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DFP-Brad-Robb-Steckel.jpg Brad Robb
Larry Steckel, UTIA weed specialist, says research indicates glyphosate in the tank with dicamba appears to increase volatility.

Glyphosate may contribute to dicamba volatility

Adding glyphosate to an Engenia or XtendiMax application may increase the likelihood of dicamba drift, according to a recent study.

Adding glyphosate to an Engenia or XtendiMax application may increase the likelihood of dicamba drift, according to a recent University of Tennessee Institute of Agriculture study.

Dr. Larry Steckel, UTIA weed specialist, Jackson, Tenn., reported in a recent podcast that farmers several years ago had observed dicamba drifting against the wind when they had glyphosate mixed in the tank with Engenia or XtendiMax.

“They didn’t see it so much if they were just spraying those straight dicamba products alone,” he said.

Steckel asked Tom Mueller, UTIA weed scientist at Knoxville, to set up a study with a humidome.

“He took XtendiMax and sprayed it on a tray of soil and compared that to XtendiMax with Roundup sprayed on a tray of soil. He did that outside of the greenhouse, away from where he was doing all these tests and then brought them into the greenhouse, put plastic covers over the humidomes (he essentially has a little vacuum that pulls air over the soil) and he monitored dicamba coming off that soil for 60 hours after application.”

The result was telling. Steckel says with Roundup in the tank with XtendiMax, from three to nine times more dicamba left that treated surface over 60 hours compared to a straight XtendiMax treatment.

Promoting volatility

“It did look from that data that he could detect that Roundup was promoting some volatility from the soil.”

Steckel said the EPA reworked the label this year to account for pH. “Anything below 5 is considered acidic enough to make these dicamba products more volatile.”

Steckel said Mueller’s research indicates that Roundup is lowering the pH. He wondered what else could contribute to lower pH. “We asked growers in the state to send us samples of water they mix from.”

With more than 12 samples, Steckel said the result shows “we have water pH all over the board across the state. I had no idea how variable they were. They run from 8.5 down to below 4.6, right out of the tap.”

He said producers need to consider water pH when they are mixing herbicide applications. “They could see some volatility with low pH and need to consider bumping the pH up.”

Steckel said producers may want to consider adding pH modifiers.

He said three products “BASF showed us did as advertised. They raise the pH of the Engenia and Roundup tank mix from 4.6 to about 5.5. They also did it for XtendiMax. So, very positive on that aspect that it does raise the pH. It should improve dicamba staying put as a result.”

Roundup effectiveness

Steckel said the research brings up more questions. One is how well will Roundup work if modifiers raise pH. “There is a reason why Roundup drives the pH down. It works better at low pH, so if you artificially raise the pH, do you lose weed control? We don’t know. That’s something we need to look into.”

Steckel said Roundup lowers pH more than AMS, which has been a key point in dicamba training — not to put AMS or any acidifying agent in the tank. They drive pH down and make the dicamba products more volatile.

Steckel said Roundup was driving pH down by 1 to 2 pH units. “That’s a lot.”

He said AMS drives pH down a half point.

“Some other aspects of ammonium sulfate besides pH may be enhancing volatility, but it just kind of goes to show, at least from this research, that Roundup PowerMax in the tank mix could be a culprit in why we’re seeing some of the drift in the fields these last three years.”

He does not recommend adding AMS.

“Anything that promotes volatility or (dicamba) leaving the field, we are definitely against. I think AMS doesn’t need to be in the tank and our recommendation from this research is we don’t encourage Roundup being in the tank, either.

“This data show that it’s lowering the pH, and from that we are seeing more volatility, according to the research Dr. Mueller has done. So, we are discouraging both AMS and Roundup.”

Roundup alternative?

If Roundup is not recommended, what can producers use?

Steckel said one of the graminicides, like Clethodim, could be an option. “It doesn’t affect the pH as much as Roundup.”

He said Roundup goes in the tank mostly for grass control. “But Roundup is not controlling a lot of the grass species in west Tennessee, whether it’s goosegrass or jungle rice or johnsongrass, and we’re having to put Clethodim in anyway.”

He said leaving Roundup out is especially important when spraying around sensitive vegetation, sensitive crops, sensitive areas. “Use one of the graminicides instead, like Clethodim, or come back with Roundup later.”

The weekly podcast “Call of the Week,” is available at

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