There has been a number of reports of poor Palmer amaranth control with post applications of PPO herbicides (Flexstar, Cobra, etc).
This is not new to this year. However what is being reported more as a problem this year than last is even 1- to 2-inch Palmer is often recovering from these post PPO applications.
I walked a number of these fields this week and indeed Palmer is recovering from applications of Flexstar, Flexstar GT and Cobra. In some cases the Palmer amaranth was just too large (4- to 12-inches tall). However, in other cases Palmer that was about 1.5-inches was putting out new growth from low lateral buds after the terminal was taken out by the PPO herbicide. This is very disappointing but not completely surprising.
The reason, I was not surprised to get these calls is that we have seen similar results with various PPO alone and tank-mixture applications on small Palmer amaranth in research here at Jackson.
Even some very hot tank-mixtures of 5.3 pints FlexStar GT with 1 percent MSO or 10 ounces Cobra plus 4 ounces Resource plus 32 ounces of Roundup PowerMax has not provided good control of 1- to 2-inch Palmer amaranth.
Some folks want to describe this with the “R” word, but I believe the extended hot/dry weather has made Palmer amaranth even more difficult to control with PPO herbicides.
So what do we do now? Hopefully, Tennessee will get a good state-wide rain to help make the soybeans more competitive and perhaps make future Palmer flushes more susceptible to PPO herbicides.
Second be cautious of follow up fomesafen (Flexstar, Flexstar GT, Prefix, Rythim, Reflex, etc) applications to try to control escaped pigweed. We saw some carryover into corn this spring from these types of sequential applications.
In most cases, if the grower is going to go back and retreat choose a PPO herbicide that will not provide soil residual that could be a problem in next year’s corn.
Cobra has performed similarly to Flexstar in our research on Palmer amaranth and it will not be a problem in next year’s corn. If fields are too covered up with escaped pigweed then destroying the soybeans and replanting would probably be the best option at this date.
When looking to treat newly emerged Palmer from here on out we have to strive to do everything correct with that first PPO application. There are no second chances with Palmer and post PPO herbicides. That includes using at least 15 gpa.
It also includes using nozzles that provide good coverage.
Finally it includes using the proper surfactants at the proper rates. After looking at my data over the past two years I believe we need to use more MSO (1 percent) with Flexstar to have the best chance to control Palmer.