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Herbicide-resistant weeds are our biggest threat

There’s no reason why the West must become an Arkansas-style nightmare because of weed resistance, but we may be heading there anyway

Neil Harker calls herbicide-resistant weeds the biggest threat to sustainable crop production in Western Canada. “We’ve been the grim reaper talking about this for ages, but this is a watershed moment,” says the research scientist with Agriculture and Agri-Food Canada in Lacombe, Alta. “There are places in North America going back to the plow and hand-weeding because herbicides are not working as they had been. We’re not there yet in Western Canada, but we’re getting closer.”

Jason Norsworthy, weed scientist at the University of Arkansas, knows where we’re heading. Arkansas is among the most severely affected states with crop rotations that often include Roundup Ready cotton, Roundup Ready soybeans and Roundup Ready corn, creating intense selection pressure for glyphosate-resistant weeds.

“Glyphosate is now completely lost for much of the southern U.S.,” Norsworthy says.

Today, only about 15 per cent of the cotton is Roundup Ready, down from nearly 100 per cent.

Growers have adapted, but at a price. “We have a really clean crop this year,” Norsworthy says. “But weed management costs have basically tripled.”

Growers use various pre-seed products to make sure the field is completely clean before seeding, and then use soil-applied residual herbicides in crop. Cotton, for example, often gets seven in-crop applications per season. Growers then hire hand-weeders to get any escapes.

The most threatening glyphosate-resistant weed by far is Palmer amaranth, which Norsworthy calls “redroot pigweed on steroids.” Literally billions of Palmer amaranth plants will emerge per acre, with emergence continuing all season long. The weeds can grow six to seven feet tall, and each plant can produce up to 1.5 million seeds. Even if soil-applied herbicides are 99 per cent effective, those escapes can quickly rebuild the seed bank.

“In Arkansas alone, we had at least 750,000 acres hand-weeded this year, and it would have been much higher if we had the labour to do it,” says Norsworthy.

Why Arkansas matters here

The fact that a single weed is causing so much trouble in Arkansas is an important message to growers in Western Canada. North America has at least 15 glyphosate-resistant weeds, but just one is enough to upset the balance. One glyphosate-resistant weed — kochia — is now confirmed in all three Prairie provinces, and for growers with a kochia problem, this is a serious concern.

The second important message from the southern U.S. experience is that no new herbicide has come along to rescue the situation. This is a huge market opportunity for crop protection companies to extract premium value for a new mode of action. “Yet there is absolutely no new mode of action in the pipeline,” Norsworthy says. So Arkansas growers are back to 50-year-old products like 2,4-D and dicamba (although weed resistance to dicamba is showing up), and they have drastically stepped up their tillage too.

The third important message from the U.S. experience is that herbicide-resistant weeds don’t have to develop on your farm to become a problem. Resistant weeds are on the move. Glyphosate-resistant Palmer amaranth was first discovered in Georgia — which makes sense given the selection pressure of the typical southern crop rotation and that Palmer amaranth is a heat-loving weed suited to Georgia’s arid conditions, Norsworthy says. “However, it is now found in 28 states, including northern states of Michigan, Wisconsin and possibly Minnesota.”

Weed seeds spread far and wide in custom combines, to give one example. Cotton hulls — and weed seeds in the dockage — are also trucked around the U.S. for dairy feed. Dairy cow manure spread on fields is a simple way to introduce seeds from resistant weeds. Then there’s the more natural method — wind — that blows seeds and seed-shedding plants all over the place. Think tumbling kochia and its 30,000 seeds per plant.

The Prairie situation

Hugh Beckie, weed scientist with Agriculture and Agri-Food Canada in Saskatoon, has been tracking herbicide-resistant weeds for the past 15 to 20 years. He is in the middle of another two-year survey of Saskatchewan. In the early 2000s, Beckie estimated the number of Prairie acres with at least one species of herbicide-resistant weed at 10.9 million. By end of that decade, Beckie’s estimate had increased to 24.4 million.

This year, Beckie revised the total to 38.0 million. The population of resistant weeds within those acres has likely also increased significantly.

Group 1-resistant wild oats are the most common. As of 2009, an estimated nine million acres were infested with Group 1-resistant wild oats. Wild oats have been found with resistance to Group 2 and Group 8 herbicides as well. More than 300,000 acres have wild oat plants with resistance to all three of these groups. Group 1-resistant green foxtail is widespread, as are many Group 2-resistant broadleaf weeds, including cleavers and kochia. In fact, weed management specialists assume that most kochia is Group 2 resistant. Glyphosate resistance is an add-on feature.

