Palmer amaranth control with all the uncertainty around dicamba

• By Larry Steckel •

palmer surviving 22 DAA
Palmer amaranth surviving 22 DAA: 22 ozs/A of XtendiMax + 32 ozs/A of Roundup PM — photos by Dr. Larry Steckel, UT

As most know by now, on June 3 the U.S. Court of Appeals for the Ninth Circuit vacated the labels for XtendiMax, FeXapan and Engenia. The question now is how do we proceed from here controlling weeds in Xtend cotton and soybean?

The folks at the Tennessee Department of Agriculture have stated that until they get direction from the Environmental Protection Agency to the contrary, growers can still apply those herbicides according to the label directions. How long this will be is uncertain. It could be just a day, several weeks or depending upon legal maneuvers, well into July.

What we do know is that both Bayer and BASF have stopped shipping any more XtendiMax or Engenia into Tennessee. Some farmers and retailers are better stocked than others with those herbicides on hand.

Our state will have roughly 2 million acres of soybean and 300,000 acres of cotton. My best estimate is that roughly 90% of those 2.3 million acres are in Xtend crops. What is most likely is even though TDA says they can still be sprayed, there will not be enough of those two herbicides to spray all the acres of cotton and soybean once let alone twice. Tavium, which is a premix of XtendiMax and Dual Magnum, was unaffected and still will be labeled. But I am told supply of that herbicide is tight.

Uncertainty just part of the issue

The uncertainty on dicamba from regulatory and supply question is only part of the issue. The word “disappointing” best describes our research on dicamba control of Palmer amaranth evaluated in the last week.

Palmer escapes DAA
Palmer amaranth escapes 23 DAA :12.8 ozs Engenia + 32 ozs Roundup PM

In this research conducted at Jackson and Milan research stations and in farmer fields in West Tennessee, the 22 oz rate of XtendiMax or the 12.8 oz rate of Engenia on 4-inch Palmer is at best providing 70% control (Pictures attached).

It is clear in order to get good Palmer amaranth control post, two herbicide applications will be needed. Examples in order of most consistent Palmer control to least: dicamba followed by Liberty = Liberty followed by Liberty > dicamba followed by dicamba > dicamba followed by Ultra Blazer or Flexstar.

Most of the cotton has had the first application of dicamba applied by this time. From here on out any post application that is targeting Palmer amaranth should be Liberty. We are clearly in the hot and humid weather conditions where Liberty works best. Those dicamba escapes in cotton should easily be cleaned up with Liberty now.

In Xtend soybeans where the Palmer and the crop have emerged, there are no options for Palmer other than dicamba. As long as TDA has given the go ahead, then an early post application of XtendiMax or Engenia followed by Flexstar or Ultra Blazer on Palmer amaranth escapes would be an option.

In our research from a few years back, we found that Flexstar and Ultra Blazer would often control Palmer amaranth that was recovering from a dicamba application. As PPO-resistant Palmer amaranth has become more prevalent, this tactic has not been as consistent.

However, it still should remove a good many particularly if applied within 10 days after the dicamba application. Remember Flexstar has a 10-month plant back to corn, so as we get closer to July and after, Ultra Blazer would be the best choice.

palmer recovering
Palmer amaranth recovering from lateral buds 22 DAA: 22 ozs of XtendiMax + 32 ozs of Roundup PM

On soybeans that have not emerged, plan to up the game on the herbicides that provide residual Palmer control. An example is the most used rates pre applied of Anthem Max is 3.25 ozs and for Zidua is 3.2 ozs/A.

Try bumping those rates up to 4 ozs/A. Another option would be to use 2.4 pts/A of Boundary if the soil type is a silty clay or silty clay loam. Then consider using higher rates of herbicides to overlap early post. A couple of examples here would be Dual Magnum at a rate of 1.33 pts/A our Outlook at 16 ozs/A.

Of course, if you are short Engenia or XtendiMax, you can look at swapping to a soybean with a different herbicide resistant trait platform. However, I have visited with several seed retailers. Most have stated that getting any supply of soybean seed in an Enlist or LLGT27 trait is difficult this late in the season.

Dr. Larry Steckel is a University of Tennessee Extension weed specialist. He may be reached at lsteckel@utk.edu.

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