Canada
September 14, 2026
New research which began in 2022 and ended in 2025, funded by Western Grains Research Foundation and SaskOilseeds, is helping canola growers determine economic thresholds on when glyphosate-resistant kochia becomes a problem, and shows that a higher-density canola stand can help fight back. Kochia has always been a tough weed to manage. But as more kochia populations develop resistance to glyphosate and other herbicides, the question for canola growers is becoming more urgent: how much kochia can a crop tolerate before it leads to economic losses?
New research conducted across Western Canada is helping answer that question, led by Dr. Charles Geddes over nine site-years at locations near Lethbridge, Alberta; Scott, Saskatchewan; and Carman, Manitoba.
Data from both irrigated and dryland production environments were provided for glyphosate-resistant kochia in canola. The study found that growers should consider taking action when kochia populations reach approximately four to seven plants per square metre (0.3–0.7 plants per square foot).
The research found a 5% canola yield loss at about seven kochia plants per square metre, the study’s action threshold. The economic thresholds were even lower: approximately four plants per square metre corresponded to a $20-per-acre yield loss, while six plants per square metre corresponded to a $30-per-acre loss.
Those numbers give growers a way to relate to kochia populations not simply as a weed-control problem, but as a potential economic loss.
The study also looked at whether canola density changes the crop’s ability to compete with kochia and found that a higher-density canola stand can help. Geddes describes this as cultural weed management, using agronomic production systems or practises to improve the crop’s ability to compete. Researchers compared canola stands targeting 50 plants per square metre with those targeting 100 plants per square metre. The higher-density canola was better able to compete with kochia and withstand competition when kochia populations became heavy.
“Seeding rates are one of the most consistent ways that you can increase the ability for a crop to compete with weeds” said Geddes, noting that canola planting has seen a reduction in seed rates because of the expense in canola seeds. Unfortunately, the economic optimum seeding rates recommended are done in weed-free conditions, “So that’s why we’re trying to quantify it.”
At high kochia densities, yield losses were about one-third lower in the higher-density canola: maximum yield loss was approximately 29.6% compared with 44.1% in the lower-density crop. That doesn’t mean simply increasing the seeding rate eliminates the need for weed control. In fact, the research found that the economic and action thresholds for kochia were broadly similar at both canola densities. But a higher-density canola stand gave the crop a competitive advantage when kochia pressure was high.
“So how does the crop actually compete with the weed, and help you to manage that weed? That’s actually where we found some of the larger benefits in this case of higher seeding rates, so for example, when we’re talking about kochia biomass overall in these experiments, the higher target density actually had over double the threshold for kochia management,” explained Geddes. “So the higher canola density meant that we could have a higher kochia density before we came in with management at a particular threshold. So essentially what that means is higher canola densities help to manage the kochia in the field. So if it turns out that you are dealing with resistant kochia that’s surviving your in-crop herbicide, then at least you’ll have a backup there of crop competition to help manage the issue.”
Yield isn’t the only concern. Kochia is notorious for its ability to produce large quantities of seed, which can replenish the soil seedbank and create problems for future crops. In the study, kochia seed production at canola maturity varied dramatically between sites, from none to almost 40,000 seeds per square metre.
In two of the three locations examined for seed production, increasing canola density reduced kochia seed production by as much as 71%. Researchers found that even a kochia population of about 13 plants per square metre could produce 5% of the maximum potential seed production, highlighting how quickly small kochia populations can contribute to the weed seedbank. That finding adds another dimension to the management decision. Controlling kochia isn’t only about protecting this year’s canola yield; reducing the number of seeds returned to the field could also help limit future weed pressure.
What does this mean for growers? The research suggests managing glyphosate-resistant kochia demands a multi-faceted strategy, not one simple fix. Growers have another tool for making management decisions: population thresholds. The study’s overall recommendation is that growers should consider controlling glyphosate-resistant kochia when populations reach approximately four to seven plants per square metre or greater. Higher canola plant densities can improve the crop’s competitiveness and reduce yield losses when kochia pressure becomes severe.
The research also has implications for herbicide strategies. Canola cultivars with both glyphosate and glufosinate resistance may allow growers to use glufosinate as a rescue treatment for kochia that survives glyphosate; but timing is critical, because kochia needs to be controlled while it is still within the effective treatment window. For growers, that makes early scouting especially important.
Geddes noted that a well-established, competitive canola stand can suppress kochia growth under some Western Canadian production conditions, reinforcing the value of crop competition as one part of an integrated weed-management program.
“So from a preventative standpoint, using higher seeding rates or promoting a competitive canola crop may go a long way to helping prevent further evolution of resistance to glufosinate, because that will be a very large challenge across the region if it does evolve,” said Geddes, explaining that there is a heavy reliance on glufosinate right now, but higher seeding rates may be a better preventative solution.
The takeaway: don’t wait until kochia is everywhere. Knowing roughly how many plants are in the field can help determine when action is economically justified; and a competitive canola stand may help reduce the damage when kochia pressure is high.
“The first recommendation would be, don’t skimp on canola seed, if you can, so it would be recommended to at least achieve the economic optimum plant density under weed-free conditions. And ideally, from a biological perspective, you would have greater than that. But certainly, you don’t want to go below those economic thresholds.
“And unfortunately, what recent surveys across the region suggest is that many canola plant densities that we’re seeing out in fields across the region end up being below that economic optimum,” Geddes noted. “This particular project is urging farmers and agronomists to also think about weed management when considering those target densities. And so this research suggests that it should be at least at the economic optimum and ideally above that.
“The most important takeaway is that higher canola target densities enhance the crop’s ability to compete with kochia, and also it can help to mitigate yield loss at very high kochia densities.”
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