Now’s the time to find resistant weeds

Herbicide-resistant weeds are on the rise and pre-harvest is a good time to find them

Herbicide-resistant wild oats are still controlled by glyphosate — for now.

Delaying the onset of herbicide-resistant weeds isn’t a lost cause.

In fact the more vigilant a farmer is the more success they’ll have, says Ingrid Kristjanson, a farm production extension specialist with Manitoba Agriculture and Resource Development (MARD).

With farmers checking crop maturity as harvest grows near, it is also a good time to scout for herbicide-resistant weeds, Kristjanson told the Crop Talk Webinar Aug. 5.

“Scouting is really important,” she said. “Take the time to check.”

Why it matters: Delaying the development and spread of herbicide-resistant weeds will give farmers more weed control options longer and at less cost.

What’s futile is believing herbicide-resistant weeds won’t affect your farm.

“If you don’t already have herbicide resistance the chances that you don’t have it are slim to none,” Kristjanson said. “It will definitely be a problem for you. And we don’t have any magic bullet herbicides that are going to come along and rescue us. It’s all going to be about good management, diversity, changing up things as much as possible and keeping an eye on and dealing with small patches before they become big issues.”

Manitoba’s first herbicide-resistant weeds — Group 2-resistant kochia and Group 3-resistant green foxtail — were found in 1988. Since then a lot of other broadleaf and grassy weeds in Manitoba have been added to the ‘official’ list created by, including wild oats, yellow foxtail, barnyard grass, wild mustard, redroot pigweed, chickweed, cleavers, hemp nettle, shepherd’s purse and stinkweed.

A number of things must occur before a new weed is added to the herbicide-resistant list, Kristjanson said. That’s why glyphosate-resistant waterhemp, which was first confirmed in Manitoba in 2017, hasn’t been added yet, even though its resistance has been laboratory confirmed, she said in an interview Aug. 6.

Weed scientist Jeff Stacher examines mature waterhemp. photo: Ingrid Kristjanson/MARD

Glyphosate-resistant waterhemp has been found in the rural municipalities of Ste. Anne, Reynolds, Whitemouth and Dufferin.

It’s a Tier 1 weed under the Manitoba Noxious Weed Act and therefore when found must be destroyed.

Manitoba also has glyphosate-resistant giant ragweed. It’s not on the weed science website list because its resistance has not been laboratory confirmed yet. It’s difficult to get giant ragweed to grow in a laboratory, but there are enough cases of glyphosate not controlling the weed in Manitoba to deem it glyphosate resistant, Kristjanson said.

Glyphosate-resistant kochia isn’t on the list either, even though an estimated 59 per cent of the kochia in Manitoba is considered glyphosate resistant.

In 2011 there were 360 unique herbicide-resistant weed cases around the world, including 51 in Canada.

In 2016 there were 514 cases globally, up 49 per cent. Canadian cases were up 25 per cent to 86.

Graphic: File/Supplied

Worldwide there are weeds resistant to 23 of 26 known herbicide sites of action (that play a role in how a weed is killed), Kristjanson said.

“We have a lot of resistance cases in Manitoba (at 30),” she said. “We are running neck and neck with Ontario (at 35).”

A 2016 survey found herbicide-resistant weeds in 68 per cent of the Manitoba fields sampled, versus 48 per cent in 2008.

“We’re going in the wrong direction,” Kristjanson said.

That survey found 78 per cent of the wild oats sampled were resistant to Group 1 herbicides, 34 per cent were resistant to Group 2 and 33 per cent were resistant to both.

There are wild oats in Manitoba that are also resistant to Group 8 weed killers, Groups 1 and 8, 2 and 8, 1, 2 and 8, 1, 2, 8 and 25 and 1, 2, 8, 14 and 15.

“That’s a pretty scary thought to be dealing with that level of resistance,” Kristjanson said. “So with those wild oats what’s left? The Group 3s — the Edges and Treflans — glufosinate, the Liberty in canola, or glyphosate. And the reason I highlighted glyphosate is it’s only a matter of time before we have glyphosate-resistant wild oats so we can’t be looking at that as always the main means of controlling some of these issues.”

Farmers should assess weed control five to 14 days after they have been sprayed, Kristjanson said.

They should do the same when checking crops for insects and diseases or assessing crop maturity before harvest.

“When you are out scouting you want to look for things that are out of the ordinary, whether it’s new patches of weeds or larger patches of weeds,” she said.

In addition to recording what herbicide was applied and where, farmers should make note of conditions at the time of spraying. Weather and weed stage can affect herbicide efficacy. Just because a weed is present doesn’t necessarily mean it’s herbicide resistant, Kristjanson said.

This spring started off cool and dry. That delayed weed germination. Crops were thinner when weeds germinated subjecting them to less competition. Some weeds have thicker cuticles to protect themselves during dry weather, making it harder for herbicides to enter, she said.

Farmers should also keep their agronomists up to date on field history.

“And of course you want to be out there double-checking as well,” Kristjanson said. “Lots of things are going on out there and certainly your history with the field is going to be of prime importance keeping track of things.”

Spotting herbicide-resistant weeds early, when they are still in small patches, makes control easier, Kristjanson said.

“Waterhemp in particular isn’t easy to manage and there will be multiple resistance over time,” she said. “It’s not just a chance of it, it will, because that’s what’s going on in North America. And it doesn’t take very long for those plants to have an issue with resistance. If you do find waterhemp in your field you want to get on it as soon as possible.”

The smooth stem of waterhemp (right) is one of the ways to distinguish it from redroot pigweed. photo: Tammy Jones/MARD

When found, waterhemp should be cut below the soil surface and either burned or composted, Kristjanson said. If left cut in a field it could regrow under the right conditions.

“You want to avoid harvesting spots with waterhemp so you’re not spreading seed and make sure that you’re cleaning your equipment before moving on to another field,” she said.

Diversity is key to delaying herbicide-resistant weeds, Kristjanson said.

“I hate to pick on anybody but if you have gone to that Roundup Ready corn, Roundup Ready soybean rotation, that’s not a rotation, that is heading for a train wreck,” she said. “You have to be looking at diversity. We can’t say often enough that, that is key.”

That means rotating crops, including switching between early and late maturing, spring and fall seeded, and annual and perennial.

Rotate herbicide groups and tank mix different herbicide groups when possible.

Also, apply herbicides at the full recommended rate and under the best conditions possible. There was a time when some farmers cut rates to save money, and often still got good weed control.

“But we know that there is a higher risk for selection pressure with that kind of weed control measure,” she said.

“If we’re going at cut rates we’re taking the chance of selecting for weeds that have some degree of resistance already. In some of those cases a low rate of resistance may be taken care of by a full rate so we don’t want to miss out on that opportunity.”

Farmers who suspect they have herbicide-resistant weeds, especially waterhemp, should contact Manitoba Agriculture and Resource Development, Kristjanson said.

The Pest Surveillance Initiative (PSI) laboratory in Winnipeg can test kochia for herbicide resistance.

Ag-Quest in Minto also tests for herbicide resistance in certain weeds.

About the author


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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