Kochia Resistance Update: Results from the 2019 to 2023 Glyphosate, Dicamba, and Fluroxypyr-Resistant Kochia Prairie Surveys 

Research Experts: Dr. Shaun Sharpe (AAFC Saskatoon), Julia Leeson (AAFC Saskatoon), Dr. Charles Geddes (AAFC Lethbridge) 

Kochia Characteristics 

Kochia is one of the most challenging weeds Western Canadian pulse growers face. Kochia is highly invasive and heat, drought, and salinity tolerant. Also, kochia leaves are downy with a waxy surface, making herbicide penetration difficult. One of the first plants to emerge in the spring, kochia can form thick mats and grow up to two meters in height. Once mature and dried down, kochia will travel as a tumbleweed in the wind to adjacent fields and marginal land, dropping thousands of seeds as it rolls. Because of its ability to spread geographically, kochia is found in cropland, pastureland, railways, ditches, and marginal land. In cropland, kochia tends to form thick patches in poorer-producing areas of the field, completely overtaking the crop and spreading throughout the field over time. Along with the natural competitiveness of kochia, this weed is also one of the top weed resistance concerns for growers. Kochia can self- and cross-pollinate, which allows for the spread of resistance traits between plants. Also, kochia’s short-lived seed bank of 1–2 years means a population turns over and evolves quickly. Finally, one plant produces 10,000 to 25,000 seeds, allowing resistant plants to multiply rapidly. Combined with kochia’s capacity to spread geographically, these capabilities have led to kochia being resistant to multiple herbicide actives. 

Kochia Herbicide Resistance Survey History 

Herbicide-resistant kochia was first identified in Western Canada in 1988 with resistance to group 2 acetolactate synthesis (ALS) inhibitors. By 2007, ALS resistance occurred in 85% of 109 fields assessed across Western Canada. Today, all kochia is assumed to be group 2 resistant.  

Glyphosate (group 9) was the next active to develop herbicide resistance, first emerging in research surveys in western Canada in 2011. Glyphosate resistance (GR) occurred at 4% of sites in a 2012 survey in Alberta, rising to 50% in 2017. In a 2013 survey, glyphosate resistance was 5% in Saskatchewan and 1% in Manitoba. 

Following glyphosate resistance, group 4 resistance was identified in 2015, with a Saskatchewan kochia population resistant to dicamba and fluroxypyr. In a 2017 survey in Alberta, 18% of kochia was resistant to dicamba, and 13% was resistant to fluroxypyr. In the same survey, 10% of samples were resistant to ALS inhibitors, dicamba, and glyphosate, with 4% of samples resistant to dicamba and fluroxypyr. Group 4 resistance has yet to be surveyed in Saskatchewan. No confirmed cases of dicamba resistance had been reported in Manitoba before this survey.  

From past survey work, kochia is an aggressive weed that quickly develops herbicide resistance, threatening Western Canadian pulse production. This research study aimed to survey the current levels of kochia resistance to glyphosate and dicamba in Alberta, Saskatchewan, and Manitoba to inform future kochia management practices. 

Survey Methodology 

The survey occurred in Manitoba in 2018, Saskatchewan in 2019, and Alberta in 2021, in regions of each province where kochia frequently occurs. The survey was a randomized stratified design. Sampling took place post-harvest in October each year when kochia plants had matured and set seed. Samples were taken at 255 sites in Saskatchewan, 300 sites in Manitoba, and 314 sites in Alberta. Plants were sampled from various land uses, including cropland, pasture, ditches, railways, and oil well sites. Ten to 20 mature kochia plants were combined into a composite sample at each site. Plants were threshed, and seed was grown in the greenhouse. After reaching 3–5 cm, the plants were screened for resistance, with glyphosate (Roundup WeatherMAX®) applied at 900 g ae ha^-1 (360 g ae ac^-1, which is 1 L of 360 formulation) and dicamba (Banvel® II) at 280 g ae ha^-1 (113 g ae ac^-1). For the Alberta samples only, fluroxypyr (Prestige™ XCA) was applied on the kochia samples at 140 g ae ha^-1 (56.7 g ae ac^-1, the 20 ac/case rate). Plants were assessed for their response to the herbicides three weeks after glyphosate application and four weeks after dicamba and fluroxypyr application. Within each sample site for each herbicide, the percentage of samples that survived, also known as resistance incidence, was assessed and grouped as low (1-20% kochia survivorship), medium (21–60%), or high (61–100%) resistance. Results were mapped to show the distribution of resistance across each province. 

