Another weed in Alberta has become resistant to herbicides.
Spiny annual sow thistle is no longer controlled by Ally in at least two fields near Ponoka, said Don Poisson, a Westco Fertilizers crop specialist in Ponoka, Alta.
Poisson discovered the herbicide resistant weeds last year after two farmers complained the herbicide hadn’t controlled their weeds.
Spiny sow thistle, similar in appearance to regular sow thistle, has become a problem in the area south of Edmonton, he said. Two farmers felt the herbicide they used earlier in the year should have controlled their weeds better.
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“I’ve never come across resistant sow thistle before,” said Poisson.
He said when he and the representative from DuPont, the company that makes Ally, checked the fields, there was no visible sign the weeds had developed a resistance to herbicide. Poisson became suspicious when the farmers told him they had used a group two chemical in the field for at least six years.
Chemicals are divided into 26 groups based on how they work. Farmers are advised to rotate herbicides they use in each field to inhibit herbicide resistance.
Group two herbicides kill plants by inhibiting production of certain amino acids. Farmers like the chemicals because they are effective and usually don’t require a second pass with the sprayer, he said.
“Some farmers find something they like and they stick with it.”
Across the Prairies, many other weeds have become resistant to group two herbicides: chickweed, kochia, Russian thistle, wild mustard, cleavers, wild oats and hemp nettle.
Poisson said he identified herbicide resistant chickweed in these fields quite quickly, but it took most of the winter to confirm the resistance in spiny annual sow thistle.
Last year agriculture specialists found a field of cleavers resistant to group two herbicides in the County of Ponoka, also a first.
Poisson worries it won’t take long for the herbicide resistant plants to spread. In the fall, the tops of the sow thistle looks like dandelions gone to seed. The feather-like seeds can be carried on the wind for miles. Like dandelions, the seed grows readily on top of the soil.
“The potential to spread is quite high. The number of fields in this area with spiny annual sow thistle – it’s going to be more of a problem weed in the future,” he said.
