas tree plantation in Mason County, Michigan. Repeated use of glyphosate to control weeds in this plantation was the main cause in the development of resistance. Even though Mason County is not a large county for agronomic crops, if resistance can be found in this county it may be present in other counties in Michigan. Possible other areas where we may have glyphosate-resistant horseweed are counties where no-till crop production is high and glyphosate is used exclusively for weed control. While resistance may not yet be present, following the practice of continuous glyphosate use without other weed control strategies will most likely lead to the development of glyphosate-resistance. So, how do we slow down the development of glyphosate-resistant weeds?

Diversity is the key. Whether it is diversity in tillage, herbicide use or cropping systems, diversity is one of the main strategies to slowing down the development of glyphosate-resistant weeds. Here are six main strategies that should be followed to help reduce the development of glyphosate-resistant weeds.

• Rotate glyphosate with herbicides that have different modes of action.
• Apply a residual herbicide before glyphosate or tank-mix another herbicide with glyphosate.
• If glyphosate is used as a burndown treatment and in-crop in the same year, tank-mix the burndown glyphosate treatment with an herbicide that has a different mode of action.
• Scout for changes in weed populations
• Use cultivation and other mechanical weed management practices, when appropriate.
• Use recommended rate for the appropriate weed height.

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 recent paper in Weed Science strongly suggests that horseweed seeds can enter the planetary boundary layer, where long-range transport is possible. In addition, we also have several horseweed populations in Michigan that are resistant to ALS-inhibitors. Since the only effective POST herbicides that can be used in soybeans are ALS-inhibitors and glyphosate, control of horseweed can be extremely difficult post-emergence in soybeans.

Key principles to horseweed management

• To effectively manage horseweed, it is important to control horseweed prior to planting.
• 2,4-D ester should be included in the burndown application. (Remember, a minimum of 7 days is needed between the application of 1 pt/A of 2,4-D ester and soybean planting.)
• 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.

For more information and specific herbicide recommendations there is a fact sheet on Controlling Horseweed on page 167 in E-434, 2007 Weed Control Guide for Field Crops. Also visit www.glyphosateweedscrops.org to view a regional bulletin on the biology and management of horseweed.