Dicamba Movement – pH Potentially Plays A Role

Photo: Debra L Ferguson

Photo: Debra L Ferguson

Weed scientists are taking a closer look at the relationship between low pH and the tendency of dicamba herbicide to volatilize.



“We have some initial data that indicates that the lower the soil pH, the more likely it is that volatility will occur with dicamba,” says Missouri weed scientist Kevin Bradley, who is in the initial stages of gathering data on the pH-volatility link. “Maybe we’ll find something we missed.”

Low soil pH could explain why some southern states are seeing higher levels of dicamba injury, Bradley adds. “Soils in the South tend to be a little lower in pH, and maybe that’s why we were finding some of those problems in that region first. We’re still looking into that. But we have done two field tests that confirm that at a lower soil pH, you’re likely to have more volatility.”

Bradley adds that liming a low pH soil “is probably something that producers should do anyway, regardless of the threat of volatility.”

Tennessee pH Studies: Looking At The Tank

In field and laboratory studies in Tennessee, tank pH is proving to be a very important factor for off-site emissions of dicamba, according to Tom Mueller, professor of weed science at the University of Tennessee. Mueller has been studying dicamba drift and tank pH for several years, thanks to funding by the Tennessee Soybean Promotion Board.

“Our results have been very consistent,” Mueller says. “As you lower the tank pH, we get more emissions, which probably includes volatility.”

It’s should be noted that Mueller’s study detects dicamba molecules, which may or may not be the result of volatility. But he can say that “the higher the pH, the fewer the emissions.”

Products added to dicamba can definitely affect tank pH, Mueller adds. “With the older formulations of dicamba such as Clarity – a DGA salt – the pH will be about 6.5 to 7.0 in normal-range water. When we add Roundup PowerMax, it lowers the pH to about 5.0.”

XtendiMax with VaporGrip – which is dicamba formulated to be less volatile than older dicamba versions – has a pH in normal-range water of about 6.0, according to Mueller.

“When you add Roundup PowerMax, it drops the pH,” he specifies, “but not as much as it does with Clarity. The entire driver of how VaporGrip works is pH.”

VaporGrip uses an acetic acid-acetate buffering system to reduce the formation of volatile dicamba acid, according to this article.

Even though dicamba can be made much less volatile, “the molecule intrinsically has the propensity to evaporate and to move, so there’s not a magic pH that switches that propensity off,” Mueller notes.

Mueller quickly points out that adding certain adjuvants to raise tank pH “may affect the efficacy of the herbicide, so we’re not making recommendations on how to raise the pH.”

Mueller’s studies do not include the effect of soil pH on dicamba volatility, but he does offer his thoughts on the subject.

“Some believe it (pH) is the most important driver,” he says. “In certain situations, it might be. But in the Midsouth when you’re spraying dicamba, most of what you’re spraying is contacting plant material, especially in soybeans. Studies indicate that there are much greater dicamba emissions from green plant material than from soil or dead plant material. So, I think what you’re hitting with the spray is a bigger factor than the pH of the soil.”

Watch That Temperature

Temperature is still a primary factor for volatility, Mueller notes.



“Our studies show that there is a threshold temperature of about 70 to 75 degrees,” Mueller points out. “Below that, emissions – and probably volatility – are greatly reduced. We can still detect emissions on our instruments, but it’s at a very low concentration. Once you reach that threshold, it’s a linear relationship – the higher the temperature, the higher the volatility.”

Dicamba can still move even when it’s applied at 70 to 75 degrees if temperatures continue to rise during the day, Mueller adds.

“We run the studies for 60 hours, sampling all day long and all night long, and it’s very clear that it doesn’t matter what the temperature is when you spray,” he says. “It’s the environment surrounding the dicamba molecule after the application. If it gets hot the next day, our data indicates it comes off.

“In the studies, if we spray first thing in the morning, we get some emissions in the morning, with the highest concentrations coming in the afternoon after the first application during the heat of the day.”

Another moving part to consider are temperature inversions, which can suspend volatilized dicamba molecules that can later move onto nearby fields.

“Inversions are common,” Bradley says. “They’re going to happen two-thirds to three-quarters of the nights during our growing season. If you have a product that can volatilize 24, 48 or 72 hours after the application, then there’s a pretty good chance that the product can get caught up the inversion as well.”

Tank Contamination

Producers made strides this season corralling dicamba injury via tank contamination, Bradley adds.



“Farmers are learning how very little of the product it takes to cause damage,” he says. “Retailers are often dedicating a sprayer just for dicamba applications.”

Greater than 90% of cotton and soybeans planted in the Missouri Bootheel in 2018 contained Xtend technology. Farmers latched onto the technology for a variety of reasons, according to Bradley.

“Farmers believe they have to have it (Xtend technology) for Palmer amaranth or waterhemp, while others plant it because of the yield,” he says. “Yet others plant it because they don’t want their soybeans injured by dicamba anymore.”

In 2018, “plenty” of off-target reports arose in Missouri outside of the Bootheel in the corn and soybean region of the state. That’s where “there were a lot of first-time adopters of Xtend soybeans,” Bradley reports.

“My opinion is that drift is probably not much different from the previous year,” he says. “But there are so many factors that make it difficult to understand what precisely contributes to it.”