A race of super weeds able to survive almost all post-emergent herbicides designed to control them has developed in pockets of Manitoba.
Their appearance has researchers reconsidering how resistance develops and how to prevent the problem.
Herbicide resistant wild oats and green foxtail have plagued some producers for years. Farmers are urged to rotate herbicide groups to avoid a buildup of resistance.
But with the discovery last year of wild oats in northwest Manitoba resistant to four herbicide groups – one, two, eight and 25 – even herbicide rotation is not a solution.
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“This really puts producers up against the wall in terms of wild oat control options,” said Hugh Beckie, Agriculture Canada weed scientist in Saskatoon.
Cause unknown
In Australia, some populations of annual rye grass have become resistant to all the herbicides registered for its control, he noted.
“But we certainly didn’t expect an analogous situation to develop in Western Canada. But it seems to be that way and we aren’t sure why it’s resistant to all these different chemistries.
Developing resistance to group one herbicides (such as Achieve, Hoe-Grass 284, Triumph Plus and Puma) usually takes five to 10 applications.
It occurs because a few naturally resistant weeds survive the chemical. They set seed and the next year more resistant weeds grow and survive. Eventually, the resistant population dominates the field.
But the super weeds in Manitoba are developing resistance to herbicides that haven’t been used in the field before, Beckie said. “That’s the alarming part.”
He theorizes that the multiple resistant weeds have changed their metabolism – the way they break down or “digest” materials that enter their system.
“Normally, for example, (crop) plants are able to metabolize a wild oat herbicide, but the weed can’t.”
Northwest Manitoba is particularly ripe for herbicide resistance because it is cool and moist, ideal for wild oats growth.
“Because of that, perhaps farmers apply more wild oat herbicides than other parts of the Prairies.”
So far, weeds resistant to all four wild oat herbicides have been found only in northwestern Manitoba. But that is the only place scientists have seriously looked for them, he added.
Weeds resistant to one or two herbicides are becoming more evident across the Prairies and the problem is probably more widespread than people realize.
Farmers often don’t know they have resistant weeds until they are full blown across a field, he added. And producers with big problems don’t like to advertise the situation because it could hurt the market value of their land.
Producers with multiple resistant wild oats can still control them with group three herbicides such as Treflan and Edge, but they are soil incorporated herbicides that don’t fit with conservation tillage. Growing herbicide tolerant canola and spraying with glyphosate or glufosinate ammonium herbicides is another option.
Although herbicide group rotation is now no guarantee of avoiding resistance, Beckie still recommends it.
Other recommendations:
- Keep records of crops and herbicides use to plan long-term weed management strategy.
- Scout fields, looking for weeds that escaped the herbicide. Resistant patches are often irregularly shaped, indicating they are not strips that were missed.
- Mow, cultivate or spot spray resistant patches. Do not allow them to set seed. Do not harvest the area with the rest of the crop.
- Practice integrated weed management. Plant competitive crops. Consider a short rotation forage crop. Use only vigorous, high-quality seed. Band nitrogen close to seed to give plant an advantage over weeds.
