Glyphosate-Resistant Palmer Amaranth Becoming A Bigger Issue In Middle Tenn.

JACKSON, TENN.
   We are getting questions from folks in Middle Tennessee now on how to control severe infestations of Palmer amaranth 6 to 8” tall in Roundup Ready (RR) soybeans. This is a big milestone because Palmer amaranth could not be found as recently as 2008 and 2009 in several surveys we ran in Middle Tenn. There was plenty of pigweed found in this part of the state then but it was all smooth and spiny. The first infestations were found in 2011 and by 2012 Palmer could be found building in many areas of Middle Tenn. In a remarkable short period of just 4 years Palmer amaranth has gone from being impossible to locate to running into overwhelming populations in some soybean fields in Middle Tenn.
   Unfortunately, as many growers have learned in west Tennessee, the only way to control large populations of Palmer amaranth that size in RR soybeans is to destroy the soybeans and pigweed in the field with tillage or Gramoxone and replant. This replant will need to have a herbicide applied at planting that can provide good residual control of Palmer. There are many candidates here but the ones I prefer are herbicides that have two effective modes of action on Palmer amaranth. Herbicides like Authority MTZ, Fierce, Boundary, Prefix, or tankmixtures of say metribuzin with Valor or Zidua would all be good candidates.
   Then early post emergence, roughly 14 days after planting, apply a herbicide application. For those folks new to this weed one might ask why the post application has to be so quick. The reason is that you do not want to let it emerge particularly in RR soybeans. Palmer amaranth with warm wet environment we have this June can emerge and grow 8” in just 13 days. If you take into account that herbicides like Prefix, Cobra and Blazer will not consistently control Palmer amaranth over 2” tall you see why the best management is to never let it emerge. That is why overlaying residuals herbicides is one of the most consistent ways to manage Palmer.
   The post emergence application would be Liberty in Liberty Link soybeans plus a residual like Dual Magnum, or Warrant. In RR soybeans I like Prefix tankmixed with glyphosate for that early post emergence application. Be mindful if Prefix was used pre plant it cannot be used post emergence. In those cases Cobra or Ultra Blazer would be good choices. Also, please take into mind the calendar date. The plant back next year to corn for herbicides that contain fomesafen like Prefix is 10 months. Again if that is a concern then Cobra or Ultra Blazer maybe better fits. Δ
   DR. LARRY STECKEL: Extension Weed Specialist, University of Tennessee