Herbicide-resistant weeds growing in Manitoba
The reduction of crop yield and quality, and the increase in additional weed management expenditures were estimated to cost Manitoba farmers $77 million annually.
The confirmation of herbicide-resistant weeds continues to grow in Manitoba, across the Prairies, and worldwide. As of 2025 in Canada, 77 herbicide-resistant weeds have been recorded in the International Herbicide-Resistant Weed Database. And at least another seven have been identified that have not been registered in this database. Globally, there are 273 weed species and 541 unique cases of herbicide resistant weeds. Weeds have developed resistance to 21 of the 31 known herbicide sites of action and have been reported in 102 crops in 75 countries.
In Manitoba, the first herbicide-resistant weeds identified were green foxtail resistance to Group 3 (ethalfluralin and trifluralin), and kochia resistance to Group 2 (ALS inhibitor) in 1988.
Herbicide-resistant weed surveys have been conducted across the Prairies over the past several decades in 2001–2003, 2007–2009, and 2014–2017. In Manitoba, the last survey was in 2016. That survey found herbicide-resistant weeds in 68 per cent of fields surveyed, equating to a field area of 6.67 million acres (2.7 million ha). At that time, eight species including wild oat, green foxtail, yellow foxtail, barnyard grass, cleavers, wild mustard, redroot pigweed and shepherd’s purse were identified as herbicide-resistant. Kochia was not evaluated for resistance due to poor seed viability.
To update the status of herbicide-resistant weeds in Manitoba, a randomized-stratified survey of 155 annual-cropped fields in Manitoba was conducted in August 2022. The objective was to determine the distribution, frequency, and impact of herbicide-resistant weeds in Manitoba with a focus on Group 1 (ACCase inhibitors) and Group 2 resistant weeds.
Fields were visited in August prior to harvest and weed seeds were collected from uncontrolled weed patches visible from a W-shaped transect. Uncontrolled weed patches were observed in 147 of the 155 fields. In each patch, about 1000 mature seeds were collected from 10 to 20 plants of each weed species.
Overall, 44 different weed species were found in the 584 weed seed samples collected in the survey. These seeds were grown in Agriculture and Agri-Food Canada’s Lethbridge greenhouse, and Group 1 and Group 2 herbicides were applied at the 1–2 leaf stage.
At least one herbicide-resistant weed species was found in 72 per cent of the fields surveyed. This grew slightly from 2016 when 68 per cent of fields had a herbicide-resistant species, and 48 per cent in 2008, and 32 per cent in 2002.
It was estimated that herbicide-resistant weeds were on 38 per cent of annual cropland at 3.652 million acres (1.478 million ha). Based on field area, herbicide-resistant weeds occupied 7.079 million acres (2.865 million ha).
Eleven different weed species were found to be herbicide-resistant, with fourteen unique weed species x herbicide site-of-action confirmed.
The frequency of confirmation of Groups 1- and 2-resistant weeds among fields tested and among all fields surveyed, and the land area and field area occupied by each unique herbicide-resistant weed.
aThis preharvest survey excluded testing of kochia for resistance to auxin mimics (HRAC Group 4), glyphosate (HRAC Group 9) and protoporphyrinogen oxidase inhibitors (HRAC Group 14) due to limited mature seed present during the sample collection time frame. However, the area occupied by herbicide-resistant kochia is included in the aggregated values since all kochia samples tested were ALS inhibitor-resistant.
b All HR weeds accounts for overlap among patches of multiple different HR weeds and therefore the total area occupied is lower than the sum of that reported for each unique HR weed.
Source: Geddes et al. 2026
Herbicide-resistant grassy weeds
Group 1 herbicide-resistant wild oat was found in all the fields where wild oats were present, in 37 per cent of surveyed fields. This equates to a patch area of 1.063 million acres (430,000 ha) provincially, and a field area of 3.632 million acres (1.470 million ha).
