Farmers are increasingly aware of the possibility of getting herbicide-tolerant weeds in their fields, but most aren’t doing enough to delay them, says Gary Martens, an agronomy instructor at the University of Manitoba.
“Farmers have not changed their behaviour based on what they know,” he said, in reaction to an Ipsos Reid poll of 500 Canadian farmers taken in March.
“I don’t think a lot of farmers have a long-term herbicide rotation plan.”
Three-quarters of the farmers surveyed from Quebec to Alberta said herbicide-resistant weeds are reducing their revenues. Most (47 per cent) said the impact was small, with 20 and seven per cent saying the effect was moderate or large, respectively. Twenty-five per cent said herbicide-tolerant weeds have no impact at all on their earnings.
Manitoba farmers seem less worried, with just two per cent saying herbicide-tolerant weeds have a big impact on their earnings, with 16 and 59 per cent saying the impact was moderate or small, respectively.
Rotating crops and herbicide groups is one strategy to delay the onset of herbicide-resistant weeds, Martens said. It looks like cutting herbicide rates is not the right approach, even though that’s the advice he used to give students, he added.
New research indicates cutting herbicide rates can stress a weed, turning off the error-correction process in the plant allowing it to possibly mutate and survive the herbicide.
“This is an evolutionary strategy and the herbicide can actually change the plant,” Martens said.
Most herbicide resistance is believed to occur through natural selection. As weeds are repeatedly exposed to herbicides with the same “mode of action,” or way of killing, over time only weeds that are naturally resistant survive and reproduce, passing their resistance on to their progeny.
Skipping a herbicide application, if appropriate, also helps to delay herbicide tolerance, he said. That might be possible, say, when wheat has germinated before the green foxtail. If the weather is cool and moisture adequate the wheat should be able to get ahead of the foxtail.
Periodically planting perennial crops, such as forages, can reduce the need for herbicides. Intercropping, such as planting canola and peas together, can have a similar effect, Martens said.
“I’m surprised that with intercropping, not more farmers are doing it,” he said.
“That fills the space in the field and you have less weeds in a situation where you have more than one crop at a time.”
Applying two or more herbicides with different modes of actions to weeds at the same time is seen as more effective in delaying resistance than rotating herbicides year to year, Martens said.
Manitoba farmers have been fighting herbicide-tolerant weeds since the 1970s starting with Group 3, trifluralin-resistant green foxtail, followed in the 1980s with Group 1- and Group 2-resistant weeds.
Glyphosate resistance is of increasing concern with the resistant kochia in southern Alberta and Canada fleabane and giant ragweed in Ontario. Twenty-one weeds globally are resistant to glyphosate.
According to some weed scientists glyphosate is the world’s most important herbicide. Michael Schaad, BASF’s business manager for Eastern Canada crop protection, says glyphosate is critical for conservation tillage.
“We need to protect that chemistry so it will be successful for a longer period of time,” he said.
“We don’t want to go back to the days when the ditches are filled with dirt.”
To that end BASF has two herbicides designed to be tank mixed with glyphosate. They make glyphosate work better, while reducing the changes of glyphosate resistance.
One is Distinct, a pre-mix product to be applied with glyphosate. It’s designed for post-harvest and chemfallow weed control. Distinct, which was just registered, contains two active ingredients — dicamba (Group 4) and diflufenzopyr (Group 19), which is a new group to Western Canada, said Chris Vander Kant, BASF’s marketing manager for herbicides in Western Canada.
“It really increases the activity of the dicamba,” he said.
Heat, a Group 14, is designed to be tank mixed with glyphosate and applied before seeding most cereals and pulses, or before crop emergence or on chemfallow. Its active ingredient is saflufenacil (Kixor).
“With the addition of Heat there will be a faster burn-down and it will give control of weeds that are resistant to glyphosate and actually improve the overall control you get with glyphosate as well,” Schaad said.
“I think farmers have to learn there are no silver bullets out there when it comes to weed resistance.”