As the 2017 spraying season winds down and field days begin to tail off, Mid-South weed scientists are commenting on how similar many of their preliminary dicamba research results appear. These results come from tests well scattered across the northern part of the region.  

One word used frequently in their findings regarding new dicamba formulations: volatility.

“We have data that supports volatility being a part of the problem otherwise we wouldn’t say it,” said Kevin Bradley, University of Missouri weed scientist, in late August. “And surrounding states and research have similar data and support for the volatility bucket. What we’re seeing isn’t much different than what’s being found in Arkansas and Tennessee.”

Hoops

In Arkansas, University of Arkansas weed scientists Bob Scott, Jason Norsworthy and Tom Barber studied “hoop” set-ups in the northeast (Keiser), the central part of the state (Lonoke) and in the southeast (Rohwer).  

“The purpose of the hoop studies was to observe if any differences existed between the old and new dicamba formulations in regards to volatility,” says Barber. “Although we haven’t analyzed all the locations together, it appears the data is going to fit together pretty well.”

The trio looked at some of the older dicamba products like Banvel, some of the older DGA products like Clarity and compared those to XtendiMax, Engenia, Roundup Xtend (a pre-mix formulation). They also had an XtendiMax treatment with AMS, or ammonium sulfate.

“We did the tests in ‘hoops.’ The hoops are about 20-feet long, covering two rows of soybean. In the middle of the hoops, we placed two standard (18 x 26 in) greenhouse trays full of moistened soil from the field the research was conducted. We sprayed the soil in the trays and then set them in the hoop for 48 hours.”

To avoid contamination, each individual treatment or herbicide was handled by a separate individual and those individuals were not allowed anywhere in the study except their specific treatment.

The hoops are made out of a PVC frame with visqueen plastic -- a miniature greenhouse out in the field. The ends are open and weather stations were used to take temperatures inside/outside the hoop.

Symptomology

The trays were left in the hoops for 48 hours before they were taken down.

“So, all the data – all the volatility from the hoop studies, anyway – were based on what came off the soil in those trays in those 48 hours.

“We took plant counts, percent injury and height data from the center of the plot in both directions, either side of the center, in increments, on two rows. Usually, with dicamba injury, symptoms begin showing up about 14 days after application. So, we collected data at 14, 21, and 28 days.

“What we were looking for was dicamba symptomology on soybean, the number of plants showing symptoms, and if there was any reduction in height. The biggest thing that stuck out in all the hoop trials was some of the first dicamba formulations like the acids or DMA salts had very high volatility. That led to very high soybean injury to the plants in the hoop as well as reduction in plant height.”

One of the highest injury-causing treatments was when AMS was mixed with XtendiMax. “The AMS caused the DGA salt in XtendiMax to disassociate from the parent acid. That allowed the parent acid to readily volatilize, resulting in a lot of injury to the plots.”

When it came to Clarity, a DGA salt, “we had less visual injury symptoms than with dicamba acids, with Banvel, or when we added AMS to DGA salts.

Statistical differences?

“At this point, we don’t know if there will be any statistical differences between Clarity, Engenia, and XtendiMax in terms of volatility because the data have not been analyzed. What we do know is they all injured soybeans to some extent in the rows where the trays were placed.

“Now, in one location, injury from Clarity may have been higher than Engenia or Xtendimax than in another but, when the data is all brought together from these three locations, I don’t expect there will be large differences. I believe the data will show anytime we put AMS with a dicamba formulation we’ll significantly increase volatility. If older formulations like Banvel are used, DMA salts or the dicamba acids, you’ll also see an increase in volatility and subsequent injury.”

Regardless of whether or not researchers are able to statistically separate Engenia and XtendiMax from Clarity, “they all volatilized enough to cause some level of injury and it was, significant enough to notice (3 to 10 percent). Remember the scale; we are talking about injury from only two 18x26 in trays of soil sitting inside the hoops for 48 hours.”

Going in, the trio was “just trying to tease out differences between the dicamba formulations,” says Barber. “The claims going in said these formulations would show a significant reduction in volatility over older products like Banvel and Clarity.

“Based on these preliminary data we have now, I agree those formulations are less volatile than Banvel, other DMA salts or dicamba acids. But in terms of soybean response, it doesn’t appear that the volatility is greatly reduced from Clarity, a standard DGA salt that is widely used. Again, this is preliminary.

However, “even if these new products show reduced volatility, they are still volatile and can cause injury.”

In other studies, the Arkansas researchers “sprayed 3 to 4 acres in a sensitive soybean field and either covered plants with buckets or inserted plants from the greenhouse we are observing volatility up to 48 hours after application.

Barber points to a term – “atmospheric loading” -- used frequently during the dicamba spraying controversy. “The research tells us that because these newer formulations remain volatile they can potentially load the atmosphere with dicamba. Is that the only way to load it? Nope. But we know when you spray a dicamba product over large acreage the amount available to volatilize, and the amount that can fill the air, can continue to increase for at least 48 hours.”