Old chemistry conscripted in battle with herbicide-resistant weeds

It’s an expensive fight in Europe that farmers here can expect to have if they don’t adjust first

Avadex and Fortress are old soil-applied, pre-emergence herbicides with a new purpose — helping delay the onset of herbicide-resistant weeds.

And fall is the right time to apply both, herbicide retailers were told at a meeting here Sept. 22 held by Gowan Canada.

Manitoba already has some herbicide-resistant weeds, including glyphosate- and Group 2-resistant kochia and wild oats, green foxtail that are resistant to Groups 1 and 2, and wild oats that are resistant to both Groups 1 and 2.

But Manitoba’s weed-resistance problems pale compared to England’s and other parts of Europe. Herbicide-resistant black-grass and ryegrass are so prevalent in some fields farmers are paying $55 to $100 an acre after applying five or six different weed killers and often only getting 80 per cent control, Gowan’s U.K.-based product development manager John Edmonds said. In some cases, the weed infestations are so heavy farmers are thinking about switching from winter cereals to spring crops, which have lower yields and therefore lower potential revenue.

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“Resistance (in Western Canada) in my mind is eight to 10 years behind where they are (in Europe),” said Gowan Canada’s general manager Garth Render. “Are we on the same set of railway tracks headed to where they are? Absolutely. We’ve had some delays in our trip because of the introduction of herbicide-resistant crops. We’ve added a couple of different modes of action they didn’t have.”

Whether herbicide-resistant weeds become as bad in Canada depends on what farmers here do over the next five to 10 years, Edmonds said.

“If you keep using the same chemistry time after time you’re only going to select out the resistant populations,” he said in an interview. “You have to think about alternating the modes of action and the products and not just alternating the product by name.”

That’s where Avadex and Fortress come in. Avadex (triallate) is in Group 8 and Fortress (triallate and trifluralin) contains Groups 8 and 3. Neither group is much used these days, although the soil-applied herbicides were popular 25 to 30 years ago.

Avadex sales have jumped in England, where farmers follow up with post-emergent herbicides to control Group 1- and 2-resistant grassy weeds.

Herbicide resistance can develop in weeds in as few as four years when a herbicide is repeatedly used on the same land, said Mike Grenier, Gowan Canada’s research and development manager. Alternating between two herbicides with different modes of action, or ways of killing a plant, can double the time before resistance occurs. Adding another mode of action through a tank mix or separate application can stretch it out to 10 years, he said.

“If you bring in the others (Avadex and Fortress) as well, hopefully you might even avoid getting resistance.”

Diversity is key, Grenier said, noting that in addition to using different modes of actions, farmers should rotate crops and use other techniques to make crops more competitive against weeds.

University of Manitoba weed scientist Rob Gulden agrees.

“All the things that help the crop help the herbicide are probably just as important, if not more important, in terms of an integrated-management approach to lessen the risk even more,” he said later in a telephone interview.

Bringing back Avadex and Fortress is a good strategy, but rotating or applying different modes of actions sequentially are not enough by themselves, he said.

“It can be a tool to delay it, but we need to think a bit broader. It’s more about having a crop helping the herbicide work more effectively.”

Farmers should also be aware that Group 3-resistant wild oats were discovered in southwest Manitoba in the late 1980s. How prevalent they are now isn’t known, Hugh Beckie, a Saskatoon-based weed researcher with Agriculture and Agri-Food Canada said in an email.

The last Manitoba weed-resistance survey done in 2008 showed 11 per cent of fields had Group 8 (triallate)-resistant wild oats.

“So overall, triallate (Avadex or Fortress) is still a good pre-emergence option to control wild oat susceptible or Group 1 or Group 2 resistant,” he wrote. “Similarly, Fortress (triallate and trifluralin) is still a good option for wild oat populations that are not Group 8 resistant, or green foxtail populations that are not Group 3 resistant. We have not documented Group 3 (trifluralin)-resistant wild oat anywhere across the Prairies, so this active ingredient is still useful for wild oat management.”

The same survey found 55 per cent of Manitoba fields had Group 1-resistant wild oats, 18 per cent had Group 2-resistant wild oats and nine per cent of fields had both.

Tips for applying Avadex and Fortress

Fall is a good time to apply Avadex and Fortress after the surface soil temperature is below 5 C or any time after Sept. 15 followed by a harrow.

Good soil contact is necessary for these herbicides to work, said Cory Bourdeaud’hui, Gowan Canada’s Manitoba sales representative.

That means ensuring crop residue or lumps of soil don’t prevent the herbicide granules from reaching the soil, he said.

“There’s no way the Avadex can penetrate those lumps,” Bourdeaud’hui said. “If there’s a lumpy field the farmer has to harrow it to break those lumps up before he applies it.”

Avadex and Fortress shouldn’t be applied to burned fields for 12 months because the carbon prevents them from working, he added.

A shallow harrowing will ensure soil contact but might not be necessary in the fall. Snowpack will help push the granules down to the soil, he said.

Harrowing in the spring will also “sharpen” control of both herbicides, he said. But farmers should avoid sweeping their fields moving piles of straw, which can pull the granules into concentrated areas leaving other areas untreated.

Application rates for both Avadex and Fortress are based on soil organic matter. Fields with higher organic matter require higher rates.

Avadex controls wild oats in barley, canola, spring and durum wheat, flax (not including low linoleic acid varieties), peas and canary seed.

Fortress controls wild oats, green foxtail and yellow foxtail in canola, flax (not including low linoleic acid varieties), mustard, barley and spring and durum wheat.

It also suppresses lamb’s quarters, kochia, redroot pigweed, Russian thistle and wild buckwheat.

About the author

Reporter

Allan Dawson

Allan Dawson is a reporter with the Manitoba Co-operator based near Miami, Man. Covering agriculture since 1980, Dawson has spent most of his career with the Co-operator except for several years with Farmers’ Independent Weekly and before that a Morden-Winkler area radio station.

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