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Pigweed resistance spreading in Texas

When Palmer amaranth or pigweed resistance to glyphosate herbicide was confirmed on the Texas High Plains in Terry County near the New Mexico border last summer and fall, it was a wake-up call.

Now that herbicide resistance by pigweed (aka “careless weed”) also has been confirmed this year in more counties, it’s a red flag for cotton growers to take action.

Kerry Siders, Texas AgriLife Extension Integrated Pest Management agent for Hockley and Cochran counties, says resistant pigweed has spread to Hockley County this year.

Key Points

Pigweed resistance to glyphosate herbicide is spreading in Texas.

Don’t encourage more resistance by practices conducive to weed resistance.

Make your strategy now for ways you are going to control weed resistance.


Wayne Keeling, noted weed expert for Texas AgriLife Research, Lubbock, collected pigweed seed in the southern part of Hockley County and subsequently tested them in the greenhouse. Keeling confirmed early this year that the greenhouse tests found the pigweed samples to be resistant to standard rates of glyphosate herbicide.Nobody was really surprised.

“If you will recall, Terry County had similar studies conducted last summer and early fall, and Palmer amaranth populations there were confirmed to be resistant as well,” Siders laments. “Along with Terry, and now Hockley counties, Hale County has also been confirmed with Palmer amaranth resistance.”

There’s likely others. “Though not confirmed, I suspect that many more locations could have Palmer amaranth resistance to glyphosate,” Siders says.

Opening the door

Siders says these encourage resistance:

herbicides that act on a single site of action

herbicides that are applied multiple times in the growing season, keeping selection pressure high

herbicides that are used for several consecutive growing seasons

repeated use of herbicides with the same site of action to the same or different crops

use of herbicides as the only weed control measure

Avoiding or managing resistance

The IPM agent says here’s ways to avoid or manage herbicide-resistant weeds:

Employ integrated weed management strategies. Use herbicides only when necessary, and combine their use with mechanical, cultural or biological methods.

Rotate herbicide use, utilizing herbicides with different modes of action.

If possible, rotate crops where herbicide rotation also is feasible.

When planting herbicide-resistant crops, limit herbicide applications and employ other weed control methods.

Scout fields regularly to determine if resistant weed populations may be present and to assess the need for herbicides.

Clean tillage and harvesting equipment to help eliminate the spread of resistant species.

Siders says it is important Texas takes action now. “Let’s not take the difficult path which the Southeast Cotton Belt has had to take. Recognize now that we can slow down and stop this resistance issue if all producers take necessary measures.”

Call Siders at 806-894-3150 or reach him by email at k-siders@tamu.edu.

This article published in the April, 2012 edition of THE FARMER-STOCKMAN.

All rights reserved. Copyright Farm Progress Cos. 2012.