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Dicamba-resistant weeds established in western Tennessee

Dicamba has been highly controversial in farm country because it can drift and damage non-target crops without genetically engineered resistance to the herbicide.

The fast-growing weed Palmer amaranth has developed a tolerance for dicamba herbicide in at least five counties in western Tennessee and likely several others, said University of Tennessee weed specialist Larry Steckel on Monday. The report was a setback for dicamba, which was introduced a few years ago as a new tool for control of invasive weeds that showed resistance to glyphosate and other weedkillers.

“So, is it time to panic? No,” wrote Steckel in a university blog. “However, it is time to reassess weed management. Herbicide stewardship is now more important than ever.” He listed six herbicides that could be used on cotton as an alternative to dicamba and said growers might follow dicamba with a glufosinate “to remove escapes,” or weeds that survive dicamba.

At present, “the level of dicamba resistance is relatively low” and succumbs when sprayed with doses two or three times higher than recommended levels of dicamba, said Steckel. “The level of infestation in any given field ranges from a small pocket where a mother plant went to seed in 2019 to an area covering several acres in a field.” Dicamba-resistant pigweed “populations are established in Crockett, Gibson, Madison, Shelby, and Warren counties and likely several others.”

Preliminary research suggests that dicamba-resistant Palmer amaranth “will also be more tolerant” to 2,4-D, a sales competitor to dicamba, wrote Steckel.

Dicamba has been highly controversial in farm country because it can drift and damage non-target crops without genetically engineered resistance to the herbicide. In February, a jury awarded $265 million to a Missouri peach farmer who sued agrochemical companies for the alleged damage his farm suffered from dicamba.

Produced with FERN, non-profit reporting on food, agriculture, and environmental health.
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