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Arkansas proves no state safe from glyphosate resistance

In more ways than one, Arkansas has become the Wild West of weed pressure. The weeds are out of control. Farmers are desperate. Some folks are facing full-blown glyphosate-resistant populations of giant ragweed, common ragweed, johnsongrass, marestail and waterhemp.

Still, public enemy No. 1 is glyphosate-resistant Palmer amaranth, commonly known as pigweed.

On many farms, pigweed can still be controlled with other chemistries, notes University of Arkansas weed specialist Bob Scott. However, folks are spraying too late.

“These chemicals don’t work like glyphosate used to,” Scott says. “One of our biggest challenges is getting people to realize that.”

Key Points

Arkansas farmers are battling six weeds with confirmed glyphosate resistance.

The toughest challenge yet has been controlling resistant Palmer amaranth.

Weed control costs have increased from $15 an acre to $80 an acre.


Today, a 2- to 3-inch pigweed is well within range. Unfortunately, Scott says too many farmers are trying to control half-foot-tall pigweed. It looks short from the cab of a sprayer, but it’s still too tall.

“You almost cannot spray too early when it comes to pigweed,” Scott adds.

Out of control

Southern Illinois farmers are just beginning to see pigweed. For the central and northern part of the state, pigweed is still a foreign concept. Scott has heard many I-state farmers say they don’t think they’ll ever see massive pigweed infestation.

“When I visit Illinois, I see Arkansas four or five years ago,” Scott warns. “A lot of these guys don’t think they’ll ever get it because they think they’re doing a great job with their herbicide program. The fact is, if you keep doing the same thing over and over again, you’re going to get resistance.”

Through the 1980s, Arkansas farmers relied on ALS chemistries to kill pigweed. These herbicides were becoming ineffective in the 1990s.

“When Roundup Ready came out in the late 1990s, it was like a miracle for us,” Scott adds. “We needed that chemical.”

In 2005, the first glyphosate-resistant pigweed was confirmed on an eastern Arkansas farm, near the Tennessee border. Then it disappeared for two years.

In 2008, the weed came back in a big way. “In 2010, it just blew up,” Scott adds. “It only took three to four years to the point where glyphosate was completely ineffective.”

Using other chemistries

Nearly 25% of Arkansas farmers planted LibertyLink soybeans this year. Malcolm Haigwood, a Newport, Ark., farmer, says the only problem is, folks treat them like they’re Roundup Ready soybeans. The LibertyLink system is good, Haigwood says. It just takes a little more management and a keen sense of timing.

“A lot of young farmers, all they know is Roundup Ready,” Haigwood adds. “They’re at a disadvantage, because you have to manage this thing completely different if you want to control it.”

Of course, herbicide costs have increased in the process. Haigwood, who custom-sprayed 65,000 acres this year, says weed control used to cost about $12 to $15 per acre. This year, he heard reports of folks paying $80 an acre. Realistically, he thinks the job can be done for about $65 per acre.

To tame this beast, Haigwood says growers must start with a good burndown program. In his part of the world, he recommends a good burndown pass in late February or early March. “You’ve got to start clean,” he adds.

He typically follows this up with three postemergence trips. He uses glyphosate tankmixed with Flexstar.

“One of the key things is, you’ve got to reduce that weed pressure so you can get good canopy closure,” Haigwood notes.

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GROWING PROBLEM: Newport, Ark., farmer Malcom Haigwood shows how quickly a Palmer amaranth problem can escalate. The weed in his right hand is near the maximum height for being controlled by herbicides (2 to 3 inches). The weed in his left hand just smiled at glyphosate and kept on growing.

This article published in the October, 2011 edition of PRAIRIE FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.