Wild oats costs farmers in Western Canada half a billion dollars to control annually.
That’s more than double any other weed species.
Applying that much herbicides on one weed species has a heavy price beyond what it costs to apply that spray. It also places wild oats under heavy selection pressure.
The wild oats that survive the herbicide onslaught pass on the characteristics that allowed their survival, and these characteristics rapidly spread in the wild oat population because herbicides prevent competition from other wild oat plants.
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Neil Harker, a weed scientist at Agriculture Canada, said this heavy selection pressure is causing wild oats to develop herbicide resistance, and growers will have significant problems managing the weed in the future.
“How long can we say that the agriculture industry is still in good shape, or the agriculture economy, when we keep losing major herbicides,” Harker said.
To illustrate how far wild oat herbicide resistance has progressed, Harker cited Dale Fedoruk’s ex-amination of the weed during a presentation at a recent herbicide resistance conference in Sask-atoon.
Fedoruk, who is an agronomist and grain farmer in central Alberta, harvested wild oat seed that survived herbicide applications and sent them to a lab for resistance testing on three Group 1 and three Group 2 herbicides.
The wild oats from each of the three fields that were tested showed resistance to the herbicides, but in one of the fields the wild oats showed minimal susceptibility to both the Group 1 and Group 2 herbicides.
“In wheat there is not wild oat control for that grower, and there will be no options for some other growers like him, so that’s a sobering story,” Harker said.
Avadex is a Group 8, soil-applied herbicide that controls wild oats as they emerge, but Harker said its prolonged use in some prairie fields will affect how long it will remain effective.
“Unless there was a fitness penalty, this should not change at all for the (wild oat) resistance to Avadex, so maybe there is two or three years left of Avadex susceptibility on some of these fields,” Harker said.
Hugh Beckie, a weed scientist with Agriculture Canada, said in 2014 that a risk assessment model he developed has shown wild oats is likely the next species to develop glyphosate resistance, which would drastically limit burn-off options for the weed.
Instead of relying on herbicides alone to control wild oats, Harker said producers should adopt an integrated wild oat management strategy.
He designed two studies that show how farming practices can reduce the selection pressure placed on wild oats.
The first study examined seeding rates, crop height and how rotating spring seeded annual crops affected wild oat pressure.
Tall barley crops had less wild oat pressure than short varieties, double seeding rates caused less oat pressure than normal rates and rotations with multiple varieties and crops had less wild oat pressure than a single barley variety grown continuously.
“By doing one thing right, you can get a two to three times reduction,” Harker said.
“By doing all three, you can get a 19 times reduction in wild oat biomass without any differences in the herbicide regime.”
Wild oats is well adapted to a continuous spring seeded crop rotation, and almost 50 percent of western Canadian acres are in a wheat-canola rotation.
Even if a pulse crop is added to the rotation, there still are three spring-seeded crops growing continuously, which wild oat favour.
“If we simply grow the same crops at the same time every year, the wild oat knows when to drop its seed so that it gets plenty in the ground,” he said.
Harker’s other study found that growing alfalfa, as well as a rotation that included winter cereals seeded at double rates and combined with an early cut barley silage seeded at double rates, both without any wild oat herbicides, provided similar wild oat control as a canola-wheat rotation grown continuously with full rates of wild oat herbicides.
“In this study, some of the wild oat seed banks did increase, but we had four of those treatments with zero wild oat herbicides three years in a row where it was not significantly bigger than the canola-wheat-canola-wheat rotation.”
Cropping systems that allow growers to forgo the use of wild oat herbicide while keeping the weed’s population in check lessens the selection pressure for wild oat resistance to herbicides and will help growers prolong the utility of wild oat herbicides.
He said wild oat resistance will continue to worsen unless growers diversify their rotations and start growing more fall seeded crops, silage or perennial crops.
“We’re really just telling these weeds, ‘you can just continue as normal.’ They have no trouble adapting or thriving in those systems,” Harker said.
Perennial forages such as alfalfa compete well against wild oat because they have ample ground cover and are cut before wild oats have a chance to set seed.
Organic growers throw wild oats off their regular life cycle pattern by delaying spring seeding with the hope that fewer wild oats will germinate later in the season to compete with their crops.
Some conventional farmers seed as early as possible so their crop can get a head on the wild oats.
“You often wouldn’t need a second application where the crop is so far ahead of the weeds,” Harker said.
Winter cereals start growing in the fall and have a strong canopy by the time wild oats is ready to emerge in the spring.
“Many winter wheat, winter triticale and fall rye growers don’t even need a wild oat herbicide.”
“You can take away wild oat selection pressure with herbicides a whole year by just having a winter cereal crop.”
However. Harker said producers who grow winter cereals continuously run the risk of downy brome pressure developing.
“Doing the same thing over and over, whether it’s winter cereals or its summer annuals, is not going to get us where we want to be,” he said. “We need to rotate, to mix things up.”
He said the development of herbicide resistant weeds hasn’t prompted greater adoption of integrated weed management on the Prairies, but that could quickly change as resistance to available herbicides increases.
