Unfortunately, due to the weather some soybean fields ran out of residual some time back and post applications have not been able to be applied timely. The question is how we proceed from here? In a few cases where the field is pretty much covered up with 2 to 3’ tall Palmer and the soybeans are Roundup Ready or conventional the answer has been a disk and replant even at this late date. 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 NC State, found that just 1 Palmer in 3 meters of row reduced soybean yield by 21%. Those results are similar to a study conducted at the University of Arkansas by Dr. Oliver, in 1994, that found 1 Palmer in 3 meters of soybean row reduced yield 17%.
The salvage treatments I describe below will provide at best 60 to 70% control. So one must 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% 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% MSO. That sequential program got us 67% 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% 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.