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The Top Four Weeds Most Likely To Develop Glyphosate Resistance

With the widespread use of glyphosate in North America, it was only a matter of time before glyphosateresistant weeds appeared. At the Saskatchewan Soil Conservation Association conference in 2007, Hugh Beckie said that, “When glyphosate resistance occurs (in Canada), it will likely be first reported in Ontario.” True to form, glyphosate-resistant giant ragweed was confirmed in a monoculture glyphosate-resistant soybean field in Ontario in 2010.

Emboldened by his predictive success, Beckie, a weed scientist with Agriculture and Agri-Food Canada based at Saskatoon, Sask., has developed a formula to estimate glyphosate selection pressure in the most common western Canadian weeds. His objective was to identify the top three weeds in the grassland and parkland regions of the Prairie Provinces that may be at the greatest risk of selection for glyphosate resistance.

Beckie’s prediction is based on a variety of factors, including: the initial frequency of resistant plants prior to herbicide use; the relative fitness of resistant versus susceptible biotypes; the average seed bank longevity; and, selection pressure.

Glyphosate selection pressure was based on the relative abundance of the weed; proportional weed emergence as a function of soil growing degree (GDD) days under conservation tillage; with glyphosate application at preseeding (250 GDD), twice in-crop (650 and 850 GDD) and post-harvest (+1,000 GDD); and, glyphosate efficacy for each weed species. Agrologists Sean Dilk, Rick Holm, Eric Johnson and Ken Sapsford provided the assessment of glyphosate efficacy.

Beckie says the main risk factors for the evolution of herbicide-resistant weeds include: recurrent applications of efficacious herbicides with the same site of action; and annual weed species that occur at high population densities, are widely distributed, are prolific seed producers and have efficient gene dissemination through pollen or seeds.

Herbicide selection pressure — efficacy, soil persistence and frequency of application — has the greatest impact on resistance evolution. In-crop applications tend to provide the greatest selection pressure compared to other timing patterns. Other factors include the naturally occurring frequency of resistance genetics prior to herbicide use, the fitness of resistant biotypes and how long the weed species’ seeds remain viable in the soil.

Beckie did a weed risk assessment on ten of the most common weeds found in western Canada, including: green foxtail, wild oat, wild buckwheat, lamb’s-quarters, chickweed, stinkweek, redroot pig-weed, cleavers, kochia and wild mustard.

In the grassland region, his top three picks for developing glyphosate resistance are: kochia, wild oat and green foxtail. In the parkland region, cleavers is swapped in along with wild oat and green foxtail.

Beckie concluded that selection pressure is greatest with in-crop herbicide applications for all weed species except kochia, which, because it emerges so early, has greater selection pressure pre-seed than in-crop. He also notes that glyphosate resistance in Kansas kochia populations was also confirmed in 2010.

If any farmers suspect glyphosate resistance in kochia, wild oat, green foxtail, cleavers or any other weeds on their farms, Beckie says they should submit a weed seed sample to a lab for herbicide resistance testing.

BillStrautmanwritesfromSaskatoon,Sask.

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