Survey Results 

*Only glyphosate was assessed in the survey

Survey results are summarized in Table 1 and displayed geographically by province in Figures 6, 7, 8, and 9. Kochia was resistant to glyphosate at 78% of sites in Alberta, 87% in Saskatchewan, and 58% in Manitoba. Kochia was resistant to dicamba at 28% of Alberta sites, 45% of Saskatchewan sites, and 1% of Manitoba sites. The first incidence of dicamba resistance in Manitoba was identified in this survey, as well as multiple resistance to both glyphosate and dicamba (0.7%). Because all kochia is considered ALS inhibitor-resistant, 10% of Alberta sites, 40% of Saskatchewan sites, and less than 1% of Manitoba sites had triple resistance. Samples in Alberta were further screened for fluroxypyr, and 44% of the samples were resistant, with 25% being triple resistant to ALS inhibitors, glyphosate, and fluroxypyr. 

Resistance incidence, the percentage of kochia plants with no herbicide injury or those with non-symptomatic regrowth (chlorosis, epinasty, cupping, or twisting of leaves) indicating they will survive the herbicide treatment, is summarized in Table 2. In Alberta, 46% of sites with glyphosate resistance had low resistance, meaning less than 20% of plants survived the glyphosate application. 37% of sites had moderate resistance (21 to 60% survivorship), and 17% showed high resistance (61 to 100% survivorship). Fluroxypyr resistance incidence levels were 74% low, 25% moderate, and 1% high. Dicamba resistance incidence in Alberta was low in 94% of samples and moderate in 6%. In Saskatchewan, 22% of sample sites had high glyphosate resistance, 31% were moderate, and 33% were low. However, dicamba resistance incidence was relatively low, with only 1% of samples at moderate to high levels. In Manitoba, incidence levels for glyphosate were 44%, 38%, and 18% low, medium, and high resistance, respectively. All incidences of dicamba resistance were low in Manitoba. 

Survey Implications and Management Considerations 

The precise outcome of this Western Canada-wide resistance survey is that herbicide-resistant kochia, especially glyphosate resistance (GR), is on the rise. Over the four to six years between each province’s surveys, GR increased by 28 (Alberta) to 82 percentage points (Saskatchewan). Incidence levels were highest in Saskatchewan, with 53% of samples showing moderate to high GR. Incidence levels were lowest in Manitoba, with only 34% of samples at moderate to high incidence. Complete ALS inhibitor resistance took 20 years to develop in Western Canada, and based on the survey results, GR in kochia appears to be on the same trajectory.  

Group 4 herbicide resistance is less extensive than GR across the Prairies, likely because dicamba and fluroxypyr resistance began four years after GR. A significant result is that Group 4 resistance incidence levels are also much lower than for glyphosate. Almost all dicamba resistance was low, and 75% of the fluroxypyr resistance in Alberta was low. However, care should still be taken with group 4 herbicides on kochia to maintain the use of these actives.  Dicamba resistance is just beginning to emerge in Manitoba; thus, growers have a chance to steward this active well.  

In addition to these survey results, group 14 resistance was identified in kochia in a mustard field in central Saskatchewan in 2021. Resistance to saflufenacil was confirmed, but the kochia may be cross-resistant to other group 14 herbicides. The pace at which kochia develops herbicide resistance means that solely using herbicides is not a sustainable management practice.  

