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Multiple resistance could be a nightmare for region's farmers

Farmers in the Mid-South and Southeast have spent a lot of money – and, in some cases, have lost entire fields – because of the development of resistance to glyphosate in prolific seed-producing weeds like Palmer amaranth or pigweed.

But how much will it cost them in added herbicide and labor expense and crop losses when weeds like pigweed or common waterhemp develop resistance to more than two or three or even more herbicide modes of action?

That was the question posed by Aaron Hager, Extension weed scientist with the University of Illinois, who spoke at the Bayer CropScience Respect the Rotation Pigposium on July 23. The event aimed at providing farmers with the latest weed control technology was held at the University of Arkansas’ Northeast Research and Extension Center in Keiser.

Dr. Hager said plants with increased genetic diversity such as Palmer amaranth and its cousin, common waterhemp, are much more likely to develop resistance to herbicides with multiple modes of action than species with a more narrow genetic base.

“For us in Illinois, one of the ways this genetic diversity is being realized is in the evolution of resistance,” he said. “As a matter of fact now, we have waterhemp populations that have evolved resistance to herbicides from five different chemical families.”

Three or four years ago, Hager said, weed specialists in Nebraska found a common waterhemp population that was resistant to auxinic herbicides – such as dicamba and 2,4-D. Worldwide, waterhemp plants have been found that are resistant to six different classes of herbicides.

“If you had a field, 40 acres, 80 acres, and you find a type of resistance in that field – pick whichever one of the size that you like – that fact by itself does what?” he asked. “It raises the level of management that you have to have in that field. You have to know which herbicides will no longer effectively control that broadleaf population.

“Unfortunately where we see this going will be much more problematic. Where we see our populations going is right here, and this is multiple resistance,” he noted, pointing to a chart. “When you say multiple resistance you have to define what scale we’re talking about.”

It would be simple if some of the plants in the field were resistant to one herbicide, others resistant to another herbicide and others resistant to a third compound. Farmers could then mix the three herbicides together, apply them and have some expectation of achieving relatively good control.

Instead, researchers are finding situations where weeds that are generally susceptible to atrazine, Cobra and Raptor can’t be killed with a combination of all three.

“This one in western Illinois, we can’t touch it with any of those three herbicides,” said Hager. “It doesn’t matter if we spray these three separately or put two of them together and tank mix them and followup with the third. Or you can put all three together in the same tank and spray it, and it doesn’t die.”

For more on resistance issues in Illinois, see

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