Researchers with the University of Tennessee Agricultural Experiment Station and UT Extension have confirmed that two populations of Palmer pigweed have survived properly applied applications of the herbicide glyphosate.

The weed populations exist in West Tennessee in Lauderdale and Crockett counties.

Larry Steckel, a UT Extension weed specialist, conducted the field trials. “We have been watching these fields since first receiving reports in 2004 of Palmer pigweed not killed by Roundup,” Steckel reports. “Our results last year indicated a very small number of pigweed plants survived our applications, but this year Palmer pigweeds at both locations survived a full 22 ounces of Roundup WeatherMax.” Steckel says plants at one location survived a doubled application rate (44 fluid ounces).

UT Weed Scientist Tom Mueller, a professor in the department of plant sciences, coordinated greenhouse and laboratory studies of the tolerant populations. “In some ways the Palmer pigweed appears to be similar to glyphosate-tolerant horseweed/mare’s tail,” he says. “All the treated Palmer pigweed plants look the same for two or three days after application. They all wilt and turn yellow.”

Mueller says about four days after spraying, the tolerant plants stop wilting and start new growth from lateral buds. “Our preliminary laboratory analysis indicates the mechanism of action, or how the plant tolerates the glyphosate, appears to be the same in the Palmer pigweed and in the glyphosate-tolerant horseweed,” he says.

Their findings provide confirmation of an announcement earlier this year by University of Georgia scientists and Monsanto. Both UT scientists agree that this is an important development for farmers throughout the state and nation. “Palmer pigweed that is not killed by glyphosate will cause major yield losses and harvest headaches for soybean, cotton and other row crop producers,” Mueller says.

Although glyphosate-tolerant horseweed spread rapidly over large areas of the Mid-South, Mueller and Steckel hope the weight of the Palmer pigweed seeds will slow the spread of the new herbicide-resistant pest. “It is less likely to spread on equipment and on the wind,” says Mueller.

Because of the weed’s widespread resistance to glyphosate, horseweed has become a major production problem, but good control options are in place for all crops, especially on fields that can be tilled. “The proper application of herbicides such as Clarity, 2,4-D, Gramoxone Max or Ignite allows farmers to produce pretty clean crops,” Steckel says. “However, we expect resistant Palmer pigweed will pose more problems for producers than horseweed.”

The University of Tennessee discovery reinforces the importance of managing weed resistance to herbicides. “It is essential to use more than one herbicidal mode of action on your fields,” says Mueller.

More details on this finding and recommendations on how to deal with glyphosate-resistant Palmer pigweed are available at the Web site: http://UTcrops.com