UT fielding calls about sketchy dicamba control of Palmer pigweed

• By Larry Steckel •

palmer pigweed sprayed with dicamba
Palmer amaranth regrowth 20 days after 22 oz of XtendiMax and 32 oz of Roundup PM

There have been a good number of calls on sketchy Palmer amaranth control with a 12.8 oz/A of Engenia or 22 ozs/A of XtendiMax mixed with a quart/A of Roundup PowerMax. I visited a number of these fields last week and indeed anywhere from 14 to 20 days after application some Palmer amaranth were recovering.

We have had similar reports in past springs but what was notably different this year is the sheer number of escaped Palmer amaranth in some fields. That is not to say the fields were grown up messes, but there were heavy patches in these fields where apparently a mother Palmer amaranth went to seed last year.

Clearly, a large number of her offspring were in some stage of recovering from dicamba last week. In past years there was a plant or two recovering from dicamba applications but in these fields many Palmer in these patches looked to be shaking off the dicamba.

The other notable difference from past years was the reports are not just coming from a field or two but from fields ranging from Lake County to Tipton County. So is this Palmer amaranth becoming resistant to dicamba?

pigweed recovering from dicamba
Palmer amaranth recovering 14 days after a 12.8 oz of Engenia and 32 oz of Roundup PM

The probability of that happening is certainly greater than zero, but research on the offspring of these plants will need to be conducted to really know. Other factors could impact dicamba performance like the week of unseasonable cool temperatures when many of these fields were sprayed.

So what now? Most will be going with a follow-up application of Liberty, Engenia, XtendiMax or Ultra Blazer to attempt to control the escaped Palmer amaranth.

The question I get is will another application of Engenia or XendiMax work this time? My answer is that I would think it would remove most of the escaped Palmer, but if there are some that survive this time they most likely have built up considerable tolerance.

I would prefer that another herbicide be used like Liberty in cotton or Flexstar or Ultra Blazer in soybean. Our research has shown those herbicides will provide good control if used as a follow-up application to remove Palmer amaranth that is recovering from dicamba exposure.

Dr. Larry Steckel is University of Tennessee Extension weed specialist. He can be reached at lsteckel@tennessee.edu

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