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Diversify rotations to slow resistance

Diversify canola-wheat rotations to save profit margins and slow
 weed resistance, before it’s too late

Swather in a field.

Scientists warn that a serious threat to sustainable crop production in Canada continues to grow. Surveys indicate the number of western Canadian fields containing Group 1 and Group 2 herbicide-resistant weeds has jumped dramatically. In addition, glyphosate-resistant kochia continues to spread rapidly in the west, while glyphosate-resistant giant ragweed, Canada fleabane and common ragweed carry on making inroads in the east. Researchers believe wild oats, green foxtail and cleavers will be next to develop resistance to glyphosate.

“Weed resistance to herbicides is one of the biggest emerging sustainability issues on the Canadian Prairies,” says Neil Harker, a research scientist with Agriculture and Agri-Food Canada. Heavy reliance on simple, cost-effective herbicide-tolerant systems and chemistries with a single mode of action has resulted in high selection pressure for herbicide-resistant weeds and their burgeoning populations across the country.

In the foreseeable future, an easy fix in terms of new modes of action is not in the cards, says Harker. Although new modes of action entering the market were common throughout the ’70s and ’80s, there have been no new modes of action available to producers for the past two decades — and there are none, at present, in the pipeline. In addition to the preservation of the chemistries currently available, it’s time to look at other weed control options, says Harker.

“Rotation plays a big role in the weed resistance problem,” he says. “What we really need is more diversity.” However, diversifying into other crops in the canola-wheat rotation may be a hard sell to producers as it means they will have to grow the most profitable crops less often, says Harker.

Yet, bitter as this pill may seem at first, it is this long-term vision that could keep Canadian producers from travelling down the costly road glyphosate resistance has taken our neighbours to the south.

Worst case scenario

In Arkansas, cotton production has dropped from one million acres to less than 300,000 due to glyphosate-resistant weeds and low cotton prices. “Farms were getting larger and larger, now they’re getting smaller because producers were forced out of the easy systems and they’re not in expansion mode anymore,” says Harker. “Producers wish they hadn’t gone to the cheap, simple systems because now the money they saved in the five or six years where they continuously planted glyphosate-tolerant crops has already been lost in the three years of resistance they’re experiencing… People have lost farms and people have left agriculture because of it.”

Weed management costs could skyrocket and control options and profit margins narrow for Canadian producers if weed resistance isn’t addressed, warns Harker. “Farmers have to decide between short-term profit and long-term vision,” he says.

“Right now weed management isn’t usually the biggest cost in farming… If farmers lose their simple, cheap herbicides to resistance, all of a sudden weed management becomes a much higher cost… that’s certainly been the case in the mid-southern United States where prices have gone up more than two to three times what weed control used to be in (areas like Arkansas).”

More from the Grainews website: Five tips for controlling weeds in canola

Rotation management

Man with glasses.
Neil Harker is a research scientist with Agriculture and Agri-Food Canada. photos: courtesy of neil harker photo: Neil Harker

Innovative and diverse rotations, such as adding perennial forages into the canola-wheat cropping system, squeeze out weeds without adding any selection pressure for herbicide-resistant weeds. For example, wild oats don’t germinate well in the spring with an established alfalfa stand above them. “Because you’re cutting the alfalfa at least twice, neither wild oats nor many other weed species can produce viable seed under those conditions,” says Harker.

Adding winter cereals at double the normal seeding rates to the canola-wheat rotation is another good option for putting summer annual weeds at a disadvantage. Very few farmers who grow winter wheat, winter triticale and fall rye need to apply wild oat herbicides, says Harker. “Not only do those producers have less cost in wild oat herbicide, they have zero selection pressure for wild oat resistance.”

If feedlots are nearby, another crop to introduce to the canola-wheat rotation to decrease wild oat populations is early-cut barley silage at twice the normal seeding rate. Early silaging takes place a week and a half earlier than normal, and the treatment prevents viable weed seed production.

Data from eight sites across Canada indicate wild oat control through diversification of the traditional canola-wheat rotation by including competitive winter cereals, alfalfa or early-cut silage can be as good as herbicide application at the full rate. “There were sites where those treatments without any wild oat herbicide for three years were as good as the full rate herbicide treatment for three years; the latter treatment exerting tremendous selection pressure for resistance,” says Harker.

Glyphosate-resistant kochia continues its swift spread throughout Alberta and Saskatchewan, and “it’s almost a certainty within a couple of years we’re going to have it in Manitoba,” says Harker. “Glyphosate-resistant kochia could be one of those weeds that changes the way people have to do things,” he says.

In general, most glyphosate-resistant weeds are found in the countries growing the most glyphosate-tolerant crops. The United States, Brazil and Argentina contain more than 80 per cent of the world’s glyphosate-resistant crop acres, which totalled approximately 330 million acres in 2009. There are 112 known instances of glyphosate-resistant weeds worldwide — 77 per cent of which are found in those three countries.

Weed resistance is forcing some countries to develop new and innovative weed management tools. For example, the Harrington Seed Destructor, already in use in Western Australia, grinds weed seeds into a powder. “In Australia they are desperate to get rid of some of their most prominent resistant weeds… Basically, it kills every seed that comes out in the chaff. It’s hooked up to the chaff part of the combine, and once the chaff comes out, it goes into the mill and destroys viable seeds. It’s better than 99 per cent effective on two of their most resistant weeds: rigid ryegrass and wild radish. They’re not spreading around those weeds with the combine and drastically reducing the soil weed seed bank,” says Harker.

As the popularity of growing summer annual crops continues across Canada, so will the increase and advancement of summer annual weeds and selection pressure for weed resistance will continue to rise. Harker says the time to take advantage of alternative weed control methods is sooner rather than later.

“We don’t want to wait too long. By the time you’re in the situation some American and Australian growers are in, it’s quite late to do something about it. There are signs there is serious weed resistance trouble ahead,” he says. †

About the author


Kari Belanger

Kari Belanger has been a writer and editor since graduating from the University of Calgary with a B.Sc. in Biology and a BA in English Literature in 1996. For more than twenty years, she has worked in many different industries and media, including newspapers and trade publications. For the past decade she has worked exclusively in the agriculture industry, leading a number of publications as editor. Kari has a particular passion for grower-focused publications and a deep respect for Canadian farmers and the work they do. Her keen interest in agronomy and love of writing have led to her long-term commitment to support, strengthen and participate in the industry.



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