Monsanto and the University of Georgia are investigating a probable case of glyphosate-resistant Palmer amaranth (Palmer pigweed) at specific sites in central Georgia. Preliminary findings of field and greenhouse trials show a lack of control at labeled rates. Additional work regarding heritability—whether the lack of control is passed on to the next generation—is needed to confirm resistance.

“There is a difference in control of this Palmer amaranth biotype compared to what is normally expected with glyphosate applications at the recommended use rates,” says Dr. Stanley Culpepper, University of Georgia weed scientist. “This does not appear to be widespread, but we are evaluating the response in a few fields surrounding the specific sites to better understand this situation.”

Resistance should not be confused with herbicide performance failures. Among the factors that can reduce weed control with glyphosate are environmental effects (such as lack of rainfall), sprayer calibration, application procedures, spray coverage, herbicide rate, and weed size. Weed size is particularly critical.

“We suspect that as pigweed initiates reproductive growth, response to glyphosate may decline considerably, making control more difficult this late in the growing season and calendar year,” says Dr. Culpepper.

“Monsanto takes product stewardship and customer service very seriously,” says Dr. Doug Rushing, Director of Technology Development for Monsanto. “We will continue to support and collaborate with the University of Georgia on field and greenhouse studies with this biotype. Heritability studies are required to confirm resistance, and these studies are underway.”

Monsanto continually evaluates its recommendations for effective weed control and will provide alternate weed control recommendations if this biotype is confirmed as resistant. When glyphosate resistant weed biotypes have been identified, they have been effectively managed with other herbicides and/or cultural practices, such as tillage.

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