Millions of Alberta acres have resistant weeds

The sight of wild oats escaping weed control is increasingly common in Alberta and could get a lot worse, says crop expert Harry Brook.

Kochia and wild oats top a long list of resistant weeds that are now present in Alberta, says crop specialist

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When it comes to herbicide resistance, producers should be looking for trouble.

The ongoing spread of resistant weeds means producers should be actively scouting for it, said provincial crop specialist Harry Brook.

“We now have glyphosate resistance in kochia in southern Alberta and it continues to spread,” said Brook. “These should serve as a wake-up call to producers to the importance of rotating different herbicide groups when treating problem weeds. Failure to take this problem seriously will eventually result in the loss of our most popular weed control products.”

Herbicide-resistant wild oats can be found in many fields in the province. Some biotypes are resistant to more than one herbicide group.

“In Manitoba in 2016, 78 per cent of fields sampled had some Group 1-resistant wild oats,” said Brook. “The majority of herbicides used for wild oat control are in this group. If wild oats is resistant to a single herbicide in a chemical group, it’s pretty well resistant to all the herbicides that use that particular mode of action.”

As well, the 2016 survey of Manitoba fields found Group 2-resistant wild oats in 43 per cent of them, while 42 per cent had wild oats resistant to both Groups 1 and 2.

“These numbers would be similar in Alberta,” said Brook. “Soil-applied wild oat control is in Group 8, which is older chemistry and has seen a resurgence in use. Cases of resistance to Group 8 herbicides is increasing, despite it not being used much in the last 20 years.”

Cleavers, kochia, chickweed, spiny annual sow thistle, hemp nettle, green foxtail, wild mustard, smartweed, Russian thistle and stinkweed have all developed resistance to Group 2 herbicides, said Brook.

“That group contain the sulfonylureas, the ‘imi’s,’ and florasulam. Weed surveys from 2014 to 2017 estimate about 7.7 million acres or more in Alberta have some weed-resistance issue.”

Producers need to investigate areas where weed control wasn’t effective.

“Rule out other factors that might have affected herbicide performance including misapplication, spray misses, unfavourable weather conditions, and misapplication of herbicide at wrong leaf stage or late weed flushes. Other warning signs include other weeds listed on the herbicide being controlled adequately; patchy control with no reasonable explanation; a history of herbicide failure in the same area; lack of signs of herbicide injury on plants; and finally, a history of using the same herbicide group on the land, year after year.”

Using the same herbicide or products with the same mode of action selects for resistant plants.

“By killing off susceptible plants, you are actually setting the stage for the resistant ones to thrive as all their competition is killed off,” said Brook.

Herbicides that have one specific mode of action are most likely to develop resistant weeds. “Group 1 and Group 2 herbicides fall into this category,” he said. “However, the most important reason for having resistance show up is due to repeated use of the same chemical. Short crop rotations and a lack of crop variety has set up the conditions to encourage weed resistance to emerge.”

Canada has resistance issues in at least six different herbicide groups. The consequences of poor crop and herbicide group rotation, along with not actively scouting, “are not cheap or pretty,” he added.

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