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Use diversity to fight resistant weeds

The latest in using integrated weed management tools for hard-to-manage weeds

Weed researchers are studying several options to control herbicide-resistant weeds. Researchers need to test each product or practice alone, and in combination.

A three-year study in Arkansas has found that a combination of fall management practices and herbicide use are the most effective ways to control herbicide-resistant Palmer amaranth, when combined with a herbicide program.

But Arkansas isn’t Alberta. So what can western Canadian farmers learn from this study?

More and more often the message in weed science “is that diversity is key,” says Breanne Tidemann. Adding a non-chemical tool is critical to managing herbicide resistance, she says. Tidemann is a University of Alberta PhD student and an Agriculture and Agri-Food Canada research scientist based in Lacombe, Alta. She has been researching harvest weed seed control, which includes strategies such as collecting and removing chaff and destroying weed seeds.

The Arkansas research was done in a soybean field that had about five to 10 glyphosate-resistant Palmer amaranth plants per square metre. Researchers tested and replicated various fall management practices from fall 2010 to spring 2014. Researchers also tried different herbicide regimes.

When it came to herbicides, a pre-emergent herbicide application helped knock back the Palmer amaranth. Pre-emergent followed by post-emergent herbicides were better than a post-emergent glyphosate application alone. A residual herbicide with glufosinate also cut seed production. And cover crops, chaff removal and seed incorporation during bed formation were the most effective fall management practices, when used with effective pre- and post-emergent herbicides.

Researchers also found synergies between some herbicides and fall management practices. For example, burning narrow windrows in the glyphosate-only program was more effective than narrow-windrowing alone. But using a pre-emergent balanced out the positive effect of burning.

But western Canadian farmers can’t expect all of the practices that worked well in this study to work on their farms. “It’s very species-specific in terms of how these types of things will work (on weeds),” says Tidemann.

Western Canada is still in the early stages of harvest weed seed control research. “We don’t really know yet how some of the different techniques are going to fit with our integrated weed management tools.”

And finding synergies between products or practices isn’t easy. Researchers need to test each product or practice alone, and in combination, Tidemann says. “So if the combination is made up of multiple products the experiment can get very large and very expensive.”

When researchers are studying how to control herbicide resistance with integrated weed management, it’s more effective as a rotational study, Tidemann says. Variables such as weather come into play, so data from a single year won’t cut it. The other complicating factor is that Western Canada has many area-specific weeds, so a study in Lacombe might not address the target weeds in Lethbridge, she adds.

Canadian research underway

Researchers embarked on a new study this year, building on Dr. Neil Harker’s work. Harker’s research set out to see if integrated weed management could match herbicide control of wild oats in canola-wheat rotations. Specifically, he studied how well crop rotations, seeding rates, early cut silage, and perennial crops could control wild oats.

The new five-year project will study many of the same integrated weed management techniques, with and without herbicides. But researchers will also try collecting chaff to simulate harvest weed seed control methods such as the Harrington Seed Destructor or narrow windrow burning, Tidemann says. She explains they’re separating the chaff stream on the combine, then bagging and removing it from the field.

Researchers are also broadening their target weeds beyond wild oats, to include cleavers, volunteer canola, hemp-nettle, kochia, shepherd’s purse and wild buckwheat. The goal is to see if there’s a combination of rotation and integrated weed management tools that controls weeds as well, and provides the same profitability, as a canola-wheat rotation with 100 per cent herbicide use, she says.

Tidemann and her colleagues have also finished the second year of a study looking at whether farmers could control wild oats by cutting the panicles once they’re above the crop canopy. Researchers clip the panicles every week to see when the seeds become viable. That would help farmers figure out the best time to cut the wild oats. Researchers have another growing season left in that study, plus data to crunch.

This year researchers also set about figuring out what they can and can’t do with a CombCut. The mower is designed to leave cereal crops unharmed while it mows weeds.

They have learned that it’s not a good idea to dig the CombCut’s knives into the soil, Tidemann says. “It does not like rough ground,” she adds. Researchers hope to start a project with the CombCut next year, led by the University of Saskatchewan’s Dr. Steve Shirtliffe.

While those projects are still underway, there are things farmers can do now to push back herbicide resistance on their farms.

“If you suspect resistance, knowing what kind of resistance you have and what herbicides are still effective is a big thing to know,” says Tidemann. That information will help properly plan herbicide applications, she says.

“But in addition to that, the more diversity you can incorporate prior to resistance, the longer you’ll be able to delay it.”

Interested in reading more? Check out “Integrating Herbicide Programs with Harvest Weed Seed Control and Other Fall Management Practices for the Control of Glyphosate-Resistant Palmer Amaranth.” Find it by searching “Integrating Herbicide Programs” at

A list of Harker’s research, along with links to abstracts, is available at

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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