Glyphosate Resistance Confirmed in Common Ragweed from Columbia County, Wisconsin

Joe Zimbric (Graduate Research Assistant) and Dave Stoltenberg (Professor), Department of Agronomy, University of Wisconsin-Madison

Common ragweed (Ambrosia artemisiifolia) is a widely distributed weed species in Wisconsin. In a survey of 64 soybean fields located in 10 counties from southwest to east-central Wisconsin, we found that common ragweed was among the most abundant broadleaf weed species, present in 53% of all fields (Fickett et al. 2013). Globally, occurrence of herbicide resistance in common ragweed currently totals 37 unique cases of resistance to one or more herbicide sites of action (Heap 2018) including acetolactate synthase (ALS) inhibitor resistance in Wisconsin (Butts et al. 2015).

Among these 37 unique cases for common ragweed, resistance to glyphosate (Group 9 EPSP synthase inhibitors) or ALS inhibitors (Group 2) have been the most common (Heap 2018). However, the most recent reports have been cases of multiple resistance to glyphosate, ALS inhibitors, and protoporphyrinogen oxidase (PPO) inhibitors (Group 14). In 2010, multiple resistance to glyphosate and the ALS-inhibitor cloransulam was confirmed in a population found in a Minnesota soybean production system. In 2016, multiple resistance to cloransulam and the PPO-inhibitor fomesafen was confirmed in a population from Michigan. More concerning is three-way resistance (glyphosate, ALS- and PPO-inhibitors) which has been found in populations from a number of eastern states.

To our knowledge, the sole instance of confirmed herbicide resistance in Wisconsin common ragweed is a population from Brown County resistant to cloransulam (Butts et al. 2015). However, a common ragweed population located in Columbia County was reported in 2017 that was suspected of being resistant to glyphosate. Field histories suggested that this population had survived repeated exposure to glyphosate over several years in a long-term corn-soybean rotation.

We conducted research during 2018 to confirm and quantify suspected glyphosate resistance in this common ragweed population and also determine if the population showed multiple resistance to the ALS-inhibitor cloransulam and PPO-inhibitor fomesafen. Seeds were collected in September 2017 from suspected glyphosate-resistant (R) and -sensitive (S) plants. We conducted dose-response experiments under greenhouse conditions on the UW-Madison campus following standard methods for herbicide resistance testing.

Glyphosate Resistance Confirmed

The results of our experiments confirmed glyphosate resistance in the common ragweed population from Columbia County (Figures 1 and 2). The population showed a 4-fold level of glyphosate resistance based on the glyphosate rate that reduced shoot biomass 50% compared to non-treated plants, and over 20-fold level of resistance based on the rate that reduced shoot biomass 90%. Even at the 10X rate of glyphosate, many of the resistance plants survived although growth was severely reduced. In contrast, there was no evidence of multiple resistance to the ALS-inhibitor cloransulam or the PPO-inhibitor fomesafen (data not shown).

These results represent the first confirmed case of glyphosate resistance in common ragweed from Wisconsin. Including this case, there are now 20 unique cases (weed species by herbicide site of action) of herbicide resistance that have been confirmed in the state (Table 1). These 20 cases consist of 13 weed species with evolved resistance to one or more herbicide sites of action.

Resistance management strategies are key to reduce the selection for herbicide-resistant weeds, and if present, to reduce their persistence and spread. These strategies include:

* Understanding the biology of weeds present and using a diversified approach to managing those weeds with the intent to prevent weed-seed production.

* Using weed-free crop seed and planting into weed-free fields.

* Scouting fields routinely to aid in identifying potential weed management issues.

* Using appropriate cultural practices that increase crop competitiveness with weeds.

* Using multiple herbicide sites of action at the labeled rates and at recommended weed heights.

* Cleaning equipment after use to prevent spread of weed-seed from field to field.

More information on herbicide resistance management can be found at https://iwilltakeaction.com/weeds and http://wssa.net/wssa/weed/resistance/.

References

1. Butts TR, Davis VM, Stoltenberg DE (2015) Common ragweed (Ambrosia artemisiifolia) confirmed ALS inhibitor-resistant in Brown County, Wisconsin. Wisconsin Crop Manager https://ipcm.wisc.edu/blog/2015/03/common-ragweed-confirmed-als-inhibitor-resistant-in-brown-county-wisconsin/

2. Fickett ND, Boerboom CM, Stoltenberg DE (2013) Soybean yield loss potential associated with early-season weed competition across 64 site-years. Weed Science 61:500-507 3. Heap I (2018) International Survey of Herbicide Resistant Weeds. http://www.weedscience.org

4. Zimbric J, Stoltenberg D, Renz M, Werle R (2018) Herbicide resistance in Wisconsin: An overview. Wisconsin Crop Manager https://ipcm.wisc.edu/blog/2018/12/herbicide-resistance-in-wisconsin-an-overview-december-2018/

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