A new chemical cocktail may stir debate among agronomists and leave herbicide resistant weeds shaking in their roots.
Francois Tardif, the scientist who is tending bar for this new style of weed control, is blending reduced rates of herbicides with various modes of action in a pre-emergent herbicide recipe.
Three years ago the University of Guelph plant researcher began treating weed resistance in the same way that doctors approach antibiotic resistant bacteria.
“One night it came to me. Resistance itself is the issue, not the weeds,” he said.
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“AIDS, peptic ulcers, TB, super bugs. All of these have resistance issues and there is a method to deal with them.”
Using the approach that doctors wield against resistant bacteria, he created cocktail formulas of herbicides that combined five or six chemicals with two or more modes of action.
Treating resistant bacteria in humans usually involves combining two or three different antibacterial compounds.
“We had to be careful to use rates in combination that were low enough not to injure the crop and high enough to kill most of the weeds,” he said.
He found that decreasing rates of individual herbicides by two-thirds often resulted in weed control rates of only 70 percent.
“When you combine more than one of these, the total control, even without resistant weeds, jumps up,” he said.
“And if you add another at (a two-thirds reduction) you can often easily exceed the effectiveness of a single mode of action herbicide … without increasing the price too drastically.”
Most researchers were skeptical when he started reducing herbicide rates, he added.
“I talked to (the federal Pest Management Regulatory Agency) and some people in the industry. It made them kind of itchy,” he said. “Even my own technician questioned the idea at first.”
Lower herbicide rates and poorly applied herbicides are often blamed for herbicide resistance in weeds.
Herbicide resistance in pigweed, ragweed, lambs quarters and foxtail barley is becoming a problem for corn and soybean growers in Ontario and while experts say it is developing at a slower rate in Western Canada, agronomists regularly emphasize the problem when advising producers.
As early as 1973, agronomists began recording instances of weeds selecting for resistance to the then-popular group 5 herbicide Atrazine, which was used on corn. Ten years ago resistance to group 2 Pursuit appeared.
“For years, agronomists and dealers have been lecturing producers on rotating their herbicides and getting involved in (integrated pest management systems), but for many farmers they have preferred to just go Round Up Ready and be done with it,” Tardif said.
“The problem is that it doesn’t end the issue of resistance or weeds. It is a Band-Aid for a year or two.”
Two years into the trials ,Tardif said there have been startling results with combinations of group 2, ALS AHAS inhibitors, group 11, carotenoid synthesis inhibitors and group 9 EPSP, synthase inhibitors.
“Some of these combinations have shown great synergies when placed together,” he said.
“We get excellent weed control and it eliminates resistance.”
He said some producers experiment with combinations of “off label use,” but in general the practice is considered too complex to “be bothered with and there are regulations to think about.”
Tardif is continuing his work and he has started talking to manufacturers about releasing the chemical recipes.
“To get minor use registration for these combinations could be expensive and difficult, but if the manufacturers could begin using them in their trials as they develop products, they would have the opportunity to sell the idea,” he said.
Resistance control is not a key focus of herbicide marketing, but Tardif feels it could become more important in the future and regulatory authorities may start to require strategies to deal with the problem as new products are released.