Kochia Resistance Management Practices  

Because of the short-lived seed bank for kochia of 1–2 years, management strategies that prevent seed set and limit the number of seeds returning to the soil will be effective. These practices include: 

• Mowing or swathing kochia patches before seed is set in parts of the field where there is complete yield loss from kochia. 
• Seed destructor mills are used on combines at harvest, and they have been shown to destroy up to 99% of kochia seeds that pass through the combine. 
• Strategic tillage in the spring to disturb early emerging kochia. Strategic tillage in the fall pushes kochia seeds to one centimetre to reduce and four centimetres to eliminate kochia emergence. 
• Utilizing mechanical forms of weed control such as harrowing, rotary hoe, and inter-row tillage. 
• Growing fall-seeded, spring-seeded silage, or perennial forage crop on a kochia-infested field. The time of harvest, chopping, or cutting coincides with kochia’s critical period for weed seed control. The harvest timing of spring annual crops already sets the seed. 

Cultural practices to improve crop competitiveness and speed canopy closure are also effective for managing kochia. Practices include: 

• Selecting cultivars that perform well regionally for a competitive crop stand. When growing peas, select a leafier pea variety to choke out kochia. 
• Growing a diverse crop rotation, including rotation between spring and winter seeded crops and annual and perennial crops, creates crop competition at varying points in the growing season and provides a broader range of herbicide options. 
• Increasing seeding rates to improve crop competition. The Saskatchewan Lentil Input Study showed that the best seeding rate in lentils for reducing weed biomass was 260 plants m^-2, targeting 24 plants ft^-2.  
• Using narrower row spacing to speed canopy closure.  
• Seeding as early as possible to help the crop establish and compete with kochia. 
• Controlling kochia throughout a diverse rotation, as pulse crops are less competitive against weeds than other crop options. 

Producers, communities, and industries should manage kochia growth on non-cropped land, such as field approaches, sloughs, field margins, bin yards, railways, and oil wells, to prevent the spread of resistant weeds. Helpful practices include:  

• Establishing perennial grasses, where possible, reduces bare ground for kochia to grow.  
• Kochia is saline tolerant; thus, establishing salt-tolerant forages in saline areas where the crop does not grow will choke out kochia.  
• Mow kochia where forages cannot be established.  
• Fences or shelterbelts are barriers that can prevent tumbleweeds from spreading. A herbicide burndown may be necessary to kill kochia the following spring and prevent it from spreading to the other side in a few generations. 
• Because kochia can spread through many land types, communities must work together to manage this weed. 

Herbicide use to control kochia is inevitable, and some herbicide options remain. If farmers and agronomists create a diverse multi-year herbicide rotation, use tank-mixing and herbicide layering with multiple modes of action, and use best spraying practices, they will help preserve the actives that remain effective on kochia. Best spray practices include using the complete herbicide and adjuvant label rates, using recommended water volume, spraying weeds when small, and using the proper speed and pressure. Herbicide options for pulse crops are summarized in Table 3. Note that soil residual group 14 pre-burn products will not kill emerged vegetation and require moisture to activate. Plan for a fall application to ensure snowfall activation. Because group 14 resistance is beginning to emerge, manage these actives carefully.

There may be restrictions on label for rates, varieties, herbicide activation, or soil types. Consult the label or manufacturer for more details. Always read and follow label directions.

Conclusion 

The survey results show that kochia herbicide resistance is rising across Western Canada. Glyphosate-resistant kochia is especially widespread, and growers should not rely on this active to kill kochia. Dicamba and fluroxypyr resistance is less severe than glyphosate but is increasing in prevalence. These survey results indicate that more than herbicide use is needed to be a sustainable strategy for controlling kochia. Growers must adopt practices that reduce the weed seed bank, speed canopy closure, and improve crop competition. Also, because kochia spreads to and from non-cropped land, community-scale management is needed to reduce the spread of herbicide-resistant kochia.