Monday, February 26, 2024

Go Back To The Future To Manage Glyphosate-Resistant Grasses

• By Larry Steckel •

larry steckel, univeristy of tennessee
Dr. Larry Steckel, University of Tennessee Extension weed specialist, compares two johnsongrass populations. On the left, a population that was controlled with the herbicide glyphosate; on the right, a glyphosate-resistant population found in a West Tennessee field — photo by G. Rowsey, courtesy UT Institute of Agriculture.

Many questions continue to be asked on how best to manage grass in cotton. The sketchy performance of glyphosate on goosegrass, junglerice and Johnsongrass in 2018 is the main driver of these questions.

One change in weed management some have gone to in cotton due to the poor glyphosate performance is to use Prowl preemergence. Using Prowl in this fashion in cotton hails back to a time before Roundup Ready cotton was on the market, where the most used PRE in Tennessee cotton was a tankmixture of Prowl and Cotoran. I am in favor of going back to Prowl preemergence again in cotton as it is a good tool to help us manage the rise of glyphosate-resistant grasses.

However, this raises the question of what to do on all the late-planted cotton that is emerging very quickly and is at times in the “cracking stage” as the sprayer gets to the field to spray Prowl and Cotoran. Will it hurt the cotton that is in the cracking stage or emerged?

The answer is yes. In these situations, I would forgo the Prowl and Cotoran and replace them with either Dual Magnum or Outlook. Both Dual Magnum and Outlook are very safe on cotton that is just emerging and can provide good residual control of GR grasses as well as Palmer amaranth.

Another often asked question is how best to use clethodim in cotton to manage grass. One of the questions associated with this herbicide is adjuvant choices. Surfactants can make all the difference with clethodim performance.

The generic 2 lb clethodim labels are based on the old Select 2EC herbicide, which called for the addition of 1% crop oil. However, using 1% COC with a tankmix of Dual Magnum or Outlook or Warrant with either Engenia or XtendiMax may burn every leaf off the cotton.

Surfactants have come a long way since that old 2 lb clethodim label was written, and many of the more modern adjuvant blends provide as good or better performance of clethodim without all the crop burn and at lower rates than the old standard 1%.

Be sure to check with your retailer or crop consultant to get the approved adjuvant partners and appropriate rates of those surfactants for tankmixes with Engenia or XtendiMax.

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

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