Glyphosate-resistant weeds a real and present danger

Canadian farmers are being warned to be careful not to lose their most precious weed-control resource

Palmer amaranth

It’s like an episode of the old TV show “The Twilight Zone” — farmers repeatedly spray their crops but the weeds refuse to die.

But that’s reality for many farmers in the mid-southern United States.

Glyphosate, “the world’s greatest herbicide,” is no longer effective there due to an explosion of glyphosate-resistant weeds caused by a lack of agronomic diversity, says University of Arkansas weed scientist Jason Norsworthy.

Canadian farmers must take steps to avoid the same fate, Norsworthy and other experts warned during Bayer CropScience’s inaugural agronomy summit here last month.

Fifty per cent of Arkansas’ cotton fields are now hand weeded and some producers have even lost their farms, Norsworthy said.

Canadian farmers are also at risk, said Neil Harker, a research scientist with Agriculture and Agri-Food Canada at Lacombe, Alta.

“If we go to the same intensity with one, single-trait rotation like RR (Roundup Ready) corn, RR cotton, RR soybean like they have, which we have the potential to do in Western Canada… we’re going to be in a similar situation,” he said.

Western Canada has some advantages over Arkansas. Most crops are grown in narrow rows, making them more competitive with weeds. The growing season is shorter and glyphosate isn’t applied as often.

“But we could be in a similar situation if we go the same direction,” Harker cautioned. “We’re just a few years behind in terms of our selection pressure.”

Protecting the resource

During his formal address Harker emphasized herbicides are “a precious, limited resource.”

“We’re approaching a cliff,” he said. “If we don’t take steps to stop weed resistance we’ll fall back on a time when all weeds were hand weeded. Every time herbicides are used in any setting, weeds evolve by developing resistance.”

Harker said the problem is that farmers grow Roundup Ready canola, which is sprayed with glyphosate, too often because it’s profitable.

A non-selective herbicide, glyphosate is the most applied weed killer in Western Canada. In 2012 a full rate of glyphosate was applied to the equivalent of 114.7 million acres in the West, Harker’s data shows. Since there are only 110 million cropped acres, many acres received more than one full-rate application.

In contrast, all the other herbicides combined were applied to the equivalent of 112.4 million acres. More glyphosate was applied to western fields than all other herbicides combined.

“That results in tremendous selection pressure so it shouldn’t be a surprise we have resistant weeds,” Harker said.

Scientists believe about one weed in every billion is naturally resistant to a herbicide. Applying a herbicide repeatedly kills the susceptible weeds and leaves or “selects” the resistant ones.

Canada already has some glyphosate-resistant weeds. The first documented case was giant ragweed in Ontario in 2009 followed by Canada fleabane in 2011, also in Ontario.

Glyphosate-resistant kochia was confirmed in Alberta and Saskatchewan in 2012.

Not only are Canadian farmers at risk of creating more homegrown glyphosate-resistant weeds, but also they are almost certain to import them in feed and equipment from the U.S., Norsworthy said.

Resorting to hoes

Glyphosate-resistant Palmer amaranth infests 61 per cent of Arkansas soy acres and 87 per cent of its cotton acres. More than 2.5 million acres are affected even though the first resistant Palmer amaranth wasn’t discovered in Arkansas until 2006 — 10 years after Roundup Ready soybeans were first introduced.

“We’ve abandoned fields as a result of resistance,” Norsworthy said. “And here’s one that’s probably going to be a shocker for everyone in this room — we’ve had individuals who completely lost the family farm as a result of glyphosate-resistant Palmer amaranth.”

So how did things get so bad?

“All we did was plant Roundup Ready crops… and we sprayed with only one herbicide and that was Roundup (glyphosate),” he said. Spraying weeds at sublethal rates to cut costs, or spraying weeds that were too big, also contributed.

Now some Arkansas cotton growers pay up to $250 an acre to get their fields hand weeded.

“If we lose a herbicide we’re right back to steel (machinery to control weeds) so we really do need to take care of our herbicides,” warned Stephen Lindell, Bayer CropScience’s lead herbicide chemist based in Frankfurt, Germany.

With glyphosate’s early success, pesticide companies wrongly assumed new herbicides were unnecessary and cut research. As a result there hasn’t been a weed killer commercialized with a new mode of action in almost 25 years.

While new herbicides are needed, they alone aren’t the answer, Lindell, Harker and Norsworthy agreed. The key, they said, is diversity.

“Using the same tactics year after year are going to fail,” Norsworthy said.

Farmers need to rotate herbicide modes of actions and crops, Harker said. Winter cereals, early silage and perennial crops should be added to the mix.

He also recommends upping seeding rates and growing taller, more weed-competitive crops.

Destroying weed seeds during or after harvest will become an important tool, he predicted.

Other options

Australian farmers are experimenting with the Harrington Seed Destructor — a mill pulled behind a combine that captures and pulverizes seeds leaving the combine. Harker predicts that machine will be placed inside the combine and sold as an option.

“Start clean and stay clean,” is Norsworthy’s advice.

“The threshold for those weeds that are resistance prone should be zero, if we’re going to try to manage the soil seed bank in resistance.”

Farmers also need to learn weed biology so they know the best time to spray.

Make a herbicide application the best it can be by applying a lethal dose, using good-quality water and applying lots of volume at a speed where weeds are well covered.

In Arkansas the mouldboard plow is sometimes used because it can bury small weed seeds so deep they can’t emerge. Instead of dealing with millions of weed seeds they are cut to hundreds, but that technique can only be done once, he said.

“Herbicides alone are not going to be the complete answer,” Norsworthy said. “Herbicides are going to be the backbone of what we do, but it’s going to take some other practices that we’re going to have to integrate… if we’re going to be successful from a resistance-management standpoint.”

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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