Resistant pigweed is a call for change

‘Change” is a word tossed about a lot nowadays in political circles. Glyphosate-resistant Palmer amaranth (aka careless weed or pigweed) found in far West Texas this growing season will require growers to change their weed management from just glyphosate herbicide alone.

Hopefully, that’s sooner — not later, when it’s too late.

Wayne Keeling, veteran Texas AgriLife Research weed specialist, Lubbock, gets multitudes of phone calls, and for some time, he was able to say resistance was a Southeast and Mid-South region problem.

Key Points

Weed resistance to glyphosate is no longer a possibility but a reality in Texas.

Growers should plan now to use more than glyphosate alone on weeds.

Other herbicides with residual activity could help in fighting resistance.


“But I can’t say we don’t have weed resistance anymore,” Keeling laments.

Keeling says weed scientists learned a few Texas cotton fields in Terry County near the New Mexico line had pigweed that survived several glyphosate treatments in August.

The scientists followed up with lab tests by cultivating the suspected pigweed in the greenhouse and then hitting it with multiple glyphosate rates. The pigweed survived even the high rates.

Keeling says the take-home message is that Texas growers can no longer rely too much on Roundup herbicide, or glyphosate, alone to control pigweed.

Slow down resistance

Now that weed resistance to glyphosate has been detected in Texas, growers need to study their options for change to deal with it in coming seasons — hopefully, slowing the spread of resistance.

“This may mean going back to residuals and even plowing some,” Keeling says. A preplant incorporated, or PPI, herbicide such as Treflan or Prowl would be a real good starting point, Keeling says.

Preemergent herbicide and other post-emergent products also could help stretch glyphosate effectiveness, possibly for years.

Since the glyphosate-resistant pigweed was found in Texas cotton fields that had been planted into terminated wheat, Keeling suspects the practice and its heavy dependence upon Roundup for weed control may make such fields more vulnerable to resistance problems.

Problem hits home

Keeling says the arrival of weed resistance to glyphosate in Texas is a sobering scene. “I guess it really hit home to me ... when I saw the crews hoeing weeds,” he recalls.

Keeling would like to hear from you if you suspect resistance. If you are a producer who sprayed fields with glyphosate several times this year, but had surviving weeds, you can call him or Peter Dotray, also with Texas AgriLife Research, at 806-746-6101.

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WEEDS WIN: Pigweeds took over this 2011-crop cotton field in Texas. Scientists have growing concerns of weed resistance to herbicide in Texas.

This article published in the November, 2011 edition of THE FARMER-STOCKMAN.

All rights reserved. Copyright Farm Progress Cos. 2011.