Researchers from the University of Guelph have confirmed they have found populations of Canada fleabane in southwestern Ontario that are resistant to glyphosate. This is the second weed species with confirmed resistance to glyphosate to be found in Canada. Glyphosate-resistant giant ragweed was confirmed in the same region of Ontario in 2008 according to www.weedscience.com, an academic-managed website that tracks all confirmed cases of weed resistance globally.

Canada fleabane, also known as marestail or horseweed, was the second species to develop resistance to glyphosate in North America, as confirmed in 2000 (Rigid ryegrass was found in 1998). It has since been confirmed in 18 states in the USA and has been documented in other world areas including Brazil, China, Czech Republic and Spain. However, only in the United States and Brazil - and now Canada - has glyphosate-resistant fleabane been found in agricultural crops. In the other countries, resistance was found in orchards and alongside railroads.

"We have seen the presence of glyphosate-resistant Canada fleabane moving north in the past decade so we are not completely surprised that we found this glyphosate-resistant weed in fields in Ontario," said Dr. Francois Tardif, Department of Plant Agriculture at the University of Guelph, who along with his colleague, Dr. Peter Sikkema, undertook the research.

The glyphosate-resistant weeds in question were found from seeds collected in soybeans fields in southwestern Ontario during the fall of 2010. Seed samples were sent to the University of Guelph from growers suspecting resistance to glyphosate. Of the 12 populations tested, eight survived the diagnostic dose of glyphosate in tests completed in the greenhouse. All resistant Canada fleabane populations were in Essex County.

"Glyphosate-resistant Canada fleabane can be problematic since it has wind blown seeds," explained Dr. Sikkema. "Currently, Canada fleabane is primarily a weed management focus in no-till crop production systems."

Monsanto is working cooperatively with the researchers at the University of Guelph to develop and communicate management strategies for the control of glyphosate-resistant Canada fleabane. The good news is there are economical and readily-available control options for farmers.

"There are already effective solutions available to farmers who are impacted," said Dr. Mark Lawton, Monsanto's Technology Development Lead in Eastern Canada. "These include products registered for use for pre-plant/pre-emergent burndown in soybeans (Eragon, FirstRate) and corn (Eragon and Banvel II). As well, there are registered options for in-crop control in both corn (Banvel II) and soybeans (FirstRate)."

Plans are in place to communicate these options to the affected growers to ensure they are aware of the available options for control of glyphosate-resistant Canada fleabane. As well, ongoing collaborative research with the University of Guelph is planned for this upcoming summer including the assessment of other weed management strategies, including the testing of Dicamba tolerant soybeans.

"The effective use of Roundup agricultural herbicides and Roundup Ready crops has continued in areas where glyphosate resistance has occurred," explained Dr. Lawton. "Glyphosate-resistant weeds have been effectively managed with good agronomic practices such as using tank mixes and/or cultural weed control methods. And in all cases, we continue to recommend that farmers use the right rate of glyphosate for the right sized weed at the right time, as well as additional weed control tools that may be necessary for the weed spectrum on their farm."

Monsanto takes product stewardship and claims of glyphosate resistance seriously and encourages growers to report suspected cases of resistance to Monsanto representatives so they can work with academics and extension services to investigate suspected cases, develop solutions for farmers and communicate the findings broadly. Monsanto's current best management practices include:

  • Start with a clean field by either utilizing a burn down herbicide or tillage to control weeds early.
  • Use Roundup Ready technology as the foundation of a total weed management program.
  • Add other herbicides or cultural practices where appropriate as part of the Roundup Ready cropping system.
  • Use the right herbicide at the right rate and apply at the right time.
  • Control weeds throughout the season to reduce the weed seed bank.
  • Be sure to include other crops (including glyphosate tolerant and non-glyphosate tolerant crops) in rotation with Roundup Ready crops to allow greater opportunity for the inclusion of other modes of action.

Farmers, who want to assess the likelihood of glyphosate resistance developing on their farm, can visit www.weedtool.com. They can also obtain additional information and stewardship recommendations in Monsanto's new pamphlet, "Best Practices for Weed Management: Start Clean and Stay Clean."