Increasing frequency of multiple herbicide-resistant kochia (Bassia scoparia) in Alberta

Abstract Multiple herbicide-resistant kochia [Bassia scoparia (L.) A.J. Scott] is a growing weed management concern for farmers in the Canadian prairies. A randomized–stratified survey of 319 sites in Alberta was conducted in 2021 to determine the frequency and incidence of glyphosate, fluroxypyr, and dicamba resistance in kochia samples four and nine years after the previous rounds of surveys. Kochia samples exhibiting resistance to glyphosate, fluroxypyr, and dicamba were found at 78%, 44%, and 28% of the sites, respectively. Triple herbicide-resistant kochia samples, resistant to acetolactate synthase inhibitors, glyphosate, and at least one synthetic auxin, were found at 45% of the sites.


Introduction
Kochia [Bassia scoparia (L.) A.J. Scott] is a problematic summer-annual tumbleweed that has become increasingly difficult to manage in the Canadian prairies. Abiotic stress tolerance allows kochia to invade and thrive in disturbed agroecosystems (Geddes and Sharpe 2022). Kochia can cause substantial crop yield losses, making it a priority weed for many farmers. In a 2017 mid-season survey of annual crops in Alberta, kochia was the most abundant weed species found in the Mixed Grassland ecoregion .
High genetic diversity (Martin et al. 2020), prolific seed production (Beckie et al. 2016), and a short-lived (1-2 years) seedbank (Beckie et al. 2018) expedite kochia evolution in response to recurrent selection pressures. Herbicides remain the predominant weed control option implemented by prairie farmers, and in response, kochia in this region has evolved resistance to up to three herbicide modes of action (MOAs) (Beckie et al. 2019). Following resistance evolution, efficient seed-and pollen-mediated gene flow in kochia hastens the spread of resistance (Beckie et al. 2016). For example, acetolactate synthase (ALS) inhibitor (Herbicide Resistance Action Committee (HRAC) Group 2) resistance was ubiquitous among all kochia samples tested in the Canadian prairies about two decades after it was discovered in 1988 (Hall et al. 2014). Glyphosate (HRAC Group 9)-resistant (GR) kochia was documented in Canada in 2011, where it was confirmed initially in southern Alberta. A survey in 2012 documented GR kochia at 4% of the sites sampled in Alberta (Hall et al. 2014), which increased to 50% just five years later (Beckie et al. 2019). The 2017 survey also documented dicamba-resistant (DR) kochia at 18% of the sites, while 13% had fluroxypyrresistant (FR) kochia (two synthetic auxin herbicides; HRAC Group 4) (Beckie et al. 2019;Geddes et al. 2022aGeddes et al. , 2022c. All of the samples were ALS inhibitor-resistant, and 16% were triple herbicide-resistant to ALS inhibitors, glyphosate, and at least one synthetic auxin. Similar surveys in Manitoba documented an increase in GR kochia from 1% to 58% of sampled sites in 2013 and 2018, respectively, while the 2018 survey also documented DR kochia at 1% of the sites (Geddes et al. 2022b).
Multiple herbicide resistance limits the chemical options available for kochia control (Torbiak et al. 2021(Torbiak et al. , 2022, which can lead to kochia evading herbicide treatments and reducing crop yields (Geddes and Sharpe 2022). Understanding the status of herbicide resistance may help producers implement integrated weed management programs on their farms. The objectives of this study were to (i) document the frequency and incidence of resistance to glyphosate, fluroxypyr, and dicamba among kochia populations sampled in Alberta in 2021, and (ii) assess how the status of herbicide resistance in kochia changed compared with similar surveys conducted in 2012 (Hall et al. 2014) and 2017 (Beckie et al. 2019;Geddes et al. 2022a).

Materials and methods
A randomized-stratified roadside survey of Alberta was conducted postharvest during a three-week period in late-September/early-October of 2021. The methodology followed similar surveys conducted in 2012 and 2017 (Hall et al. 2014;Beckie et al. 2019;Geddes et al. 2022a). Surveyors visited 319 sites covering the area where kochia was abundant based on a 2017 mid-season weed survey . The sample sites were selected by spotting kochia from the road and stratified based on the cultivated area within each ecodistrict. Sites included cropland, pastureland, and ruderal areas (railway rights-of-way, ditches, approaches, and oil well sites). The stubble of each crop was classified as corn (Zea mays L.), oilseeds, pulses, small-grain cereals, or others.
Aboveground reproductive biomass was collected from 10-20 kochia plants and combined in a composite sample representing the kochia population at each site. The samples were dried at 30 • C, seed threshed under isolated conditions, and then stored at 4 • C until use. Each sample was evaluated for herbicide resistance following the single-dose screening methodology outlined by Geddes et al. (2022aGeddes et al. ( , 2022bGeddes et al. ( , 2022c. Each kochia sample was planted shallow in separate 26 cm × 26 cm × 5 cm greenhouse flats filled with Cornell soilless growing medium (756, 958, and 505 mg NPK L −1 mixture). The flats were placed in the greenhouse at 20/18 • C, under 16 h photoperiod supplemented with 100 μmol m −2 s −1 light, and watered daily. When plants reached 3-7 cm in height, the flats were treated with either glyphosate (Roundup WeatherMAX , Bayer CropScience Inc., Calgary, AB), fluroxypyr (Prestige TM XCA, Corteva Agriscience Canada Company, Calgary, AB), or dicamba (Banvel II, BASF Canada Inc., Mississauga, ON) at 900, 140, and 280 g ae ha −1 , respectively. Kochia densities were determined prior to treatment. At least 40 plants from each sample were evaluated with each herbicide. Herbicides were applied using a movingnozzle cabinet sprayer fitted with a flat-fan TeeJet 8002VS nozzle (Spraying Systems Co. Wheaton, IL) at a pressure of 275 kPa. The nozzle traveled at 2.4 km h −1 delivering 200 L ha −1 spray solution in a single pass 0.5 m above the plants.
The response of each sample to glyphosate, fluroxypyr, and dicamba was evaluated at 21, 28, and 28 days after treatment, respectively. Different assessment timing for glyphosate and the synthetic auxins was based on the time frame resulting in the greatest differentiation between resistant and susceptible biotypes when treated with these herbicides (Geddes et al. 2022a(Geddes et al. , 2022b(Geddes et al. , 2022c. Each plant within each sample was evaluated as resistant (no injury, or some injury with new regrowth) or susceptible (nearly dead, or dead). The number of resistant plants as a percentage of the total number of plants treated within each flat was considered the resistance incidence. Susceptible, low, moderate, and high resistance included samples with an incidence of 0%, 1%-20%, 21%-60%, and 61%-100%, respectively (Geddes et al. 2022b). Resistance frequency was the percentage of samples containing resistant plants within a given area or site classification. Maps of kochia resistance incidence and frequency were developed using QGIS 3.16 (QGIS Geographic Information System, Open Source Geospatial Foundation). The distributions of resistance incidence among sampled populations were plotted relative to those of the previous 2012 and 2017 surveys (Hall et al. 2014;Beckie et al. 2019;Geddes et al. 2022a) using the ggplot2 package of R v.4.2.1 (R Core Team, Vienna, AU).

