Palmer amaranth problems couldn’t be found in Middle Tennessee as recently as 2009.. But now the region has problems. Palmer problems were building in Middle Tennessee by 2011 and growing.

“We are getting questions from folks in Middle Tennessee on how to control severe infestations of Palmer amaranth 6 to 8 inches tall in Roundup Ready soybeans,” says Larry Steckel, weed specialist with the University of Tennessee Extension.

Growers in Middle Tennessee can learn a bit from the troubles of those in West Tennessee, he said, where growers learned the hard way that “only way to control large populations of Palmer amaranth that size in Roundup Ready soybeans is to destroy the soybeans and pigweed in the field with tillage or Gramoxone and replant,” Steckel said.

A herbicide will have to be used at replant, one that provides residual control, he said. Like Prostko, Steckel has a list of good candidates and prefers those with two modes of action like Authority MTZ, Fierce, Boundary, Prefix, or tank-mixtures of metribuzin with Valor or Zidua.  

Roughly two weeks after planting, get a herbicide application in the field. “For those folks new to this weed one might ask why the post application has to be so quick. … You do not want to let it emerge particularly in Roundup Ready soybeans,” Steckel said.

Palmer amaranth in warm wet conditions can grow eight inches in two weeks. “If you take into account that herbicides like Prefix, Cobra and Blazer will not consistently control Palmer amaranth over 2 inches tall (in Tennessee), you see why the best management is to never let it emerge.”

For a LibertyLink soybean, the post emergence herbicide would be Liberty plus a residual like Dual Magnum or Warrant.  For Roundup Ready soybeans, Steckel recommends Prefix tank-mixed with glyphosate for early post-emergence application. 

If Prefix was used preplant it cannot be used post emergence. In those cases Cobra or Ultra Blazer would be good choices. The plant-back time the following year to corn for herbicides that contain fomesafen like Prefix is 10 months.  If that is a concern then Cobra or Ultra Blazer might work better.

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