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Ontario Grain Farmer Magazine is the flagship publication of Grain Farmers of Ontario and a source of information for our province’s grain farmers. 

Herbicide resistance takes centre stage at weed conference


Weed scientists from across Canada met this week in Niagara Falls at the Canadian Weed Science Society annual conference. This is a great opportunity for public and private scientists alike to come together to discuss important issues and highlight recent breakthroughs in weed science and control.
Naturally, herbicide resistance was a large focus of the conference and attendees heard from scientists on weed resistance to glyphosate, atrazine, ALS inhibitors (such as Pursuit) and auxinic herbicides like Dicamba and 2,4-D.
All of these herbicides with their different modes of action have some resistant weeds recorded going as far back as the 1980s although some have more extreme levels of resistance than others.
An historical look
Dr. Francois Tardif from the University of Guelph provided an excellent history lesson in his presentation on resistance to triazine herbicides like atrazine. “Farmers in the 1970s thought all of their weed control problems were solved,” said Tardif to the gathered scientists. “But,” he continued, “protection comes at a cost.” Atrazine was used at high rates and it was often the sole herbicide in the 1970s and as a result, farmers saw high levels of resistance develop.
The number of resistant weed species is not growing at as high of rates as they were in previous decades and atrazine is still an effective option for many farmers. But, it’s still important to remember this history as we deal with new chemicals so as not to repeat it, especially in light of the growing number of weed species developing resistance to glyphosate.
Managing resistance
Tardif outlined several options to prevent the development of resistance. He first recommended that perhaps we should stop using herbicides altogether as “abstinence is the safest way,” to many chuckles from the audience.
As this is obviously not a viable solution, Tardif listed several other solutions. First, he talked on the benefits of rotating chemicals. When a weed is not killed by herbicide A one year, it will be killed by herbicide B the next year. This can also be accomplished through crop rotation as rotating crops often demands the rotation of chemicals.
Although rotation is an important part of the solution, it does have flaws. By leaving weeds unmanaged one year to be managed the next year, those weeds are given the opportunity to set seed and add to the weed seed bank in the soil. “Unfortunately,” said Tardif, “rotations just delay resistance.”
Another more effective option is mixing herbicides. By applying more than one type of herbicide if a weed is not killed by herbicide A it will be immediately taken care of by herbicide B without having the opportunity to add to the seed bank.
Mike Owen from Iowa State University covered the importance of mixing herbicides or, “redundant weed control” in his presentation as well. He emphasized that farmers should use multiple modes of action that are actually effective on troublesome weeds. He provided the example of tank-mixing two herbicides to control waterhemp. If the weed is resistant to one of the chemicals in the tank already, only one effective chemical is being applied reducing the efficacy of tank-mixing to avoid resistance. In this situation, there is an increased risk the waterhemp will eventually become resistant to this new chemical. It’s important, claims Owen, to use mixtures of “effective chemicals.”
Cross-discipline collaboration
The conference strayed from traditional weed science territory as they invited Shiona Glass-Kaastra to speak on the parallels between antimicrobial resistance in humans and herbicide resistance in weeds. Glass-Kaastra’s talk was eerily similar to Tardif’s historical account of resistance in weeds.
Penicillin became widely available in 1940 and the first case of a person being resistant to the beneficial impacts of penicillin was recorded in 1946, Glass-Kaastra told the attendees.
The largest difference between antimicrobial resistance and herbicide resistance that Glass-Kaastra noted is the research pipeline. There are no new antimicrobial drugs in the research pipeline as these types of drugs are not attractive or lucrative for pharmaceutical companies to develop. Most development is focused on drugs for chronic diseases like high blood pressure and diabetes.
In comparison, says Glass-Kaastra, the agricultural industry has a very full pipeline of new weed control options. She urged the scientists and the companies in the crowd to learn from the pharmaceutical industry’s mistakes and continue research and development in weed control technology.

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