Results and discussion
Kochia samples from 314/319 sites had enough viable seeds for resistance evaluations. Of these, 78% (245/314) had GR kochia present (Figs. 1 and 2). This compares with GR kochia found at 4% and 50% of the sites sampled and tested in 2012 and 2017, respectively (Hall et al. 2014;Beckie et al. 2019). Of the 245 GR samples, 114 (46%) had low resistance, 90 (37%) had moderate resistance, and 41 (17%) had high resistance (Fig. 1). As the survey timeline progressed from 2012 to 2017 to 2021, both a greater percentage of kochia samples (frequency) and a greater number of plants within samples (incidence) were GR (Fig. 1). Therefore, GR kochia is impacting more farms in Alberta and also having a greater impact on farms in which it occurs. GR kochia was found in all of the 18 counties sampled. The GR kochia frequency was greatest in pulses (95%), followed by small-grain cereals (83%), pasture (80%), oilseeds, corn, and other fields (75%), and ruderal areas (66%) (data not shown).
least one synthetic auxin) (Figs. 2 and S1). This represents a large increase from 16% documented in 2017, which were the first known cases of triple herbicide-resistant kochia in Canada (Beckie et al. 2019;Geddes et al. 2022a). In 2021, 10% of the sites had kochia with resistance to all herbicides tested. Of the 183 sites (58% of total) that had resistance to at least one synthetic auxin, 52% (94/183 or 30% of total) were FR but not DR, 24% (45/183 or 14% of total) were DR but not FR, and 24% (44/183 or 14% of total) were both FR and DR. Therefore, it is more likely that a synthetic auxin-resistant kochia population in Alberta is resistant to one of these active ingredients and not the other than it is resistant to both. This further Fig. 2. Frequency of (A) glyphosate-, (B) fluroxypyr-, and (C) dicamba-resistant kochia confirmed within each county sampled during a 2021 survey of Alberta, and (D) a Venn diagram showing the overall frequencies of resistance to these herbicides among the 314 sites evaluated. Base layers: municipality boundaries (Altalis Ltd., Calgary, AB, www.altalis.com); map projection: NAD83/Alberta 10-TM.
supports the same conclusions made by Geddes et al. (2022aGeddes et al. ( , 2022c and suggests that fluroxypyr and dicamba resistance in kochia in Alberta are likely conferred by separate mechanisms. The current study documented the continued increase in kochia populations resistant to glyphosate, fluroxypyr, and dicamba in Alberta. While the frequency of resistance to these herbicides has grown rapidly, many of the sites with FR and DR kochia exhibited low resistance due to a lower number of resistant individuals within the samples. This likely is an indication that synthetic auxin resistance will continue to grow in kochia populations absent of implementing alternative and truly integrated management programs. Future surveys should aim to incorporate assessments of integrated weed management adoption with evaluations of herbicide resistance to determine linkages between management practices and the frequency or incidence of herbicide resistance among farms. High genetic diversity (Martin et al. 2020) combined with unfettered seed-and pollen-mediated gene flow in kochia (Beckie et al. 2016) contributes to rapid evolution and spread of herbicide resistance traits. Therefore, a community-based approach is necessary to mitigate kochia re-infestation in fields where effective management tactics are implemented. The short-lived (1-2 years) kochia seedbank (Beckie et al. 2018) represents an opportune target for weed management strategies that mitigate kochia seed production and return to the soil seedbank (Geddes and Davis 2021). With increasing resistance to multiple herbicide MOAs, it is clear that nonchemical approaches, such as growing competitive crops, increasing crop seeding rates (Geddes and Kimmins 2021), and judicious--yet strategic--tillage (Obour et al. 2021), will have a prominent role in preserving the efficacy of the remaining herbicides available for kochia control.

Article information
History dates