Palmer Amaranth (Amaranthus palmeri) Showing Reduced Sensitivity to Glyphosate

Researchers with the Tennessee Agricultural Experiment Station and UT Extension have confirmed two separate populations of Palmer amaranth (pigweed) that survived properly-applied applications of Roundup herbicide. These two populations are located on cotton fields in Crockett and Lauderdale Counties. This past summer we conducted numerous field and greenhouse trials confirming the reduced sensitivity of these Palmer pigweed populations to glyphosate. Last year a very small number of pigweed plants survived a glyphosate application in these fields. This year some Palmer pigweeds at both locations survived a full 22 oz/A of Roundup WeatherMax, and at one location some plants survived a 2X (44 fluid ounces) rate.

County Extension Agents Richard Buntin of Crockett County and Jerry Parker of Lauderdale County alerted us to these fields in 2004. This past summer with their help we conducted field trials on Palmer pigweed in four different fields in two counties. One field in each county was chosen due to suspected glyphosate-resistance. The other field in the county was chosen to help ascertain the level of glyphosate tolerance in a random field. As it turned out the random chosen field in each county contained a "normal" population of Palmer pigweed that was readily controlled with glyphosate. The normal Palmer pigweed fields were separated from the glyphosate-tolerant fields in Crockett County by 3 miles and in Lauderdale County by 15 miles. All the Palmer pigweed we evaluated had been in fields that were continuous cotton. The Palmer pigweed in these fields was about 3 to 5 inches tall at the time of application. The "normal" Palmer pigweed populations showed 100% control with all rates of Roundup WeatherMax applied including the low 1/2X (11 fluid ounces/A) rate. The two tolerant Palmer locations showed 10 and 23% Palmer pigweed survival at the 1/2X rate, 2 and 5% survival at the 1X (22 oz/A) rate, 0.5% and 4% survival at the 2X (44 oz/A) rate and complete control at the 4X (88 oz/A) rate (Figure 1.).

Though overall Palmer pigweed control was slightly better, the greenhouse analysis reflected the results in the field. Palmer pigweed grown from seed in these fields showed two-fold (GR 50 = 2X) more tolerance to glyphosate than Palmer pigweed grown from seed collected from a known susceptible Palmer population from Jackson. In some ways the Palmer pigweed appears to be similar to glyphosate-resistant horseweed. All the treated Palmer pigweed plants look the same for two or three days after application; they all wilt and turn yellow. However, at about 4 days after spraying the tolerant plants stop wilting and start new growth from lateral buds.

Dr. Tom Mueller, weed scientist at UT Knoxville, conducted the laboratory analysis. The preliminary data indicates that the mechanism of action, or how the plant tolerates the glyphosate, appears to be the same in the Palmer pigweed as in the glyphosate-resistant horseweed. It appears to be metabolism-based.

Tom Mueller, 865-974-8805, tmueller@utk.edu

Larry Steckel, 731-425-4705, lsteckel@utk.edu