Monday, August 2, 2021

Palmer amaranth control now that June 30 dicamba cutoff has passed

• By Larry Steckel •

arkansas palmer pigweed
Palmer amaranth

It is July 1, so the June 30 cut-off to spray dicamba over Xtend soybean has passed. The questions of the week have been once July has arrived what alternatives are there to control Palmer amaranth in soybean?

If the soybeans are Xtend, then options for post-emergence control is very limited. A high rate of Ultra Blazer or Cobra with MSO will likely provide limited control. A survey we conducted in 2019 found that more than 80% of the Palmer amaranth population in Tennessee is resistant to the PPO-inhibiting herbicides like Ultra Blazer and Cobra.

Therefore, the best approach is to try to keep the Palmer amaranth from emerging. To do that, start with using a herbicide that provides good residual Palmer amaranth control applied behind the planting press wheel.

Moreover, use the highest rate allowed by the herbicide label for the given soil type. Then 14 to 21 days later apply another herbicide that gives good residual Palmer amaranth control.

Another alternative in Xtend-Flex soybean is Liberty. The weather forecast for the first full week of July is hot and humid, which should be ideal for Liberty performance on Palmer amaranth. Just a quick refresher is that Liberty works best under hot and humid weather and applied during the middle of the day (9 a.m. to 4 p.m.).

Retailers have informed me that generic glufosinate has long since run out and the name brand Liberty formulation is growing increasingly tight. I have yet to hear that a farmer has not been able to get Liberty, but that may change as we get further into July.

Therefore, another option if soybeans have not been planted yet would be to plant an Enlist soybean variety and use Enlist One. In our research, we have consistently seen that Enlist One provides better Palmer amaranth control if tankmixed with Liberty. So, if Liberty is not available, then plan on a second pass of Enlist One no later than 14 days after the first application to help control any Palmer amaranth escapes.

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

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