Palmer Amaranth Management In Soybean Over The Next Two Weeks


   The large rain event over the weekend really was needed and from a weed control perspective greatly helped activate PRE applied herbicides.  Unfortunately, the long dry spell before that allowed a lot of weed escapes in some fields and managing them has become a concern. 
   The main problem in managing Palmer in the near future is that the 10 day forecast calls for a lot of rain.  It may be 2 weeks before we can get to some fields to spray. Palmer amaranth will likely be very large by then in fields where PREs were not activated. Moreover, all this rain in fields where PRE applied herbicides were activated will accelerate the degradation. The rate they will play out varies by herbicide and soil type. Herbicides like Valor, Sharpen, Reflex and metribuzin will dissipate very rapidly (<14 days) under saturated and warm soil conditions.  Other herbicides like Dual Magnum or Outlook will last a little longer. Zidua will likely last the longest.
   The question is how to proceed on weed control when we can get back into the field. In cases where the field is pretty much covered up with very large (>6”) Palmer and the soybeans are Roundup Ready or conventional the answer may be disk and replant.
   In other cases where the Palmer is still in that 2” to 4” range and not just covered up then a sequential treatment has a chance to salvage Roundup Ready or conventional soybeans. I define salvage as suppressing the Palmer enough that it can be harvested. 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 percent. 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 percent.
   The salvage treatments I describe below will provide at best 60 to 70 percent control. So one must weigh the expected yield loss from the Palmer that will escape these treatments to the yield penalty associated with a later soybean planting date if you elect to disk up and replant. Last year, we examined a number of sequential PPO treatments where the first application was the maximum rate of FlexStar GT plus 1 percent MSO followed a week later with a sundry of follow up applications. The FlexStar GT applications applied to Palmer in that 4” 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 through. 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. We also saw no improvement in Palmer amaranth control when tankmixing Cobra in with a Flexstar product. ∆
   DR. LARRY STECKEL: Extension Weed Specialist, University of Tennessee
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