Green foxtail resistant to Group 1 herbicides was found on 27 per cent of all fields surveyed. This covered a patch area of 2.095 million acres (848,000 ha), on a total field area of 2.894 million acres (1.171 million ha).
Group 1 herbicide-resistant yellow foxtail was found in 12 per cent of all fields surveyed, mainly in the south-central/eastern region of Lake Manitoba and Interlake Plains.
Wild oat that was resistant to Group 2 (ALS inhibitor) was found in 30 per cent of all fields surveyed. All wild oats with Group 2 resistance also had Group 1 resistance. This covered a patch area of 978,500 acres (396,000) ha representing a field area of 2.926 million acres (1.184 million ha).
Group 2 herbicide-resistant yellow foxtail was found in three per cent of all fields surveyed, all in south-central Manitoba. A single green foxtail sample from southwestern Manitoba exhibited ALS inhibitor resistance.
Herbicide-resistant broadleaf weeds
Kochia that was resistant to Group 2 herbicides was found in all fields where viable seeds were collected. Estimates put 19 per cent of Manitoba fields to have Group 2 resistant kochia.
Group 2 resistant redroot pigweed was found on nine per cent of all fields surveyed. Pale smartweed resistant to Group 2 herbicides was found in 10 per cent of all fields surveyed. Group 2 resistant spiny sowthistle was found in seven per cent of all fields surveyed. Other broadleaf weed species with Group 2 resistance were Powell amaranth, false cleavers, shepherd’s purse and common chickweed.
Four of these weed species, common chickweed, pale smartweed, Powell amaranth, and spiny sowthistle had not been observed to have herbicide resistance in the 2016 survey.
Millions of dollars in cost
In 2016, Manitoba farmers estimated that they incurred a loss of $27/ha because of herbicide-resistant weeds due to lost yield and quality, and additional weed control expenditures. Applying this cost to the current survey, the loss on the estimated field area of herbicide-resistant weeds would be $77 million in 2022.
While this survey focused on Group 1 and Group 2 herbicide-resistance, many other herbicide-resistant weeds/Groups have been confirmed in Manitoba, and across the Prairies. For example, herbicide-resistant wild oat has been confirmed to Group 15 (triallate) and multiple combinations of Groups 1 & 15 (triallate), Groups 2 & 15 (triallate), Groups 1, 2 & 15 (triallate), Groups 1, 2, 15 (triallate) & 25, Groups 1, 2, 14, 15 (triallate and pyroxasulfone) (MB).
And an emerging problem is herbicide-resistant kochia. Not only is it resistant to Group 2 herbicides, but resistance has also been confirmed across the Prairies to Group 4 (dicamba and fluroxypyr), and Group 9 (glyphosate), and multiple resistance to Groups 2 & 9, Groups 2 & 4, and Groups 2, 4 & 9. This in addition to the confirmation of Group 14 resistance in Saskatchewan and Alberta, and recently confirmed in Manitoba as well.
The researchers concluded that “the increasing impact of herbicide-resistant weeds emphasizes the critical need for adoption of integrated weed management where non-chemical tactics augment contemporary herbicidal weed control.”
This research was funded by the Governments of Manitoba and Canada through the Canadian Agricultural Partnership, a federal provincial, territorial initiative, with cofounding from Manitoba Canola Growers Association, Manitoba Crop Alliance, and Manitoba Pulse and Soybean Growers. This research was also funded by the Saskatchewan Ministry of Agriculture – Agriculture Development Fund through the Sustainable Canadian Agricultural Partnership with cofounding from Alberta Grains, Saskatchewan Wheat Development Commission, and Western Grains Research Foundation.
Dr. Charles M. Geddes, Ms. Mattea M Pittman, Mrs. Kim Brown, and Mrs. Julia Y. Leeson. Distribution, frequency, and impact of herbicide-resistant weeds in Manitoba. Canadian Journal of Plant Science. Open Access https://doi.org/10.1139/cjps-2025-0197
