Multiple-Resistance Weeds: Coming to a Field near You?

Farmers aren't concerned unless it's in their fields.

Glyphosate-resistant weeds are on the rise says Kevin Bradley, weed scientist at University of Missouri, speaking at a Bayer CropScience meeting in Nashville this week.

Of even greater concern is that some weeds are becoming resistant to several modes of herbicide action – not just glyphosate.

"Glyphosate resistance is a concern and we want farmers to change their ways," says Bradley. "I am probably equally scared about multiple resistance as just glyphosate resistance. Some waterhemp is now resistant to glyphosate, ALS and PPO-inhibiting herbicides.

"We're trying to scare people to death in order to change their ways, but the shock value is going away," he says.

Using different modes of action will lessen the impact of resistant weeds. But it's an uphill battle to get farmers to change their ways says Bradley. For one thing, the best systems cost twice the cost of a typical 2X glyphosate application.

Liberty Link soybeans is one way to battle resistance, he says. And we may have dicamba-resistant soybeans by 2013, which would add another weapon in the weed arsenal.

Resistance happens within the natural population of weeds. "What we do wrong is spray glyphosate over and over and allow that biotype to survive until sooner or later, you have a patch of resistant weeds among the patch of killed weeds," says Bradley. "Then those weeds multiply."

In Missouri, a recent retailer survey revealed that 51,000 acres may be suffering from glyphosate resistance.

Where we're headed is more Roundup Ready acres whether we want it or not, he says. Most seed companies say in the next three to five years we will have 80% Roundup Ready corn-Roundup Ready soybean rotations.

"If we get into these rotations, we're going to be in trouble," he says. "We're already 90% roundup ready soybeans. These continuous glyphosate systems are what we're worried about."

The list of resistant weeds includes marestail, common ragweed, giant ragweed, amaranth, waterhemp, hairy fleabane, Italian ryegrass and rigid ryegrass, he says. And most of it is in the eastern Corn Belt as well as the mid-south. You can get more information at You can also learn more about glyphosate resistant weeds at:

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