Response of Horseweed Accessions from Wisconsin and Northern Illinois to Spring Burndown Herbicides

Horseweed (aka marestail) Erigeron canadensis:

  • annual weed species that emerges in the fall or early in the spring
  • prolific seed producer (up to 200,000 seeds per plant)
  • seeds are commonly dispersed by wind
  • can be a troublesome weed in no-till systems
  • often resistant to glyphosate (Group 9) and Group 2 herbicides

A greenhouse study was conducted at the University of Wisconsin-Madison in the fall of 2020 to evaluate the response of 21 horseweed accessions from Wisconsin and northern Illinois to spring burndown herbicides (Figure 1). Horseweed seeds from suspected glyphosate-resistant accessions were collected by farmers, consultants and Extension educators.


The treatments selected for this greenhouse screening represent common corn and/or soybean spring burndown herbicides used by US Midwest farmers adopting no-till. Herbicides were sprayed at the label rate (except for Roundup PowerMAX which was also sprayed at 3X the label rate) when horseweed plants at the rosette stage reached approximately 3.5 inches in diameter (Table 1). Visual control and biomass reduction data were collected 21 days after treatment.


Key findings from this research:

  • Sharpen, Liberty, Gramoxone, AAtrex, Callisto, and Xtendimax provided effective horseweed control (>90% control at 21 DAT). Enlist One was slightly less effective than the aforementioned options (Figures 2 and 3).

  • Roundup PowerMAX (1x and 3x) and Pursuit did not provide satisfactory (<90%) horseweed control (Figures 2 and 3).

  • Glyphosate (Roundup PowerMAX, Group 9) and imazethapyr (Pursuit, Group 2) resistance appear to be widespread in horseweed accessions from southern WI and northern IL.


For full study details, check out the UW-Mad About Weeds YouTube summary video “Response of Putative Glyphosate-Resistant Horseweed from Wisconsin to Spring Burndown Herbicides”:

Always read, understand and follow the pesticide label.

Acknowledgments: we would like to thank the Wisconsin Soybean Marketing Board for sponsoring this project. Many thanks to the stakeholders who contributed to this project by collecting and submitting horseweed seeds.


  • Alexandre Rosa (former UW-Madison Weed Science Postdoctoral Research Specialist)
  • Nicholas Arneson (UW-Madison Weed Science Outreach Specialist)
  • Maxwel Oliveira (UW-Madison Weed Science Research Analyst)
  • Rodrigo Werle (UW-Madison Extension Cropping Systems Weed Scientist)