New ‘super weed’ crisis emerging? Dicamba herbicide failing to stop growth of Palmer amaranth pigweed in greenhouses experiments

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Palmer amaranth infestation. Credit: University of Illinois

University of Tennessee weed scientist Larry Steckel has spent the past two months coaxing Palmer amaranth weeds to grow from seed collected in 2019 — so he could try to kill them.

But after a labeled rate of the dicamba herbicide XtendiMax on two-inch tall weeds, in the well-controlled environment of a greenhouse, a lot of that pigweed did not die.

“I called the farmers and retailers who found some of these and I told them — dicamba isn’t going to control Palmer amaranth in these fields anymore,” he said. “They were not surprised at all.”

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His colleague, University of Arkansas weed scientist Jason Norsworthy has found the same thing in his greenhouses with samples of Palmer amaranth from Tennessee, which saw widespread reports of dicamba performance failures in 2019.

Related article:  Federal judge dismisses 'key' claims against Monsanto in dicamba drift lawsuit

Norsworthy said some of the Tennessee pigweed populations start to recover four to five days after an application of a half-pound-rate of dicamba. “They resume active growth, and 14 to 21 days after application, there appears to be little symptomology present from auxin herbicide,” he explained.

“Just as we expected, further research is confirming that dicamba is in the initial stages of failure against Palmer amaranth,” Norsworthy concluded.

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