Residual herbicides ran out in some Tennessee soybean fields a while back and growers haven’t been able to get post applications out in a timely fashion. What can be done as Palmer amaranth plants get too high? Can growers ‘rescue’ a field?

In a July 1 blog, University of Tennessee Extension weed specialist Larry Steckel says in fields where Palmer has reached higher than 3 inches and the soybeans are Roundup Ready or conventional, well, the disk and replant has been the answer, even this late in the season:

“This has mostly happened in a few soybean fields in Middle TN where Palmer is still relatively new.

In other cases where the Palmer is still in that 4 to 8” range and not just covered up then a sequential treatment has a chance to salvage RR or conventional soybeans.  I define salvage as suppressing the Palmer enough that it can be combined. Remember that yield loss has already occurred in these fields. A study published in 2012, conducted at North Carolina State found that just one Palmer in three meters of row reduced soybean yield by 21 percent.

The salvage treatments I describe below will provide at best 60 to 70 percent control.  Weigh the expected yield loss from the Palmer that will escape these treatments to the yield penalty associated with a July soybean planting date if you elected to disk up and replant.

We examined a number of sequential PPO treatments where the first applications was the maximum rate of FlexStar GT plus 1% MSO followed a week later with a sundry of follow up applications.  The FlexStar GT applications applied to Palmer in that 4 to 8” range provided about 30 percent control if no sequential herbicide was applied. The best sequential program was FlexStar GT followed a week later by Ultra Blazer applied at 1.5 pts plus 1 percent MSO. That sequential program got us 67 percent Palmer control.  This of course is not great but in a soybean field that can be enough to get a combine across.

We tried spiking in 4 ozs of 2,4-DB in with that sequential application of Ultra Blazer and found it did not provide any more Palmer control and really beat up the soybeans.  Other sequential applications of Cobra or Cobra plus 2,4-DB provided 57 percent control. Again the 2,4-DB caused excessive soybean injury and provided no significant pigweed help.

Please remember it is now July and any fomesafen containing herbicide could carryover into corn or grain sorghum next year. If that is the case then Ultra Blazer followed by Cobra would be a comparable alternative.”