Arkansas farmers seeing possible PPO-resistant pigweed

In Delta Farm Press

by Bob Scott, University of Arkansas Extension Weed Specialist

There is a scientific process that weed scientists go through before we declare a weed resistant to a given herbicide. It involves looking at the suspect weed in comparison to a known “non-resistant” biotype of the same weed, looking at various rates and making sure that the resistance is heritable. The heritable part is why we grow plants out for two seasons and make sure the progeny are just as resistant as the parents.

My research counterparts at the University of Arkansas Campus in Fayetteville are working on these tests right now for PPO-resistant Palmer pigweed in Arkansas. They are not quite done yet, although it is becoming very apparent in the field that it’s here and possibly in other Mid-South states.

The PPO chemistry (group 14) includes: Blazer, Cobra, Flexstar, Prefix, Sharpen, Valor and Spartan/Authority. These products make up the more active ingredient on pigweed in many premixes such as: Envive, Prefix, Broadaxe/Authority Elite, and Fierce. These products, especially fomesafen or Flexstar, have basically been making it possible to grow conventional and Roundup Ready soybean for the past seven to eight years in the face of pigweed that is already resistant to the ALS chemistry, glyphosate and even Prowl and Treflan.

We have relied heavily on the PPOs for some time. A close second are the mitotic inhibitors such as: Dual, Outlook, Warrant and Zidua, to which little resistance has been documented thus far.

PPO-resistant pigweed species have already been documented in the Midwest and other areas around the U.S. and the world. But up until this year we seem to have been spared this problem. This year I am getting calls from all over.

I have been in several fields where it does appear that either Valor PRE or Flexstar POST, or both, have failed. The plants are burned and partially controlled but re-growing from the terminal and not being controlled. This can and does happen when applications are made too late, but I can verify at least two or three fields where the pigweed were legitimately 2 inches tall at application with good conditions for control. We retreated some in the field, and they also were “burned” but not controlled. Therefore, I am convinced it’s here.

Read the rest of the story in Delta Farm Press.

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