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Managing Group-2 resistant weeds in pulse crops across Western Canada

Cleavers, mustard, kochia and sow thistle are among the weeds getting harder 
to control, but there are methods, starting with good agronomy

Farmers check out pea plots during the Western Applied Research Corporation’s annual field day at Scott, Sask. Weed control is a perennial problem for pea producers.

Group 2 resistance is something that is really a big problem across Western Canada,” says Dr. Chris Willenborg, a researcher at the University of Saskatchewan’s plant sciences department.

Willenborg has worked on several research projects that inform weed resistance in pulses. The bad news is that resistance is spreading. And the more farmers rely solely on Group 2s as in-crop herbicides, the more Group 2 resistance will challenge us in the future, Willenborg says.

But farmers still have tools left to manage resistant weeds in pulses. Read on to find out how to best use them.

Spot resistant weeds

There’s no shortage of problem weeds in pulses. Cleavers are “top of mind” for growers in the black soil zone, Willenborg says. And Group 2-resistant wild mustards and kochia are also problems, he says.

“Both of those tend to be problematic in the brown and dark brown soil zones. And a lot of that issue has not been helped with Clearfield varieties, in particular where we’re not necessarily managing the technology,” says Willenborg. Over 90 per cent of the kochia population sampled is resistant in Western Canada, says Willenborg, so farmers should treat all kochia as resistant.

But with other weeds, resistance is more localized. Willenborg says farmers shouldn’t give up on Group 2s with sow thistle, as resistance is patchy. Resistant cleavers are also spotty, but the problem is growing.

Resistant wild mustard is a problem in southwestern Saskatchewan. Farmers will know if they’ve got resistant wild mustard in lentils if it “sort of looks like an intercrop with mustard. A lot of those fields, we can assume, have Group 2-resistant wild mustard.”

If a herbicide fails, farmers should look first at the application, says Willenborg. For example, cleavers have a narrow application window, so the weeds may have been out of stage at spray time.

Willenborg recommends scouting a couple of weeks after spraying to examine the weeds. If they haven’t been controlled or are coming back, the herbicide may have failed. In that case, the best thing to do is send a sample to the crop lab, Willenborg says. If farmers can find an effective product that’s still on label, they can spray again.

“And that’s where we can get into a bit of trouble. Sometimes by the time we come back in, a lot of the products are out of scope or the crop’s too large to have it applied.”

Start with good agronomy at seeding

Willenborg quips that there are 29 modes of action, not 28. “And the 29th is good agronomy. Good agronomy goes a long way to managing resistance.”

Higher seeding rates are part of that good agronomy package. To manage resistance, pea growers should bump seeding rates higher than the general recommendation. “We really think that you need to be in that 90 to 100 plants per metre squared if you’re targeting a competitive stand.”

When it comes to row spacing, Western Canada is sliding backward, says Willenborg. But he acknowledges that there are practical reasons for wider rows.

“The fewer openers, the fewer shanks we have on our drills, the less horsepower required, the bigger we can make them, the more land (we can cover).”

Many row spacing studies are done in the absence of weeds, Willenborg says, and that concerns him. “We can grow things on 30-inch rows. We can grow things on 50-inch rows. But when you factor weeds — and in particular resistant weeds — into that equation… you’re going to lose weed control by going to wide-open rows. And we know that narrowing the row spacing always results in better weed management.”

The bottom line is that wider rows put more selection pressure on herbicides. That means farmers need excellent weed control. “Otherwise, you’re giving the opportunity to the weeds to have a large space of light.”

Rotating versus mixing herbicides

Rotating modes of action is critical, says Willenborg. Producers using a two-pass system should rotate groups within the same crop year, he adds. “That second product, that in-crop, really needs to be another mode of action, or at the very least, a herbicide mixture.”

Using herbicide mixtures that contain different modes of action is also key. Such mixtures are actually more effective than chemical rotation alone at reducing resistance. That’s because it’s very unlikely that a weed will evolve resistance to two chemical groups simultaneously, Willenborg explains.

But it’s important to use mixtures before resistance develops, Willenborg says. “If you’ve already got Group 2 resistance, and your solution is to mix in two products, but one of those is a Group 2, you’re not really using a mixture.”

“The other place we can run into trouble is where we advocate mixtures for a certain weed, but only one of those mixtures has good efficacy,” he adds. If one component is a “laggard for control,” it’s not an effective mixture, Willenborg says.

Plan your rotation

Group 2 resistance in peas is a central issue to most farms, Willenborg says. He advocates setting up rotations to control resistant weeds prior to the pulse crop coming into rotation.

One strategy is to grow herbicide-resistant canola to control Group 2-resistant weeds. Willenborg suggests slipping a cereal crop between the canola and pulse to alleviate disease issues. Both oats and barley are good options because they’re very competitive. Farmers should pick barley over oats if they have Group 2-resistant wild oats, he adds.

Cereals also allow farmers to use herbicides from other groups — such as Group 4s, 6s and 27s — to control some of the Group 2-resistant weeds. “So you’ve got a lot of options if you build a cereal into that rotation prior (to the pulse).”

