Wednesday, June 23, 2021

Managing dicamba- or Enlist One-resistant Palmer amaranth

• By Larry Steckel, Clay Perkins and Delaney Foster •

Judging from research tests and walking a few farmers’ fields, many of the preemergence-applied herbicides in soybeans and cotton played out about a week or so ago. Timing is everything on trying to do the best job on Palmer amaranth that has broken through the pres.

With respect to Palmer amaranth that has low-level dicamba or 2,4-D resistance (2x to 3x), timing is even more important. Moreover, herbicide selection is also critical in controlling resistant Palmer.

In our research where dicamba or 2,4-D are applied to resistant Palmer amaranth less than 4 inches tall, the weeds grow laterally fairly slowly (0.5”/ day) for 10 to 14 days after application (DAA).

herbicide-resistant palmer amaranth
(Picture 1) 20 DAA: Palmer amaranth escapes from 22 ozs XtendiMax + 16 ozs Clethodim + 32 ozs glyphosate applied on 1-inch to 4-inch weeds — photos courtesy University of Tennessee

By 14 DAA, they appear to shake off the dicamba and start growing like Palmer can (>2”/day) (Picture 1).

However, if the resistant Palmer is just a little larger at time of application (4 inches to 8 inches), they still grow sideways for a few days but the rate of growth looks to be faster (>1”/day) and they recover quicker from the dicamba or 2,4-D (Picture 2).

With all the wet weather this week, Palmer will likely be larger by the time a sprayer can get across the fields. Therefore, if Palmer is not controlled with an application of dicamba or Enlist One, expect it to recover within seven days or so.

larger herbicide-resistant palmer amaranth
(Picture 2) 14 DAA: Palmer amaranth escapes from 22 ozs XtendiMax + 16 ozs Clethodim + 32 ozs glyphosate applied on 4-inch to 8-inch weeds

Follow-up applications will need to occur no later than seven to 10 days after the initial application for the best chance of success. If possible, that follow-up herbicide needs to be glufosinate under good temperature and time of day conditions.

If the crop is not glufosinate tolerant or if you cannot get glufosinate due to supply issues, then a re-treat with dicamba or Enlist One on Xtend and Enlist crops, respectively, is an option.

In fields where Palmer is not tolerant to dicamba or 2,4-D, the follow-up applications should clean up the field. However, based on last years’ field observations, in fields where resistance is present, there will be survivors.

Dr. Larry Steckel is Extension weed specialist; Dr. Clay Perkins is a post-doctoral research associate and and Delaney Foster is a graduate research assistant. All are with the University of Tennessee.

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