It’s a tough fight, but there is no reason for Palmer amaranth to kick a soybean grower’s butt.

Some growers continue to struggle, but soybean farmers now have the tools to win against it. So how can farmers not lose against pigweed? Two Southeast weed specialists have good game plans.

Even though the tail end of harvest for this year is in sight, it’s never too early in Georgia for farmers to get pigweed strategies lined up for next year's row crops, including soybean. Get a good plan and stick to it as best able, said Eric Prostko, weed specialist with the University of Georgia Cooperative Extension.

“As it has been said before ‘the devil is in the details,’” Prostko said. “In my opinion, there are three details that growers must use if they ever expect to get a handle on Palmer amaranth control.”

No. 1: Start clean. You will lose if you plant soybeans into a stand of Palmer amaranth. Till it, burn it down with herbicides or use rye cover crops, but start Palmer–free.  If there will be a long delay between tillage and planting, use of a residual herbicide to prevent Palmer emergence during that time period.

No. 2: At least one or even two residual herbicides will be needed for the season.  “Sadly, the good old days of just spraying glyphosate or Liberty are over (in Georgia),” he said. “There are many effective residual herbicide choices.”

Authority MTZ, Boundary, Canopy, Dual Magnum, Envive, Tricor, Warrant and Valor all work.  “I would prefer herbicides like Reflex or Prefix be applied postemergence. These are the only over-the-top herbicides that will control small emerged Palmer plants and provide residual activity,” Prostko said.

Even if a farmer uses a LibertyLink soybean system, he still needs a residual herbicide.  But a residual is only good if it can be activated timely with irrigation or rain. If not activated, the soybean farmer will lose against pigweed.

No. 3: Postemergence herbicide application must be made before the biggest pigweed hits 3 inches. If the plants are bigger, well, you know, a farmer will lose against pigweed. Scout fields once a week or more, he said, and do it from outside of the truck.

“My greatest wish is that I never again receive a phone call about how to control 12-inch Palmer amaranth in soybeans.  That is a question for which there is no answer. … Palmer amaranth control does not have to be the major management issue.  I will be the first to admit that it is not easy.”

Middle Tennessee soybean farmers get Palmer problems

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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