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Management of glyphosate resistant and new, invading weeds in Ontario

Principal Investigator

Peter Sikkema

Research Institution

University of Guelph

External Funding Partners

BASF, Bayer CropSciences, Dupont, Monsanto, Pioneer, Syngenta, Valent

This project was funded in part through Growing Forward 2(GF2), a federal-provincial-territorial initiative. The Agricultural Adaptation Council assists in the delivery of GF2 in Ontario.

Project Start

April 2015

Project End

October 2017


  • Survey the distribution of glyphosate resistant weeds in Ontario.
  • Develop strategies for the control of glyphosate resistant waterhemp in soybean in Ontario.
  • Investigate the mechanism of resistance in glyphosate-resistant common ragweed.
  • Develop strategies for the control of glyphosate resistant Canada fleabane and common ragweed in soybean.
  • Determine the effect of cover crops seeded after winter wheat on Canada fleabane establishment.
  • Develop control strategies for new, invasive weeds in corn, soybean and wheat as they are identified.


  • The development of effective weed management programs for Glyphosate resistant (GR) common ragweed, GR waterhemp, GR giant ragweed and GR Canada fleabane in cropping systems will allow farmers to remain competitive in the global market and effectively use pesticides used in production.

Scientific Summary

Disclaimer: The information presented here does not constitute a recommendation by the researcher or Grain Farmers of Ontario. Always read and follow the pesticide label before use. Always ensure that you have the most current label.

One of the greatest challenges facing Ontario farmers is the control of glyphosate resistant (GR) and the control new, invasive weeds for which there is little or no efficacy data for registered herbicides. These new weeds are a result of: a) the widespread adoption of Roundup Ready crops and the associated dramatic increase in the use of glyphosate for weed control, b) the increased adoption of no-till production systems and therefore the increased use of glyphosate as a burndown, b) reduced use of herbicides with an alternate mode-of-action, and d) greater movement of farm machinery and produce which facilitates the rapid movement of weeds. There are five GR weed species in Canada: giant ragweed (Ambrosia trifida) [Ontario], Canada fleabane (Conyza canadensis) [Ontario], common ragweed (Ambrosia artemisiifolia) [Ontario], kochia (Kochia scoparia) [Alberta and Saskatchewan], and waterhemp (Amaranthus tuberculatus) [Ontario]. In studies conducted on Ontario farms yield losses in soybean due to Canada fleabane, giant ragweed and common ragweed competition was as high as 99, 96 and 92%, respectively.

This research, and the corresponding knowledge transfer, will increase the awareness of glyphosate resistant and invading weeds problem for Ontario farmers. This research program will strive to develop cost effective control strategies for producers who have these weed biotypes on their farms and will help reduce the spread of glyphosate resistant and invading weeds in Ontario. Glyphosate resistant (GR) common ragweed has been confirmed in Essex county. In soybean, the most effective soil applied herbicides are Sencor (87%) and Lorox (85%), and the most effective post-emergence (POST) herbicide is Reflex (75%). GR waterhemp has been confirmed in Essex, Kent and Lambton Counties. Of great concern is that 3-way multiple resistance to Group 2, 5 and 9 has been found on Essex and Lambton counties. In soybean, based on one year’s results, the most effective preemergence (PRE) herbicides are Fierce (97%), Focus (96%) and Authority Supreme (91%). GR giant ragweed has been confirmed in Essex, Kent, Lambton, Elgin, Middlesex and Huron counties. In soybean, the best control is provided by a preplant application of Roundup + 2,4-D. In corn, Marksman (88%) were the best herbicide applied POST. In winter wheat GR giant ragweed can be controlled with MCPA (89%), Trophy (92%), 2,4-D (94%) or Estaprop (93%). GR Canada fleabane has been confirmed in 28 counties in Ontario from Essex county in the southwest to Glengarry county on the Quebec border. Of great concern is that in 22 counties there is multiple 2-way resistance to Group 2 and 9. This makes control in soybean very difficult. In corn, GR Canada fleabane can be controlled with a PP application of Banvel, Marksman, Callisto + atrazine or Integrity which all provide greater that 90% control. Banvel, Distinct, Marksman or Pardner + atrazine applied POST provided greater than 90% control in corn. In soybean a three-way mix of Roundup + Eragon + Sencor provides greater than 90% control. The herbicide of choice in winter wheat is Infinity (92% control).