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dfp-ron-smith-larry-steckel.jpg Ron Smith
University of Tennessee Extension weed specialist Larry Steckel discusses weed management during a weed tour stop last summer.

Dicamba resistant pigweed identified but 'no time to panic'

"No time to panic," says University of Tennessee weed specialist Larry Steckel, responding to research showing Palmer amaranth resistance to dicamba herbicide.

"No time to panic," says University of Tennessee Institute of Agriculture Extension weed specialist Larry Steckel, responding to research showing Palmer amaranth resistance to dicamba herbicide.

"It is time to reassess weed management, however. Herbicide stewardship is now more important than ever," he blogged July 27 (https://news.utcrops.com/2020/07/dicamba-resistant-palmer-amaranth-in-tennessee-stewardship-even-more-important/).

Steckel says greenhouse experiments over the winter and spring and in ongoing field research this growing season show dicamba-resistant (DR) Palmer amaranth occurring in several Tennessee counties. "DR populations are established in Crockett, Gibson, Madison, Shelby, and Warren counties and likely several others," Steckel says.

Resistance, so far, is not high, about 2.5 times he says. "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."

Steckel compares current dicamba resistance to early documented glyphosate-resistant Palmer amaranth found in Tennessee back in 2006. Back then, most farmers were still seeing relatively good Palmer amaranth control with glyphosate; others were noticing escapes in their fields.

"When the Xtend crops first came on the scene, it was not uncommon to see a stray Palmer amaranth escape dicamba here and there," Steckel explains. "These escapes would grow very little if at all for two or three weeks. Then most would be covered up by the crop, never to be seen again."

He says now DR Palmer amaranth in some fields start growing again in about 10 days "and in unprecedented numbers."

He says DR weed greenhouse screening conducted at the University of Tennessee and the University of Arkansas revealed that some Palmer amaranth escapes from several fields in 2019 in Tennessee were more than two times more tolerant to dicamba than Palmer amaranth grown from seed collected over a decade ago in Arkansas and Tennessee. "Subsequent greenhouse tests conducted at Texas Tech University showed that a population collected from Shelby County, Tenn., was more than 2.4 times more tolerant to dicamba than Palmer amaranth sourced from Lubbock, Texas.

"Replicated field trials conducted on several of these suspect Palmer amaranth populations in Tennessee mirrored the greenhouse screens, showing the labeled one-time application of dicamba (0.5 lb/A) provides 40% to 60% Palmer amaranth control. Follow-up applications of dicamba only marginally improved control in a couple of these trials."

Herbicide stewardship

Steckel says several growers reported that they needed to spray the same Palmer amaranth three or four times to get control. "Those reports unfortunately indicate that this greenhouse and field research reflects what some Tennessee consultants, retailers and farmers are seeing in the field."

Identifying resistance, he says, emphasizes the importance of herbicide stewardship.

"That is why we have stressed using hooded applications with herbicides such as paraquat, glufosinate, Valor, diuron, pyroxasulfone and MSMA in cotton. As we look forward to 2021, a pre-applied residual that is effective on Palmer is now a necessity."

He recommends timely applications of Liberty shortly after a dicamba application to remove escapes. "Preliminary research would suggest that DR Palmer amaranth will also be more tolerant to 2,4-D. This makes Liberty the most important herbicide in the weed management system in both Xtend and Enlist crops."

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