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Managing herbicide resistance

Forty-three per cent of farmers think herbicide-resistant weeds have taken root on their farm, according to a recent Ipsos Reid poll carried out for BASF Canada. Ipsos Reid also reports that 47 per cent of surveyed farmers believe glyphosate alone isn’t effective for controlling weeds.

“Glyphosate resistance is here in Canada. And it will continue to increase as time goes on. And the pace of that will depend on how we manage it,” says Clark Brenzil, provincial weed specialist with Saskatchewan Agriculture.

Herbicide-resistant kochia has been grabbing headlines, but it’s not the only weed farmers need to keep an eye on.

“We’re now hearing of resistance issues with other weeds, particularly wild mustard in the Yorkton, Rosetown, Moose Jaw areas. A lot of those are focused in the areas where we grow lots of lentils and lots of pulses. So that Group 2 component is very frequent in the rotation,” Brenzil says.

Dr. Hugh Beckie’s research shows kochia, wild oats and green foxtail are most likely to develop herbicide resistance in the brown soil zone, says Eric Johnson, weed biologist with Agriculture and Agri-Food Canada. Farmers in the Parkland region should watch for resistant wild oats, green foxtail, and cleavers.

Glyphosate is a Group 9 herbicide, but resistance I also developing to other herbicide groups. Farmers shouldn’t count on Group 2 herbicides to control kochia, either, Brenzil says.

“Once a weed is resistant to a chemical family, it will always be resistant to that chemical family. There’s no penalty to being resistant and there’s no going back,” Brenzil says.

Managing herbicide resistance

Chem fallow is a major source of glyphosate-resistant weeds. Other areas that aren’t cropped, such as driveways, irrigation channels, and fence lines, are also potential reservoirs.

“If you’re going to chem fallow, you better be tank-mixing the glyphosate with something else. Something very effective,” says Johnson.

By pre-seed burn-off time, 60 per cent of the kochia has already emerged. “That early emergence emphasizes the need for early season weed control,” says Johnson. “Because we have some late emergence, we also maybe need some residual control, if possible, particularly in crops that aren’t that competitive, like lentils.”

Rotating herbicide groups can also help slow resistance. Farmers need to make sure they rotate chemical groups rather than switching to chemicals within the same group. But even so, resistance can develop.

“You’re still applying a fair amount of selection pressure when you’re just applying one (herbicide) at a time, even though you’re rotating it,” says Johnson.

Tank mixes with two or three different active ingredients do a better job of delaying resistance. “It’s difficult, then, for that plant to develop resistance to all three,” says Johnson.

If weeds have developed resistance to a chemical, it shouldn’t be used in the mix. Farmers should rotate tank mixes year to year as well, if possible.

When farmers are picking their herbicides, Brenzil suggests starting with the crop with the fewest options. “Pick your herbicide for that crop. Move on to the next most limiting crop, and keep the process going, trying not to repeat groups as you go along.”

Herbicide management one piece of resistance puzzle

“You want your crop to be your best herbicide,” says Johnson.

Weeds grow best in crops that are similar to themselves, so adding diversity to the rotation keeps weeds from becoming entrenched.

“If you keep doing the same thing over and over again, you’re increasing the probability of not only resistance, but you just give the weeds an advantage. So what you have to do is keep switching things up in your management,” says Johnson.

Farmers should physically remove any weeds they think are resistant. Brenzil recommends working these pathes under. “Don’t wait to find out if you’re wrong because it expands so quickly out of that patch stage to the entire field, that you’ll have lost the race before you even get started.”

More weed seeds also promote resistance. A chaff catcher, which is an old technology, can help cut the seed bank. Australians can now use a machine, called a Harrington Seed Destructor, to grind seeds coming out the back of the combine. Brenzil says they’ve had to develop the machine because some areas in Australia have no other options.

Swapping land with a livestock producer once in a while is also a strategy. “You’ll get the benefit of weed control. He’ll get the benefit of the increased phosphate on the land to improve the growth of his forages when he moves back to his land. And everybody wins,” says Brenzil.

Tillage may become a useful tool in some situations, such as chem fallow, Johnson says. “I don’t think we’re at a point yet with resistance where we have to do a lot of tillage, but it’s something that may have to be considered.”

Farmers can evaluate their own risk for developing glyphosate resistance by visiting The website, developed by Monsanto, has farmers answer 10 field-related questions before spitting out a risk rating.

Above all, farmers need to keep in mind that herbicide resistance is here to stay. “As long herbicides are used for weed control, you will have herbicide resistance continuing to develop,” says Brenzil. †

About the author

Field Editor

Lisa Guenther

Lisa Guenther is field editor for Grainews based at Livelong, Sask. You can follow her on Twitter @LtoG.



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