With the off-target dicamba movement in recent years, Tennessee soybean growers asked my colleague, Dr. Tom Mueller, and me to vet some of the directions on the XtendiMax and Engenia labels.  The goal was to come up with information to help mitigate off-target dicamba. In a series of blog posts this week, we will share our most current findings.

In one series of tests, we examined the effect on tank-mixture pH when glyphosate is added to either XtendiMax or Engenia. Labels for both products recommend that applicators monitor spray tank pH since low pH can result in increased volatility of dicamba.  This, coupled with the fact that glyphosate (which typically decreases spray tank pH) is tankmixed with most dicamba applications leads to a question—does adding glyphosate to tankmixtures that contain XtendiMax or Engenia lower spray-tank pH?

In our research, we took water that Tennessee applicators actually use for mixing from 12 different sources across the state. To these water sources we added one of the two low-volatile dicamba formulations and then added Roundup PowerMax.  The starting water pH from this survey ranged from 8.4 to 4.6.  The addition of Roundup PowerMax to Engenia across the water sources decreased the measured pH from 1.0 to 2.1 pH units.  The addition of Roundup PowerMax to XtendiMax decreased the measured pH from 1.0 to 1.5 pH units.  Across the water sources the average final pH after Roundup PowerMax was added to Engenia was 4.6 and after addition to XtendiMax was 4.8.

Recently, Bayer shared their research on the effect of adding Roundup PowerMax to XtendiMax and they reported a 0.2 to 0.3 pH unit decrease.  Clearly, we were unable to replicate those results. Please keep in mind the new labels for both of these dicamba products recommend that spray tank pH should be above 5.0 to minimize potential volatility.

This research led the University of Tennessee weed manual committee to make the following recommendations on the 2019 Tennessee Weed Control Manual PB1580: “UT research indicates that tankmixing glyphosate with these low-volatile dicamba formulations will lower the solution pH which can result in increased dicamba emissions.”

In our next post, we’ll cover our research on the effect of spray solution pH when AMS is added to Engenia or XtendiMax.