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Herbicide resistance on the rise


November 30, 1999
By Hugh McElhone

The list of weeds that are resistant to herbicides is growing much faster than the list of new herbicides coming to the market, says Kristen Callow the weed management program lead for the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) based out of Ridgetown, Ont.

The list of weeds that are resistant to herbicides is growing much faster than the list of new herbicides coming to the market, says Kristen Callow the weed management program lead for the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) based out of Ridgetown, Ont.

This is not a localized problem related to one species, in one area, or to one industry. On the contrary, it’s global.

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“In Ontario alone, there are numerous weed species resistant to seven groups of herbicides spread across the province,” says Callow.

In the U.S., weed species resistant to glyphosate-based products such as Roundup, covered some 11.4 million acres in 2010. This represents a ninefold increase from 2.4 million acres in 2007. She noted this area is now roughly equal to the size of Ontario’s entire production area.

Callow defines general resistance as “the inherited ability of a plant to survive and reproduce following application of a herbicide that would normally be lethal to individuals of the same species.”

Cross resistance is the “resistance to a herbicide to which the plant has not been previously exposed, but having a similar mode of action as the original herbicide,” she says.  The hardest to combat is multiple resistance, which is “resistance to more than one group of herbicides with very different modes of action where more than one basis for resistance is involved.”

Callow says nature itself supplies the four main reasons for resistance, those being human, resistance, herbicide and weed natures.

For human nature, “if something is successful, we tend to repeat it,” she says. Callow knows of one producer who applied Gramoxone to his crop for 30 years straight before he noticed a persistent problem with Eastern black nightshade.

“All pests, including weeds, have the ability to adapt to repeated practices,” she says.

For resistance nature, Callow explains “that within all weed species, a few individual plants may have altered genes that allow them to be resistant to the herbicide.” These plants will thrive in subsequent generations.

Of herbicide nature, Callow says not all herbicides are created equal, nor are they effective against herbicide-resistant weeds. For example, the “old chemistry” Group 4 products, such as 2, 4-D and MCPA, have been in use for more than 50 years and are successful but with a limited selection of resistant weeds. On the other hand, Group 2 products, such as Glean and Ally, were on the market for only a few years before resistance was detected.

Weed nature and resistance is simply based in the plant’s vast genetic diversity. Callow says plants that cross-pollinate, rather than are self-fertilizing, are more likely to contain an individual with a resistant mutation.

“Herbicides need to be viewed as a valuable resource,” she says, and all producers need to rotate crops and herbicides as the best way to manage weed resistance.

“Many herbicide options may be gone if a weed biotype is resistant to more than one herbicide, (both) cross and multiple resistance,” she says.

The development cost and registration of new herbicides is prohibitive.

“There are no new products coming to market in Ontario anytime soon, so we don’t want to lose the ones we have now,” she says.

On the global stage, Callow said some 347 resistant biotypes have been identified, of which, 114 species are broadleaves and 80 are grasses. Some 340,000 fields worldwide have been affected.

Perennial rye grass was the first species to be found with resistance to Roundup (glyphosate) in Australia. Of the 20 species identified since that discovery, 10 Roundup resistant species have been found in the U.S. affecting 18 states. Canada fleabane is the only resistant species found so far in Canada.

Vegetable producers in the Holland Marsh, near Bradford, Ont., suffered a near crop failure last year with pigweed resistant to Lorox, says Callow. Lorox, a Group 7 product, is one of two herbicides registered for pigweed and ragweed control.

“All three species of pigweed were found there,” she says, while showing slides of fields so choked with pigweed that the carrots, onions and lettuce could not be seen.

“We are working with carrot and onion growers in Quebec who have a similar problem but with ragweed,” she says. Both provinces are working on a joint research plan to submit for funding.

Callow mentioned two weeds to watch for, the first being wild parsnip, which will cause a rash to unprotected skin. The second is Kudzu, which she says the Americans describe as “the vine that ate the South.” This prolific vine smothers crops, trees and buildings and is noted for its mitten-style leaf and purple flower.

The best way for producers to combat resistance is to rotate crops and herbicides and tank-mix products that have multiple modes of action. Other strategies include cultivation, manual plant removal and spot spraying.
“If you suspect resistance, don’t let it go to seed,” Callow cautions. “The University of Guelph will test for herbicide resistance. It’s discreet, and it’s free.”

For more information visit  http://www.uoguelph.ca/plant/resistant-weeds/resources/resistancetesting.html .