Beckie estimates the cost of herbicide-resistant weeds at $1.1 billion to $1.5 billion per year on the Prairies. The cost is from a combination of related factors, according to grower surveys. These include added herbicide cost due to increased tank mixing required, added overall weed management (cost of tillage or crop rotation, for example) and lost yield due to increased weed competition.

So what do we do?

Gregory Sekulic

Gregory Sekulic

Gregory Sekulic is an agronomy specialist with the Canola Council of Canada and he spends a lot of time thinking ahead about sustainability challenges to canola production on the Prairies.

“With resistant weeds, the first step is to think about weed management as a whole,” Sekulic says. “Herbicides alone are not the solution.

“Herbicide-resistant canola has saved growers a lot of time and effort when it comes to weed management, but we are at risk of losing that tool,” Sekulic says. “Best practices for weed management need to be applied to save these tools for the long term.”

This includes immediate action and long-term prevention. Scout for escapes and suspicious patches and then act quickly to contain them. Cut those patches before the weeds set seed — use a mower or perhaps take that part of the crop for silage — or spot spray with a different mode of action.

To reduce the selection pressure for resistant weeds, look at rotating herbicide-tolerant canola systems, especially if you’re in a two-year canola rotation, Sekulic says. Use product tank mixes in the pre-seed, in-crop and harvest windows. And take steps that increase efficacy — such as hitting weeds when they’re small, when they’re actively growing, and at label rates and water volumes.

Then adopt practices that aren’t just about herbicides, Sekulic adds. Winter cereals are a great rotation crop because they’re highly competitive and provide this competition at different times of the year (i.e. fall and early spring) than spring-seeded crops. Also winter cereals are harvested earlier, which means many weeds are cut off before they set seed.

Perennial alfalfa and clover are dynamite. Neil Harker led a five-year study, concluding in 2014, into integrated crop management systems for wild oats. He found that three years of alfalfa took wild oat populations down to almost nothing with no herbicides required after the crop was established. “What we need to do better,” Harker says, “is to apply economics to these rotations to see how they can work into a grower’s profitability objectives.” He concedes that with the move away from mixed farms, these crops don’t seem to make economic sense for all growers. He’d like to have numbers to show how these crops could work into an integrated long-term approach to weed management, especially on fields where resistant weeds are at critical numbers and the higher investment required for herbicides has changed the economic picture.

Sekulic would much prefer growers look at rotation and diversity before tillage. “I don’t see a widespread return to tillage as the solution,” he says.

Norsworthy encourages growers to get in front of the issue. He gives the example of one Arkansas grower who noticed a few Palmer amaranth escapes back in 2004. Numbers were low — about one escape per acre — but the grower still felt it worthwhile to pay $2 to $3 per acre to have people walk fields and hand-pull these escapes. As a result, he kept his soil’s seed bank of Palmer amaranth very low. Now, while neighbours pay $150 per acre for hand-weeding, his costs are more reasonable and his profits are healthier, Norsworthy says. “He still has Palmer amaranth, but he can still effectively control it with glyphosate and his hand-weeding costs are only $4 to $5 per acre.”

Arkansas is researching harvest strategies, including weed mills that destroy seeds that pass through the combine, and windrow burning. These tools will reduce the seed bank and reduce selection pressure on herbicides. Communities have also taken on the job of managing weed escapes near ditches, creeks and bridges.

Necessity has forced them. “We’ve had folks go bankrupt over herbicide-resistant weeds,” Norsworthy says.

That said, it’s not a complete writeoff. “I’m seeing the cleanest crops I’ve seen in seven or eight years, and soybeans achieved a state record for yield last year,” Norsworthy says. “But every acre will have glyphosate-resistant Palmer amaranth, and every grower wishes he had been proactive to prevent this problem in the first place.”

Jay Whetter is a communications manager with the Canola Council of Canada. For more on Neil Harker’s wild oat study, read study 6.1 in the Canola Digest. For much more on herbicide-resistant weeds around the world, visit

This article was originally published as “Our biggest risk” in the November 2014 issue of Country Guide.

ID problem weeds early

trail of herbicide-resistant kochia

The photo above shows bands of glyphosate-resistant kochia in a field. Mature resistant plants had tumbled through, shedding seed along the way. The following season all other kochia plants were controlled by an application of glyphosate, but not these ones. This is a clear sign of resistance. Any weed patches that should have been controlled by a herbicide, but were not, suggest a potential resistance situation. Live weeds beside dead weeds of the same species is another indicator. Early detection followed by spot spraying or even hand-weeding will keep these resistant patches contained. Labs in Western Canada can test weeds for resistance if you want to be sure. A list of them can be found at

About the author


Jay Whetter is communications manager for the Canola Council of Canada.



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