Weed resistance close to ‘onslaught’

It’s almost scary how serious the problem is, says Bob Nichols of Cotton Incorporated.

Weed resistance to multiple herbicides has created a situation “where we’re on the precipice of an onslaught of resistance,” says the senior director of Agricultural Research at Cotton Incorporated.

Key Points

• Use preemergence herbicides this season. No excuses.

• Pigweed’s resistance to glyphosate is serious business.

• Scientists advocate a program to manage resistance.


Here are the hard facts:
• Some 120 million acres have weed resistance to glyphosate alone.

• Some 20 weeds are resistant to glyphosate.

• Four principal modes of action are resistant to weeds.

“This is hard data,” Nichols says.

One field of pigweed in North Carolina is indicative of the problem. Pigweeds there drank in 88 ounces of glyphosate, and it didn’t even faze them.

“It’s like one seed head of the pigweed sticks up above the others as a middle finger and says, ‘This is what I think of modern technology,’” Nichols says.

“We have few alternatives in soybeans and cotton. Atrazine is holding together postemergent weed control. If we lose atrazine, it will be very serious.”

In cotton, only PPO herbicides stand between pigweed and the use of Ignite.

The next mode of action could be 10 years down the road. The last new mode of postemergent herbicide came in 1993.

Despite the seriousness of the problem, however, an official effort to fund research was recently rejected. Weed scientists put together a definitive statement in support of the need for research, but a grant that would have funded an initiative was rejected, Nichols says. Weed researchers are currently working on a paper that states the magnitude of the problem.

In the Mid-South, pigweed, or Palmer amaranth, is dominant, but resistance is fast coming in Italian grass, ragweed and other weeds. Besides being resistant to glyphosate, pigweed is often also tolerant or resistant to ALS herbicides.

Pigweed, which is native to the Southwest, has taken over the South, replacing the dominant smooth pigweed species. Starting in 2004 on a single field in Macon County, Ga., it showed documented resistance to glyphosate in Tennessee in 2005, and in Arkansas in 2006.

It’s invaded fields where farmers had come to rely exclusively on the miracle of glyphosate, and it has even compromised conservation tillage.

It requires a new management program, Nichols says, and a concerted effort to save conservation tillage. “We need to implement a resistance management program,” he says.

That involves using preemergence herbicides before planting. “There are no excuses, no questions asked when it comes to applying preemergence herbicides this year,” Nichols says.

“We’re facing an exponential expansion of resistance to pigweed … if we don’t use preemergence herbicides before we plant,” Nichols says.

He advises using different modes of action and even considering the use of other technologies to fight resistance.

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SERIOUS BUSINESS: Bob Nichols is matter-of-fact when he recommends farmers use a preemergence herbicide before planting this season. “No excuses.”

This article published in the January, 2011 edition of MID-SOUTH FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.