December 06, 2005

Resistance to popular weed killer may be growing

by K.C. Jaehnig

CARBONDALE, Ill. -- Facing a weed species that glyphosate can't kill, Illinois farmers should start circling their spray rigs.

"We will certainly have more weeds in the future become resistant to glyphosate — the only question is when," said Bryan G. Young, point man on weeds for Southern Illinois University Carbondale's College of Agricultural Sciences.

"Our challenge is to make that a long time from now instead of in the near future so that glyphosate continues to be an integral part of our crop production system."

The active ingredient in the popular weed killer Roundup, glyphosate kills a broad range of annual and perennial grasses and broadleaf weeds. Farmers use glyphosate-based herbicides not just in fields but in orchards and vineyards, too.

University of Illinois researchers earlier this year reported finding a field population of marestail that seemed glyphosate resistant. Follow-up tests over the summer confirmed that suspicion.

"Adjoining states (Indiana, Kentucky and Missouri) confirmed resistance to glyphosate in marestail several years before us," Young said.

"I'm not sure if that's an indication that we have managed our glyphosate use better or whether it has more to do with tillage. Glyphosate resistance in marestail has been more commonly associated with no-till, and our adjoining states have a longer history and greater adoption of no-till practices."

Glyphosate-resistant weeds became a hot topic of conversation in July when Monsanto officials and researchers from the University of Georgia reported the discovery of a form of Palmer pigweed that looked to have resistance. In September, after a number of tests, Monsanto announced that this particular biotype can pass on its resistance to subsequent generations.

Bad news for Georgia, but not necessarily for Illinois. While Palmer pigweed grows here, glyphosate — at this point, anyway — readily fries it.

"All of the Palmer pigweed that I've sprayed has been easier to kill than waterhemp," Young said.

Unfortunately, waterhemp could well turn out to be this region's Palmer pigweed.

"Problems with achieving consistent control of waterhemp with glyphosate have escalated over the last several years," Young said.

"In addition, a recent report from the University of Missouri has indicated that a population of waterhemp in northwest Missouri is suspected to be resistant to glyphosate.

"We've had a problem with not getting complete control of waterhemp for several years, but to date, our greenhouse studies haven't confirmed true resistance in any suspected populations in Illinois. However, we will continue to screen problematic weed populations as we believe glyphosate-resistant weeds may be present. It's just a matter of identifying the populations if they increase."

Farmers can help in the battle to keep those numbers down by cutting down on their glyphosate use — something Young knows will be hard for them to do because it's so cheap, so easy to use and so effective.

"But it's a numbers game," Young said.

"The frequency of any glyphosate-resistance traits in weeds is extremely low based on the fact that glyphosate has been used successfully since the 1970s. However, the more plants you try to kill with glyphosate, the greater the chances that you will find one with that trait (and leave it standing to go to seed while all those weeds without resistance die). Anything that reduces the number of individual plants we try to kill with glyphosate can help delay the onset of widespread resistance."

Farmers also can team herbicides having residual soil activity with glyphosate-based weed killers.

"If the residual herbicide effectively prevents a major portion of the weeds from emerging, that should reduce the odds of finding weeds with that low-frequency trait that might allow for glyphosate resistance," Young said.

By using less glyphosate now, farmers may be able to extend their future use.

"It's the single most effective post-emergence herbicide we have," Young said.

"It's truly been the deciding factor for some growers on whether they're profitable or not."

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