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Glyphosate resistance draws attention to herbicide stewardship

One final test remains to confirm whether a giant ragweed population in Ontario is glyphosate resistant. If confirmed, it will be the first case of glyphosate resistance in Canada. That would be a dubious distinction for Ontario, although the herbicide would join the ranks of others that now fail to control certain weed biotypes.

December 16, 2009  By Heather Hager

 If glyphosate resistance is becoming a problem in Ontario, it
signals something of a new era in weed management.
Photo courtesy of Dr. Peter Sikkema, University of Guelph.


One final test remains to confirm whether a giant ragweed population in Ontario is glyphosate resistant. If confirmed, it will be the first case of glyphosate resistance in Canada. That would be a dubious distinction for Ontario, although the herbicide would join the ranks of others that now fail to control certain weed biotypes. This situation is a significant reminder that good herbicide stewardship practices should be at the forefront of growers’ minds.

Dr. Peter Sikkema, weed management specialist at University of Guelph’s Ridgetown Campus, was first notified in 2008 of poor control of a giant ragweed population in a field in Essex County, in the southwestern corner of Ontario. “The field had a history of glyphosate use, but it wasn’t that excessive,” says Dr. François Tardif, a University of Guelph plant scientist who is involved with the herbicide resistance testing program. The field had been planted with glyphosate-tolerant soybeans for six of the prior seven years. Tardif says that, unlike situations in which growers have used glyphosate-tolerant soybean technology continuously since its introduction in 1996, the grower had used the technology in the field only six times.


To test for resistance, giant ragweed seeds were collected from the field and grown in a growth chamber along with a known glyphosate-resistant giant ragweed population from Ohio and several susceptible populations. When sprayed with glyphosate, suspected and known resistant plants survived, whereas known susceptible ones died, says Tardif. Seeds were then collected from the suspected resistant plants that survived. The testing takes some time because giant ragweed seeds must be treated for two months to break dormancy, and plants surviving the first test must be grown to maturity to collect seed for the second test. If the second generation of giant ragweed plants survives testing, glyphosate resistance will be confirmed. Tardif estimates that the final testing should be completed by late January 2010.

 Weedy checks are often grown out in research plots, but the process for confirming resistance also requires extensive laboratory
Photo courtesy of Dr. Peter Sikkema, University of Guelph.


Minimizing herbicide resistance
Experts agree that the best strategy for minimizing the risk of herbicide-resistant weeds is agronomic diversity. “I think that if farmers have a diversified crop rotation and they use multiple herbicide modes of action over time, those would be the two best strategies,” says Sikkema. “They can introduce those different herbicide modes of action in different crops or even within the Roundup Ready crop using a pre-emergence residual herbicide or a tank mix of glyphosate and another effective herbicide.” Using an additional herbicide reduces the selection intensity for glyphosate resistance by reducing weed populations before or concurrent with glyphosate spraying.

Over the years, Monsanto has developed herbicide stewardship recommendations that have been fine-tuned by studying the situations underlying previous occurrences of glyphosate resistance, says Dr. Mark Lawton, Monsanto Canada’s technology development lead for Eastern Canada. The recommendations include starting with a clean field and staying on top of weed control, using the full labelled rate at the right time for the weeds of interest, and adding other herbicides or cultural practices where appropriate in the production system. “I think those three simple reminders are good advice for success this year and for many years to come. For the giant ragweed site, if those tactics would have been an option for the grower, we might be dealing with a different situation,” he remarks.

The good news for Ontario growers is that this discovery of potential glyphosate-resistant giant ragweed should raise awareness of herbicide stewardship, says Tardif. “Resistance doesn’t suddenly appear over the whole field. It very often starts as small pockets of weeds that look like maybe the spray didn’t reach that spot for some reason.” These should be knocked down with a different herbicide early on, or flagged or coded into a GPS at harvest and double-checked the following year after spraying to ensure they have been killed. Growers should consult their agricultural input suppliers or the Ontario Ministry of Agriculture, Food, and Rural Affairs (OMAFRA) for guidance with weed control issues.

Beans on beans
Crop rotation is not just a good idea to minimize herbicide resistance; it also has other agronomic benefits. “We’ve learned that a continuous production practice of the same crop in the same way over a number of years can be troublesome,” says Lawton. “Rotating provides benefits for soil structure, disease pressure, weed control and insect control. It’s not specific to any one issue; it’s just good practice to remain successful over many years in a field.”

Although it is not a recommended practice, some growers in Ontario do plant soybeans in the same field continuously or for multiple years in a row, says Horst Bohner, soybean specialist with OMAFRA. Many of those would be found in Lambton and Essex counties and the Niagara region. In these areas, says Bohner, the heavy clay soils can hinder corn planting in spring, and getting good yields can be difficult. 

Planning for rotation does not always work in the heavy clay soils. “You might plan to plant corn and then it rains all of April and three-quarters of May, and you may not have the opportunity. Some of those clays take a month to dry out.  So you’re forced back into growing soybeans,” explains Bohner. Market fluctuations are also a factor. Plans to plant wheat might be scuttled because of low prices, giving soybeans a short-term economic advantage on paper.

Regardless of difficulties in introducing regular crop rotation, growers should be careful to introduce herbicide diversity. “It’s not a good idea to use the same chemistry consistently,” says Bohner. “But people grow glyphosate-tolerant soybeans year after year because it’s a simple system that’s easy to keep clean.” 

Going for what is easy and looks good economically in the short term can set growers up for grief down the road.  It is a matter of weighing the short-term profit from soybeans with the less tangible longer-term agronomic gains from planting an alternative crop. So far, the suspected glyphosate-resistant giant ragweed is proving difficult to control (see sidebar: Giant ragweed control options). “I think growers that have this problem may have to consider the economics of planting an alternative crop relative to trying to control glyphosate-resistant giant ragweed in soybeans, where there are not many control options,” says Sikkema.

 Resistance to glyphosate and Group 2 herbicides is another
issue facing growers, researchers and industry stakeholders.


Giant ragweed control options
Dr. Peter Sikkema, weed management specialist at University of Guelph’s Ridgetown Campus, has tested herbicide options for controlling the conventional type of giant ragweed in corn and soybeans. In corn, for pre-emergence herbicide application, Marksman and Banvel gave better control than the other options registered in Ontario, he says. Post-emergence, the best control was obtained with Marksman, followed by Banvel and Distinct.

In soybeans, for pre-emergence herbicide application, FirstRate gave the best control. “All of the other options gave pretty disappointing control that farmers would not be happy with,” says Sikkema. Post-emergence, the two best options are FirstRate (94 percent control) and Pursuit (88 percent control); Classic gave only 68 percent control, and the remaining options gave less than 50 percent control.

In Roundup Ready corn or soybeans, Sikkema says that two applications of glyphosate typically gives good control, whereas control is variable with one application. 

Controlling glyphosate-resistant giant ragweed, however, is another matter. Sikkema says that the data indicate that good control might be achieved in corn using a tank mix of Roundup plus Marksman. In soybeans, he suggests that Roundup plus FirstRate should be quite effective. “Having said that,” he continues, “the population that we think is resistant to glyphosate also appears to be resistant to the Group 2 herbicides, and FirstRate’s a Group 2 herbicide, so it doesn’t work. So control in soybeans will be a huge challenge if it’s resistant to both glyphosate and Group 2 herbicides.”


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