Perennials and cover crops are going to have to play a major role in dealing with herbicide resistance, Willenborg says. The argument against cover crops has always been their moisture use, says Willenborg, but some farmers have seen excess moisture in the last few years. Options include tillage radish, annual ryegrass and blends.

“Adding perennial crops to the rotation are, I think, critical because they suppress seed production,” says Willenborg. That’s good news for farmers dealing with resistant weeds, with the exception of wild mustard. Willenborg says researchers have documented wild mustard seeds surviving for 15 to 20 years.

Willenborg suggests alfalfa because it’s easier to fit into an annual cropping system — and terminate — than grass stands. There are several hay markets in the U.S. that pay a premium, he says. But alfalfa growers need to know their customers’ preferences. For example, U.S. dairy producers typically favour large squares, he says.

One commonly cited reason for avoiding perennials is the opportunity cost lost when farmers don’t have that annual, says Willenborg. Farmers tie the economic benefit directly to the hay they’re exporting, and don’t want to take a hit on it, he adds.

But farmers should ask themselves why they’d include a perennial in the rotation. Reduced weed seeds, nitrogen credits and improved soil organic matter are all benefits bestowed by perennials.

“If you take the value of those things and put them back on the perennial crop, it’ll pay for itself.”

Pick competitive varieties

Willenborg and his colleagues are pushing for seed guides to list weed competitiveness so growers can pick competitive pea varieties.

But Willenborg and his colleagues have researched weed competitiveness in peas. While most varieties clump in the middle, they did find a few more competitive varieties.

“Varieties like CDC Patrick, CDC Dakota, and Centennial all tend to have a greater competitive ability than some of the varieties like Reward, Camry or Stratus,” says Willenborg.

Pea growers wed to weakly competitive varieties can mix in a highly competitive variety to boost performance, Willenborg says. But varieties in the middle won’t see much benefit unless farmers go with a 70/30 mixture, he says.

One caveat with varietal mixtures is to think about the end use when selecting varieties, Willenborg says. For example, don’t mix yellow and green peas.

Willenborg’s pea variety research is due to be published in Weed Science in the new year.

Layer your herbicides

Giving up on herbicides is a knee-jerk response to resistance. Instead, farmers should try to start with clean fields, says Willenborg. And layering herbicides is one way to do that.

“It’s estimated that only 20 per cent of fields see a pre-emergence or pre-plant herbicide. Yet these are key to managing resistance because they help to keep the weed population low for the in-crop application.”

Willenborg says they’ve found applying a residual herbicide in the spring keeps the field clean until they can apply an in-crop herbicide. Willenborg says one reason farmers don’t apply a residual is because they don’t see many weeds that early. But residuals are more effective on weeds before emergence, and they can play a major role in limiting weeds, he adds.

Residuals are also a useful tool for farmers dealing with cleavers in the Black soil zone. Typically, farmers will hit Group 2-resistant weeds with a Group 14. But once organic matter tops six per cent, Group 14 products tend to use their efficacy. In such conditions, the Group 14s might come close to suppression, but never control, says Willenborg.

But growers in the black soil zone can start by applying a pre-emergent such as Edge, Authority or Heat, Willenborg says. By coming back in with something like Viper, they could get 90 to 95 per cent control, he adds.

“We don’t obviously strongly advocate fighting resistance with more herbicides. But if you’re able to come in with a residual herbicide that’s a different mode of action than your in-crop, and both of those have some effect on your resistant weeds, I think you’re going to end up with really good weed control.”

Stop seed production as a last resort

If all else fails, farmers can focus on stopping seed production.

Growers have typically resorted to mowing that patch, although that’s easier to do in cereals than peas, Willenborg says. Grazing is another option, but not the best choice for pulses, he adds.

Canadian farmers can look to Australian growers for methods. For example, if the weeds pop above the crop canopy, farmers can hit them with a cutter bar, in a process called topping. Willenborg says the University of Saskatchewan recently bought a comb cutter for this purpose. It works like a sickle bar, cutting the heads of anything taller than the crop canopy. It doesn’t damage the crop and prevents seed production, Willenborg adds.

Weed wicking is also a possibility. Farmers will need a tank mounted to the tractor and a long wick that’s saturated with a systemic herbicide, such as glyphosate. The wick touches the weeds above the canopy, brushing herbicide on the plants. The systemic herbicide moves through the plant. Willenborg cautions that weed wicking wouldn’t work with a contact product.

Despite the problem weeds pulse producers face, Willenborg is still relatively optimistic.

“I’m not prepared to give up on Group 2 technology. But you have to be aware of whether you have those weeds in your field.”

Information on sending samples to Sask’s Crop Protection lab is online at AgQuest also tests for herbicide resistance. Visit to learn more.

About the author

Field Editor

Lisa Guenther

Lisa Guenther is field editor for Grainews based at Livelong, Sask. You can follow her on Twitter @LtoG.



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