Glyphosate-resistant weeds in Canada
by Dr. Charles Geddes
The many advantages of glyphosate for weed control, the introduction of glyphosate-resistant (GR) crops in 1996, and reduced cost of this herbicide following patent release in 2000 contributed to glyphosate becoming the most popular and most used herbicide in the world. In the last 20 years, however, 40 unique weed species developed resistance to glyphosate worldwide. Currently, five species with GR biotypes emerged in Canada; four in eastern Canada and one in western Canada. Giant ragweed (Ambrosia trifida), found in 2008 in Essex County, Ontario was the first GR weed in Canada. Essex county is the southern tip of crop production in Canada. Following this, Canada fleabane (Conyza canadensis) (2010), common ragweed (Ambrosia artemisiifolia) (2011) and tall waterhemp (Amaranthus tuberculatus) (2014) also showed up near this region. The first Canadian cases of GR in these weed species were found in GR soybean and can be linked to a general overuse of glyphosate in crop rotations.
Glyphosate is a group 9 herbicide active-ingredient, most commonly known by its original trade name Roundup. Following application of glyphosate and uptake by the plant, glyphosate binds to and inhibits the activity of the enzyme enolpyruvylshikimate-3-phosphate synthase (EPSPS). Inhibition of EPSPS blocks the function of the shikimate acid pathway responsible for the production of aromatic amino acids; which causes plant death by starvation.
Glyphosate is a non-selective, translocating herbicide that has activity on almost all plant species. It has been referred to as a “once in a century herbicide” (Duke and Powles, 2008) due to its many advantages and the large impact that this herbicide has had on agricultural crop production. Perhaps most importantly, pre-emergence use of glyphosate has contributed to an adoption of reduced- or zero-tillage in western Canada. Reduced tillage has led to a reduction in soil erosion, loading of pesticides, nutrients and/or sediments into waterways, greater soil moisture conservation and saved time that would have been spent on soil tillage.
In western Canada, GR kochia first cropped up in chemical-fallow fields near the Warner-Milk River area of southern Alberta. Since then, GR kochia biotypes showed up in all three Prairie Provinces. The populations from Alberta and Saskatchewan were in chemical-fallow or fields with frequent use of glyphosate pre-emergence. In Manitoba, however, most of the GR kochia populations have been found in the GR crops corn and soybean.
It is clear that an over-reliance on any herbicide mode of action (MOA) can select for herbicide-resistance. This too, is the case for glyphosate. Like all herbicide MOA, it is important to limit the frequency of glyphosate use in crop rotations and maintain diversity from both a herbicide MOA and non-chemical weed management standpoint. It is clear from other countries like the United States (currently total of 17 GR weed species) that maintenance of diversity in our cropping systems is a large contributor to reduced selection for glyphosate-resistance.
The production of GR crops in western Canada is increasing. As we grow these crops more frequently in our crop rotations, it is important to include additional tools for weed management, even if this means greater immediate cost prior to the development of GR weeds in your field.