Soybean growers may want to reconsider how they use some of their standard pre-emergence herbicides as they make plans for what experts believe could be an extremely challenging weed control season in 2022.

Weed scientists aren’t ready to say Palmer amaranth has developed resistance to the herbicides in question – Dual Magnum and Warrant – but studies show they may not always be as effective as they once were on pigweed.

“You will notice we have relatively poor pigweed control in that plot this year, although our grass control is very good,” said Dr. Tommy Butts, Extension weed scientist with the University of Arkansas System Division of Agriculture, who spoke during the virtual Arkansas Rice and Soybean Field Day last fall.

Butts was referring to a study at the Jackson County Extension Center near Newport, Ark., comparing Group 15 herbicides that are commonly applied prior to planting or in combination with postemergence products in soybeans.

One of the first plots he discussed had been sprayed at planting with a single application of 1.33 pints per acre of S-metolachlor or Dual Magnum. (To demonstrate the efficacy of the residual herbicides, none of the plots received a postemergence application.

“I attribute that low pigweed control to maybe developing a little bit of tolerance,” he said. “I’m not going to say we have full blown resistance on this farm, but it may be that we're evolving some tolerance to S-metolachlor after repeated use year after year.

“Instead of getting maybe three weeks of control, we were only getting a week to 10 days. If we are evolving that resistance in our fields, that's what we expect it to look like: It will break early, and we will have a lot of escapes compared to what we’ve been used to in the past. So overall S-metolachlor didn't do so great for us this year on the pigweed front.”

Good grass control

It provided good grass control, but was weak on morningglories, according to the results of the replicated study at the Jackson County Extension Center, he said.

In contrast, an application of pyroxasulfone or Zidua alone at 4.0 ounces per acre provided 90% to 95% control of Palmer amaranth. Zidua, which has only been on the market a few years, also eliminated most of the grasses and the morningglories in the plot located adjacent to the S-metolachlor application.

The other herbicides Butts discussed – dimethenamid-P or Outlook at 16 ounces and acetochlor or Warrant – also saw pigweed escapes, but both provided good to decent grass control.

“One recommendation is that if we’ve relied on Dual or Warrant for a number of years you may want to try rotating those Group 15 herbicides just to break that cycle and delay some evolution of resistance,” he said.

“In areas where we may have higher tolerance or resistance developing, I would say the encapsulated formulation of acetochlor tends to lag behind our other Group 15s. So we need to be timely if we’re using Warrant, to apply that overlapping residual application a little bit sooner.”

Butts said pre-emergence pre-mix products with multiple effective sites of action have also controlled pigweed effectively in the study. Those include Boundary, a combination of S-metolachlor and metribuzin and Fierce, a mixture of flumioxazin or Valor and pyroxasulfone, along with the trivent herbicides.

“Typically, all of those will get a diverse weed spectrum,” he noted. “One might be better than another on teaweed or prickly sida, morningglories or yellow nutsedge. But all will help with more than just one species, and all will help with grass control.”

To view the presentation, visit https://aaes.uada.edu/videos/2021-soybean-weed-control/.