Glyphosate Resistant Giant Ragweed Suspected in Ontario

A giant ragweed population in Essex County, is the first suspected Canadian case of resistance to the popular herbicide glyphosate.

Preliminary testing at the University of Guelph has shown that the population is able to survive glyphosate rates that completely kill normal susceptible plants. These results represent a major problem for growers, as glyphosate has become a tool of choice for the control of this extremely competitive weed. The appearance of glyphosate resistant populations complicates management of this species.

Resistance evolves after a weed population has been subjected to intense selection pressure in the form of repeated use of a single herbicide. Where this population has been identified, glyphosate has been the main herbicide for over a decade. The herbicide controls all the susceptible weeds leaving only those that have a resistant gene to reproduce. Over time the uncontrolled weeds come to dominate the population. There are eight different weed species resistant to glyphosate in the United States and 15 species worldwide.

Control strategies for glyphosate resistant giant ragweed

Non-glyphosate products that have provided good control of giant ragweed are shown in soybean (Table 1a) and corn (Table 1b). Dr. Francois Tardif and Dr. Peter Sikkema will be examining management options for glyphosate resistant giant ragweed.

If you suspect that you have a population of glyphosate resistant weeds notify our public weed researchers by going to http://www.ontarioweeds.com/weedID/add.php or phone 1-877-424-1300.