AgWatch


Late Palmer Amaranth Management









DR. LARRY STECKEL

JACKSON, TENN.
   This has proven to be a very challenging year to manage Palmer amaranth.I do not think I have spoken to a crop consultant this year who does not know of a couple soybean fields that were disked up and replanted due to Palmer amaranth. Even tillage at times did not work as one tillage pass typically will not provide good control of foot tall Palmer.  It really takes a couple passes at a reasonable depth separated by about 3 days to do an adequate job.
   The last several springs PRE applied herbicides have worked very well and even when they did not fields could often be sprayed timely as they were not muddy. This was not the case in 2015. The prolonged wet period this spring and now summer has greatly accelerated how quickly PRE applied herbicides played out. This coupled with muddy fields made it a struggle to spray Palmer amaranth prior to it getting 3” tall. As a result we have too many grown up messy Palmer amaranth soybean fields this summer.
   I typically get less calls on pigweed management in early July but that is not the case this time as folks are trying to control large Palmer amaranth in soybean. Unfortunately, I am not much help as there are no good options for >6” Palmer in soybean. A sequential application of FlexStar followed by Ultra Blazer or Cobra most folks are going with in these situations. This strategy can provide 70 percent Palmer amaranth control if applied on the weeds in that 3 to 6” range but is sadly lacking on Palmer amaranth that is >12” tall.  At this point it is best to try to keep any more Palmer amaranth from emerging and getting a chopping crew lined up for the large ones.
   As we move into the middle of July we really need to be aware of what crop will likely be planted in that field next year. Any herbicide that contains fomesafen (FlexStar, Reflex, Prefix, Marvel, Intimidator, etc) has a 10 month plant back to corn or grain sorghum. This will become even more important for the soybeans that are being planted today and for those fields that may be planted closer to August as the Mississippi River goes down. ∆
   DR. LARRY STECKEL: Extension Weed Specialist, University of Tennessee
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