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More than one herbicide needed for Palmer amaranth

Larry Steckel discusses the effectiveness of overlaying residual herbicides.

Forrest Laws

August 19, 2020

This year’s online Milan No-Till Field Day gave speakers an opportunity to showcase different herbicides and combinations of herbicide treatments at growth stages that normally would not be available at a single tour stop.

One of those discussed by Larry Steckel, professor of weed science at the University of Tennessee, involved a relatively new technique in which farmers overlay multiple residual herbicides in dicamba- and 2,4-D-tolerant cotton and soybeans.

“One of the treatments I really want to highlight here is actually a very pleasant treatment that is working quite well,” said Steckel, who has spent the last 17 years at the West Tennessee AgResearch and Education Center in Jackson, Tenn., primarily working on controlling problem weeds like Palmer amaranth in herbicide-tolerant crops.

“And that is overlaying residuals. You've heard me time and time again talking about overlaying residual herbicides to get good — in particular — Palmer amaranth control,” said Steckel, who earlier discussed approaches with single herbicide applications that haven’t performed as well.

When the new dicamba-tolerant crops were first introduced, farmers often were able to control Palmer amaranth or pigweed with one or two postemergence-only applications of the new formulations of dicamba. University of Tennessee weed scientists have now documented cases in which dicamba formulations did not provide control of pigweed in 2019 and 2020.

“What we have here is Authority MTZ applied pre-emergence, which did a very good job,” he said. “Then we came over-the-top with Anthem Max, Roundup and XtendiMax or Enlist E3 over the top. So we got a residual (Anthem Max) on top of a residual (Authority MTZ), and we had very few pigweed to have to deal with when they normally would have come up.

“In this case, the Anthem Max with the dicamba did a very good job taking out those small pigweeds that happened to get through the Authority MTZ.

Steckel said 2020 was “a very good year for pre-emergence herbicides in West Tennessee” because of the frequent rains that helped activate the herbicides at planting and again when growers applied residual herbicides such as Anthem Max over-the-top.

In the video from the Milan Field Day, Steckel moved to another treatment in which researchers applied a quart of Roundup and 22 ounces of the Xtendimax dicamba formulation per acre postemergence at about the V-3 to V-4 growth stage of the soybeans. “And you can see we leave a lot to be desired.

“This particular treatment a few years ago was actually more effective on pigweed than what we're seeing here today,” he said. “We're seeing more and more of them kind of struggle through. It shows using an effective residual herbicide upfront and keeping the numbers down can help any postemergence herbicide.”

Next: Tank mixes may be causing antagonism 

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Palmer AmaranthResiduals

About the Author(s)

Forrest Laws

Forrest Laws spent 10 years with The Memphis Press-Scimitar before joining Delta Farm Press in 1980. He has written extensively on farm production practices, crop marketing, farm legislation, environmental regulations and alternative energy. He resides in Memphis, Tenn. He served as a missile launch officer in the U.S. Air Force before resuming his career in journalism with The Press-Scimitar.

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