Nip Glyphosate Resistance in the Bud

Scout and catch glyphosate-resistant weeds before they're costly.

Published on: Apr 16, 2009

If you spot horseweed (marestail), waterhemp, giant ragweed, even lambquarters raising their ugly heads after a spring glyphosate application, act swiftly to put them down before they go to seed.

Better yet, burn them before you plant if you saw escapes last season. You may have a resistance problem in the budding that won't be fixed by more glyphosate.

Reports of glyphosate-resistant weeds have continued to rise. And not all reports are from corn and soybean fields utilizing Roundup Ready genetics. A joint university study by Iowa State, Purdue, Mississippi State, North Carolina State, University of Nebraska and Southern Illinois University Carbondale confirms that farmer reliance on the technology is weakening the herbicide's ability to control weeds.

Bill Johnson, Purdue weed scientist, explains: Farmers who plant Roundup Ready crops and spray glyphosate-based herbicides almost exclusively are finding that weeds such as marestail and giant ragweed have developed resistance.

"It's a pretty major issue in the Eastern Corn Belt. (Giant ragweed) can cause up to 100% yield loss."

"Farmers don't think resistance is a problem until they actually have it," he adds. "And they think the chemical companies can turn on the spigots and produce a new herbicide whenever they want. But since Roundup is so effective, there's not been any money for new herbicide discovery."

Take preemptive action

After noting that most of the marestail he saw in Roundup Ready soybean fields last summer were glyphosate resistant, Mark Loux, Ohio State weed specialist strongly urges taking out glyphosate-resistant marestail before planting the crop. And, flushes of new emergence must be prevented before the new crop canopy closes to shade out weeds.

Loux says the best option is to add a residual herbicide and 2,4-D ester to paraquat and use this mixture to burn down marestail while it's still fairly small, in the rosette stage.

Loux and colleagues at the University of Illinois, Penn State University and Purdue University have found that paraquat (Gramoxone Inteon), with or without metribuzin was the best option for controlling volunteer corn up to more than 10 inches tall. Best results were with seedlings less than 6 inches tall. The alternative non-selective herbicide glufosinate (Liberty) was only effective on very small seedlings.

Rotate crops and herbicides

Rotating crops consistently and using different herbicides will slow the development of glyphosate-resistant weeds. "That's the best short-term solution," Johnson says. "You want to minimize the exposure that any weed population has to Roundup. If you diversify a little bit, you'll extend the life of the technology."

Penn State Extension Weed Specialist Bill Curran urges reporting any suspect resistance problem immediately to a county Extension ag agent immediately. Swift action with alternative herbicides and other physical control methods can eliminate the plants. Once they go to seed, it's too late.

"Investigate the causes and factors," he says. "How big was it at application? What rate or applications were applied? Is there a spray pattern associated with the surviving plant? Are the plants displaying different levels of herbicide injury? Just because you didn't kill the weed doesn't mean it's resistant. In fact, frequently it's not."

Even lambsquarters are showing increased tolerance to glyphosate, notes Curran. "There have been performance problems with common lambsquarters and other weeds on Maryland's Eastern Shore and in portions of the Midwest.

"There have been more resistance problems with the ALS-inhibitor family - a different family of herbicides - than with any other family. We're seeing more of these problems in the Northeast."

So plan, keep your eyes peeled and prepare!