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Manage Resistance In Herbicide-Tolerant Crops

About 90 per cent of canola acres grown in Western Canada contain either the Liberty Link or Roundup Ready trait. Canadian canola producers have so far dodged the weed resistance bullet that is causing problems for many American soybean and corn growers.

“They have big problems in the southern U. S., particularly in the soybean and cotton belt, because they have Roundup Ready (RR) cotton and RR soybeans and it’s such an easy, effective treatment that they do it (apply glyphosate) too much,” says Neil Harker from Agriculture and Agri-Food Canada’s Lacombe Research Centre in Alberta. “(Canadian) canola growers seem to have a little more restraint and are putting canola in at least a one in two or three year rotation, and because they are willing to do that, even though it’s not always the most cost effective thing to do in the short term, they exert much less selection pressure for glyphosate resistance.”

Most canola growers are already familiar with herbicide resistance. They have seen it to a certain degree with some Group 1 and Group 2 herbicides over the past few years. They are, for the most part, acutely aware of the possibility of further resistance developing to other products, such as glyphosate, and try to make preventing it a part of their overall crop management strategy, says Barry Chappell of the Manitoba Canola Growers Association (MCGA).

“I would say that farmers as a whole are pretty good stewards in trying to manage weed resistance,” says Chappell. “On our own farm we do a two year rotation with RR canola and two years after that Clearfield canola and go strictly with a Group 2 product, we don’t use a Group 2 product on our wheat or cereals, we save our Group 1’s for that. We know that continuous use could definitely cause some problems with resistance. Never say never, but I am not aware of any glyphosate resistance here.”

The U. S. has confirmed glyphosate-resistant populations of common ragweed, common lamb’s-quarter, waterhemp and Canada fleabane resistant to glyphosate. There is also a glyphosate-resistant giant ragweed in southern Ontario. Comparing the experience of Canadian and U. S. growers, however, is difficult because of differing plants, conditions and practices.

“Here in western Canada we spray at different times of the year and we are targeting different weed spectrums,” says Chappell. “I am not sure of their application timing in the U. S. and our different climactic conditions here probably do play somewhat of a part.”


Rotating canola varieties and varying herbicides with different modes of action definitely helps to avoid resistance. “With canola there are lots of other alternatives that are very good for weed control,” says Rob Pettinger, president of the MCGA.

Other important factors in a weed’s potential to develop glyphosate resistance are the frequency of glyphosate use and the application rate. Repeated use of glyphosate increases the risk that some resistant biotypes will survive, set seed and multiply. It is important to note that herbicides do not cause resistance, rather some biotypes have a rare genetic trait that may exist in the natural weed population which allows a plant to survive glyphosate application

“Resistant weed biotypes are naturally occurring in the environment, but we didn’t know they were there because we didn’t previously apply selection pressure,” says Peter Sikkema of the University of Guelph, Ridgetown Campus. “What happens is when you repeatedly use a herbicide with the same mode of action the only plants that survive and return seed to the soil are the resistant biotypes, and so that biotype increases in frequency until it becomes dominant over time, while the susceptible biotype decreases in frequency.”


Application rates may also affect weed resistance, depending on the weed biotype. Maintaining full glyphosate application rates may help reduce the selection of some types of resistant weeds. Weeds that have target-site resistance are not usually influenced by low application rates. There is also an emerging theory that varying application rates in addition to herbicide modes of action may also be a good practice. By applying the lower labelled rate for two years and then using the high labelled rate the third year, then going back to the lower rates on the label and so on, weeds may be confused, making it another possible strategy to try and delay the selection of herbicide resistant biotypes.

As long as canola producers remain vigilant and make the kind of management decisions needed to help prevent resistance from developing, they should be able to avoid the economic hardships that have beset some of their US counterparts.

“Whether you are growing a Clearfield, Roundup Ready or Liberty Link products you don’t have to sacrifice yield,” says Chappell. “Producers can grow all three technologies to help rotate in and out and prevent resistance problems.”

Angela Lovell is a freelance writer based at Manitou, Man.

About the author


Angela Lovell

Angela Lovell is a freelance writer based in Manitou, Manitoba. Visit her website at or follow her on Twitter @angelalovell10.



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