Giant Ragweed Resistance to Glyphosate in Wisconsin

Giant Ragweed Resistance to Glyphosate in Wisconsin

Dave Stoltenberg (Weed Scientist, Dept. of Agronomy, UW-Madison), Melinda Yerka and Courtney Glettner (Graduate Research Assistants, Dept. of Agronomy, UW-Madison), Jim Stute (Crops and Soils Educator, Rock County, UW-Extension), Tim Trower (Senior Outreach Specialist, Dept. of Agronomy, UW-Madison)

Giant ragweed (Ambrosia trifida) with resistance to glyphosate has been confirmed in several Midwest states, including our neighboring states of Minnesota and Iowa.  In Minnesota, as well as Ohio, giant ragweed populations have evolved resistance to more than one herbicide mode of action (i.e. multiple resistance), specifically, resistance to glyphosate and ALS (acetolactate synthase) inhibitors.

As reported at the 2012 Wisconsin Crop Management Conference in January, our research has confirmed giant ragweed with resistance to glyphosate in Rock County, Wisconsin.  This giant ragweed population was identified in a typical corn and soybean cropping system with a history of glyphosate use for weed management.  We found resistant plants to have more than nine-fold resistance to glyphosate compared to plants susceptible to glyphosate. The glyphosate-susceptible plants used in our research were from seeds collected on the same farm where resistant plants were identified.  This level of resistance means that plants in a resistant population can survive glyphosate applied at rates much higher than would normally be lethal to susceptible plants (Figure 1).  Although the glyphosate-resistant plants may show injury symptoms, we’ve observed that they typically survive treatment, continue to grow, mature, and produce seeds.

Figure 1. Rock County giant ragweed 4 weeks after treatment with 3.0 lb ae/glyphosate under greenhouse conditions at UW-Madison.

Preliminary results suggest that resistance of this Rock County giant ragweed population is due to an altered enzyme target site, such that glyphosate is less effective in inhibiting activity of this enzyme.  Our research also suggests that glyphosate-resistant plants were not resistant to ALS-inhibiting herbicides (e.g. cloransulam).

These findings represent the first confirmed case of weed resistance to glyphosate in Wisconsin.  What can we learn from these findings?  First, glyphosate and glyphosate-resistant crops are valuable technologies to many growers in Wisconsin, and are utilized on millions of acres.  This means that there is wide-spread selection for weed resistance to glyphosate.  Second, to protect the long-term value of these technologies, we need to implement proactive weed management practices that reduce the risk of selecting for glyphosate-resistant weeds (as well as resistance to other herbicide modes of action).  Several best management practices for reducing the risk of weed resistance to herbicides have recently been put forward by the Weed Science Society of America ( The strategy is to consider utilization of all cultural, mechanical, and herbicide options available for effective weed control in each management situation:

  1. Understand the biology of the weeds present.
  2. Use a diversified approach to weed management focused on preventing weed seed production and reducing the number of weed seeds in the soil seedbank.
  3. Plant into weed‐free fields and then keep fields as weed free as possible.
  4. Plant weed‐free crop seed.
  5. Scout fields routinely.
  6. Use multiple herbicide mechanisms of action that are effective against the most troublesome or herbicide‐resistance‐prone weeds.
  7. Apply the labeled herbicide rate at recommended weed sizes.
  8. Emphasize cultural practices that suppress weeds by utilizing crop competitiveness.
  9. Use mechanical and biological management practices where appropriate.
  10. Prevent field‐to‐field and within‐field movement of weed seed or vegetative propagules.
  11. Manage weed seed at harvest and post‐harvest to prevent a buildup of the weed seedbank.
  12. Prevent an influx of weeds into the field by managing field borders.

Our current research is investigating other instances of suspected giant ragweed resistance to glyphosate and ALS-inhibiting herbicides.  These best management practices should be promoted and used to reduce the likelihood of further cases of weed resistance to herbicides from occurring in Wisconsin.  Weed resistance to glyphosate and other herbicides will be discussed at the Pest Management Field Day on July 11 at the UW-Arlington Agricultural Research Station.  For more information on the field day see