It’s been a fairly common practice for cotton and soybean farmers to add Roundup to the tank mix to broaden the spectrum of weed control when they were spraying glyphosate — and dicamba-tolerant crops.

That may not have been such a good idea, according to scientists such as Dr. Larry Steckel, professor of row crop weed management at the University of Tennessee and a widely-recognized expert on the spraying of dicamba herbicide.

When EPA renewed the registration for the use of Engenia and Xtendimax herbicides on dicamba-tolerant crops last October, it added several new requirements to the product labels. One of those was that applicators must keep the pH of the spray solution above 5.0 to reduce the potential for volatility.

“The lower the pH the more likely the herbicide is to become volatile and leave the field days and hours later,” said Steckel, who discussed the new requirement in a presentation at the Winter Production Meeting held by the Extension staffs in Fayette and Hardeman counties at Lone Oak Farms near Middleton, Tenn.

Steckel presented a series of slides about what happens to the pH of a spray solution when Roundup or the generic equivalent of the product is added to different formulations of the dicamba herbicides and water.

“We tested some water from Weakley County that had a pH of 7.8 all the way down to 6.1,” he said. “When we pulled the 7.8-pH water and added Engenia to it, it went to 6.9. When we added Roundup, it went below 5.0 (to 4.8).

“With Xtendimax it went to 6.5, and if we added Roundup it dropped below 5.0 to 4.8. With Clarity it went to 77.9. Add Roundup to it, and it’s a 5.0.

Temperature impact

Temperatures can also have an impact — the higher the temperature the more likely the risk of volatility.

Dr. Tom Mueller, professor of weed science at the University of Tennessee, has run tests measuring the amount of dicamba being released into the atmosphere at different temperatures using humidomes.

“You can see that at these high temperatures (above 86 degrees) when you add Roundup to Xtendimax you greatly increase the amount of dicamba that’s collected in the humidome over the next 48 hours,” said Steckel. “And that’s reason for concern.”

Unfortunately, for farmers and commercial applicators, EPA did not provide guidance on how to lower the pH of the spray after Roundup is added to the recommended dicamba formulation.

“The new label recommends you test for spray solution pH and add a buffering agent if the spray solution pH is less than 5.0,” said Steckel. “I’ve made a number of calls on what we can throw in the tank to raise the pH. I have yet to get an answer from anybody.”

The other side of the coin on the issue is that if you raise the pH of a spray solution containing Roundup above 5.0, the efficacy of the Roundup drops significantly. “Roundup doesn’t like high pH,” Steckel said. “That’s why it’s formulated to drop the pH.

“This is the reason we are not recommending that anyone tank mix Roundup with a dicamba formulation. I’d much rather you use clethodim (Select). It doesn’t mess up the pH, and it will pick up the grass. It’s safer to use, especially around specialty crops, than if you put Roundup in the tank.”

New regulations

Some weed scientists are saying they don’t think the new regulations on dicamba herbicides go far enough. They favor restrictions such as those adopted in Minnesota in 2018 which banned spraying the new dicamba herbicides on dicamba-tolerant soybeans after June 20 and after temperatures reach 85 degrees.

“The positive thing is they’re admitting volatility is a contributor to off-site movement,” says Dr. Bill Johnson, professor of weed science at Purdue University. “That’s much different than what they’ve been willing to admit to for the past two years.”