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The problem with problem weeds

Different regions, different crops and different species turn ‘control’ into ‘management’

It’s a message filled with contrasts. Weed management is simple yet complex. It’s all about improving your bottom line, but it adds a cost. And although it’s all about controlling all your weeds, it often gets judged on a single weed species.

In spite of all the differences that come with farming in different regions, with different crops and weeds, the one thing that’s familiar everywhere is the definition of a problem weed.

It’s the weed that’s tough to control, and sometimes can’t be controlled.

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Dr. François Tardif takes that definition a couple of steps further.

“Either it is not on the label, or it is on the label but the level of control is not satisfactory, or it’s variable depending on crop stage or conditions,” says Tardif, a professor in the department of plant agriculture at the University of Guelph. “Or it could be a new weed that jumped into a field after a grower controlled the normal weeds. It could also be a weed that farmers inquire about a lot because they’re not happy with the control.”

Tardif remembers the days when Pursuit was the dominant molecule in soybeans, and problem weeds were three-seeded mercury and field violet. Then Roundup Ready technology in soybeans took care of them.

Earlier than that, in the 1980s, quackgrass was the primary headache. “They had two national conferences on how to deal with that weed,” recalls Tardif. “Then fast forward 20 years with new molecules in corn — like Ultim — and cheaper glyphosate… quackgrass is something a farmer shouldn’t have.”

In the past 20 years, weed science has also evolved in its understanding of weed dynamics, including the critical weed-free period. Add to that, too, the adaptability of all weed species, and according to some, it’s meant the end of the one-pass, single mode-of-action approach to controlling weeds. In fact, during that time, the word “control” has given way to “management.”

It bears repeating

It’s for these reasons that one crop adviser believes the message on problem weeds needs to be repeated. The lack of new molecules — of a glyphosate-type revolutionary development — means the methods of dealing with those new or evolving species must change. Mervyn Erb refers to a problem weed as “anything that’s where it shouldn’t be,” and adds that there’s too much at stake to worry about overusing the term.

Much of the press on problem weeds centres on Canada fleabane or waterhemp, but growers must be aware of unwanted weed seed in their cover crop blends as well.
photo: Courtesy Dr. Peter Sikkema, University of Guelph

“It’s not a hard sell to use soil-applied herbicides on Roundup Ready crops. People are quite willing to do it and they understand why they need to do it,” says Erb, who operates Agri-Solve Inc. in Brucefield, Ont. He adds that the ag press has done a decent job of delivering the message. “We have to keep it on everybody’s mind so they don’t forget.”

Erb says he doesn’t find it too hard to convince growers to adjust their practices to deal with problem weeds. When growers pay $300 for a bag of corn seed, they don’t want to hear they also have to add some soil-applied product up front. So they may grumble, adds Erb, but they realize they have to pay attention.

Erb also notes the increased use of cover crops, noting that he’s read U.S. reports about weed seed mixed in with cover crop seed. Where much of the product originated from the U.S. Northwest — Oregon and Idaho — some is now coming into Ontario from Ohio and Indiana, with the potential for introducing some bad weeds to the mix.

For Erb, part of the message on problem weeds is that what’s a problem for some isn’t a huge issue for others. What’s growing in the field — or how it’s used — can have a lot to do with what’s defined as a problem.

“You wouldn’t normally call Italian ryegrass a problem weed, but I have a grower who established grass waterways and part of that mixture was perennial ryegrass,” he says. “It’s a low-growing, well-rooted grass and doesn’t grow as tall as orchard or brome grass, and it’s nice to use in a grass waterway. For some reason, somebody who was making up his grass mix on two of his farms threw Italian ryegrass into the mix instead of perennial ryegrass.”

That left Italian ryegrass scattered across both farms following two wheat harvests, and Erb says it’s become a problem weed for those farms. Glyphosate doesn’t kill it but fortunately Focus, a grass herbicide, has some activity.

Different species, different regions

Much has been made about the four glyphosate-resistant weed species (Canada fleabane, giant ragweed, common ragweed and waterhemp) that challenge growers, primarily in Ontario. The story of glyphosate-resistant Canada fleabane has been a particular focus, both because of its resistance to multiple modes of action and its rapid spread from Essex County in 2010 to the Quebec border in 2015. But what happens if any of those aren’t the problem in a particular region?

What’s the problem weed then?

For Clare Kinlin, horsetail is the problem weed in his part of eastern Ontario, not fleabane or ragweed. When it comes to glyphosate-resistant weed species, he sees more volunteer canola that’s glyphosate resistant, and resistant to the imidazolinone herbicides such as Pursuit. That leads to some interesting reflections on how farmers are approaching their particular weed management issues. Asked how things have changed in the past 10 to 12 years, Kinlin responds that grower awareness has changed.

The arrival of the Xtend system in parts of Eastern Ontario will help in the battle with glyphosate-resistant canola.
photo: Supplied

“There’s more awareness of chemistries and modes of action, and a better overall awareness,” says Kinlin, an agronomist with MacEwen Agricentre in Maxville. “It doesn’t mean we’re doing it better, but it means we’re more aware of it.”

At the same time, Kinlin insists weed management is more complicated than it was even five years ago. Yes, glyphosate and Roundup Ready technologies simplified weed management for a time, and he estimates that up to 20 per cent of the soybeans in his part of the province are still sprayed with glyphosate only. Yet Kinlin is quick to note that conditions that challenge growers in southern Ontario are not the same ones hindering growers in the east. For one, he disagrees with the default mindset surrounding a two-pass program, stating that he can use Halex once a season and get good weed control.

“If I have a really good soybean program and a really good corn program, I don’t need to spray it twice, as long as I’m on top of the weeds every single year, and I don’t take a year off,” says Kinlin. “In eastern Ontario and western Quebec, it’s a huge post-emerge market, and we don’t have growers wanting to lay down Primextra before the season.”

Those who do rely on pre-emerge or more than one spray are usually the ones who know every weed species on their farms, and are willing to employ multiple modes or more than one application. The rest are content to spray once and would like herbicides that can control 99 per cent of their weeds. There are those who find Group 2 resistance a challenge, especially in soybeans. And with IP soybeans, he says there is a need for a two-pass herbicide program, because weeds there cannot be controlled with a single pass.

The Roundup Ready soybean program is a good residual with the glyphosate, depending on the weed species in a field, adds Kinlin. For any broadleaf weed control residual activity, the Xtend system will be a good fit. In corn, growers can still use Roundup Ready technology, and when it comes to the volunteer canola issue, he believes the arrival of the Xtend technology will also be a huge benefit.

Says Kinlin: “We have the tools, but we need to use the whole toolbox, not just the top drawer,” he says.

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CG Production Editor

Ralph Pearce

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