Herbicide-resistant weeds growing in Saskatchewan

Group 1 and Group 2 herbicide-resistant weed patches were widespread in Saskatchewan covering an estimated 15.315 million acres (6.198 million ha) in a total field area of 28.211 million acres (11.417 million ha). Given the linear increase in herbicide-resistant weeds, every annually cropped field in Saskatchewan could have at least one type of herbicide-resistant weed species by 2027.

Herbicide-resistant weeds are a major threat to food production across the world. As of 2023, 72 countries had confirmed cases of herbicide resistance covering 272 weed species, and Canada is third in the world with 56 confirmed cases, behind the United States (132 cases) and Australia (89 cases). Of the 31 known herbicide sites-of action, 21 have confirmed resistance to a weed species.

Herbicide-resistant weed surveys have been conducted across the Prairies over the past several decades. In Saskatchewan, surveys using the same preharvest methodologies have been conducted in 2003, 2009 and 2014/2015.

The most recent randomized-stratified preharvest survey was conducted in 2019 and 2020 covering 419 fields in Saskatchewan. The objective was to determine the distribution, frequency of occurrence, and impact of herbicide-resistant weeds with a focus on acetyl-CoA carboxylase (ACCase: Group 1)- and acetolactate synthase (ALS: Group 2)-inhibiting herbicides at greatest risk for resistance selection.

Fields were visited in August prior to harvest and uncontrolled weed patches visible from a W-shaped transect were sampled for weed seeds. In each patch, about 1000 mature seeds were collected from 10 to 20 plants of each weed species.

Fifty-one different weed species were found in the 1,651 weed seed samples collected from the 419 fields surveyed. Wild oat was found in 61% of fields, followed by kochia at 39%, common lambsquarters at 27%, wild buckwheat at 26% and green foxtail at 25%.

Seeds were grown in Agriculture and Agri-Food Canada’s Lethbridge greenhouse, and herbicides were applied at the 1 to 2 leaf stage at rates that would indicate herbicide resistance.

Herbicide-resistant grassy weeds

Wild oat is the most prevalent herbicide-resistant weed in Saskatchewan, and 82% of the 250 fields where wild oats were found had herbicide resistance. This was up from 65% in the 2014/15 survey.

Group 1 resistant wild oat were found in 77% of fields tested and 46% of all fields surveyed. This was similar to 45% of all fields surveyed in 2014/15 but up from 32% in 2009 and 10% in 2003.

Group 2 resistant wild oat were found in 30% of 74 fields tested, which was 18% of all fields surveyed. This was similar to the 2014/15 finding of 21% of fields surveyed, but much higher than 7% in 2009 and 4% in 2003.

Multiple herbicide-resistant wild oat to Group 1 + 2 herbicides was found in 26% of fields tested and 16% of all fields surveyed. Currently, no post-emergent herbicide for wheat, durum, barley, lentil and field pea remain to control wild oats with blanket resistance to all active ingredients in Groups 1+2.

Green foxtail resistance to Group 1 was confirmed in 34% of the 100 fields tested, or 8% of the 419 fields surveyed. This has grown from 0% of surveyed fields in 2003, 4% in 2009, and 5% in 2014/15.

Three fields with yellow foxtail were tested for resistance, and all had resistance to Group 1 herbicides. These fields were found in the southeast corner of Saskatchewan.

Broadleaf weed resistance

Twelve broadleaf weed species were found to have resistance to Group 2 active ingredients. The top five based on frequency and field area impact were led by kochia at 100% of fields tested representing 39% of fields surveyed. Spiny sowthistle was confirmed in 11 of 64 fields tested or 17% of fields tested and 3% of all fields surveyed. Redroot pigweed was found in 57% of fields tested and 4% of all fields surveyed. Group 2 resistant false cleavers was found in 42% of 31 fields tested and 4% of all fields surveyed. Rounding out the top 5 is Russian thistle with 3 of 4 fields with viable seed testing positive for Group 2 resistance.

The frequency of confirmation of acetyl-CoA carboxylase (ACCase)- or acetolactate synthase (ALS) inhibitor (HRAC Groups 1 and 2)-resistant weeds  among fields tested and among all fields sampled, and the land area and  field area occupied by each unique herbicide-resistant (HR) weed.

Geddes et al. 2024

Spiny sowthistle, pale smartweed, common lambsquarters, and common hempnettle were documented with Group 2 resistance for the first time in these recurring surveys of Saskatchewan.

At what cost?

When extrapolating the random survey to the total annual-cropped land in Saskatchewan, the researchers estimated that 15.3 million acres (6.2 million ha) were occupied by herbicide-resistant weed patches in a total field area of 28.2 million acres (11.4 million ha). To put that into perspective, total annual-cropped land according to Statistics Canada (2023) was 38.053 million acres (15.4 million ha). The estimated weed patch area with herbicide-resistant weeds increased by 29% since the 2014/15 survey, and the total field area impacted increased by 31% in the five years between surveys.

The surveys show that the area infested with herbicide-resistant weeds is increasing linearly by 0.74 million acre/year (0.3 million ha/year), and the affected field area is also increasing linearly by 1.48 million acre/year (0.6 million ha/year). At this rate, every annually cropped field in Saskatchewan could have at least one type of herbicide-resistant weed species by 2027.

During the 2014/15 survey, 250 farmers indicated the mean perceived cost of herbicide-resistant weeds in increased weed control expenditures and lost revenue from lower yields and grades was $12.15/acre ($30 per ha). For the current survey, this estimated cost would result in the loss of $343 million annually. This highlights the need for integrated weed management strategies that use cultural, physical and biological management strategies for weed control.

This research was funded by the Governments of Manitoba and Canada through the Canadian Agricultural Partnership, a federal, provincial, territorial initiative; in addition to the Alberta Wheat Commission, Manitoba Canola Growers Association, Manitoba Crop Alliance, Manitoba Pulse & Soybean Growers, Saskatchewan Ministry of Agriculture, Saskatchewan Wheat Development Commission, and Western Grains Research Foundation.

Dr. Charles M. Geddes, Ms. Mattea M Pittman, Dr. Shaun M. Sharpe, and Mrs. Julia Y. Leeson. Distribution, frequency and impact of herbicide-resistant weeds in Saskatchewan. Canadian Journal of Plant Science. Just-IN  https://doi.org/10.1139/cjps-2024-0017

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