Dicamba injury complaints have been one of the hottest topics in the Mid-South in 2016 and 2017. The second hottest is what caused the injury – dicamba drifting onto sensitive crops or volatilizing and moving to them after application?
That question is at the heart of the debate over whether states should continue to allow growers to make in-season applications of the new Xtendimax, Engenia or FeXapan herbicides or ban them from being applied after April 15 as Arkansas plans to do in 2018?
(Monsanto, the manufacturer of XtendiMax with VaporGrip Technology, has asked the Arkansas State Plant Board to reverse its April 15 cutoff decision. A group of seven growers in northeast Arkansas has also filed a petition along with a series of recommendations for guidelines for applying the herbicides as late as May 25. See http://bit.ly/2hkDPHg.)
“We have spray requirements that address drift, and we hope our formulation addresses volatility,” says Bob Montgomery, technical development representative with Monsanto. “We do these kinds of tests to make sure our product does what we think it does.”
Dr. Montgomery and his son, Garrett Montgomery, who joined Monsanto after earning his Ph.D. degree in weed science under Dr. Larry Steckel at the University of Tennessee last spring, are standing in a field of non-dicamba-tolerant soybeans near East Prairie, Mo. The soybeans were sprayed with a label rate of Xtendimax with VaporGrip technology a few weeks ago.
Two Mid-South locations
The field is one of two in the Mid-South where they and a team of researchers have been conducting trials with Xtendimax. Researchers at the land-grant universities in Arkansas, Missouri, Tennessee, Mississippi, Nebraska and Indiana are also conducting similar tests.
The trial at the field in Missouri was rather simple in scope, but not so simple to execute. A label rate of XtendiMax with VaporGrip Technology, Roundup PowerMax and Intact was applied on Aug. 22. It was made in accordance with XtendiMax with VaporGrip Technology application requirements. See www.XtendiMaxApplicationRequirements.com.
They applied the tankmix to a 300 x 300 foot area near the center of the field when the wind was blowing at 4.25 mph from the southwest. “Normally, we would not recommend you spray when the wind is blowing in the direction of sensitive crops, but this was for research,” said Dr. Bob Montgomery.
Before they sprayed the tank mix, the researchers placed 14 8X10-foot tarps over the soybeans at 33 feet, 110 feet and 220 feet from the application on the downwind sides and at 33 ft. and 110 ft. on upwind sides.
The researchers removed the tarps 20 to 30 minutes after the application and marked the areas that had been covered with green flags on each corner of the area the tarp had covered.
Examining for injury symptoms
A few days later they surveyed the fields to determine the amount of injury outside the spray area where the soybeans had been killed. They marked an area downwind from the application zone where spray drift had caused damage of at least 5 percent.
The 8X10 blocks that had been covered with a tarp in the area where the spray drift occurred had no visible injury. Nor did the areas that had been covered on the other sides of the application zone exhibit any signs of dicamba injury.
“We made the application, and we let the spray settle for 20 minutes,” said Bob Montgomery. “Research would say the primary volatility will occur within 48 hours so we took this off after 20 minutes. Thus, we know we have the drift separated from the volatility in this area here.
“The difficult thing when you’re investigating drift and volatility is the two can occur in the same field, and there’s no way to separate them after the fact unless you do something like this. People will see the symptoms and some will call it one thing and some will call it something else.”
Montgomery says the part of the field he’s showing an editor is just one plot “and certainly you can’t draw conclusions as to how the world works with one plot, but you can see that in this field on this day that these symptoms we see here are totally related to drift and not volatility.
XtendiMax application requirements
“We have spray requirements that address drift, and we hope our (VaporGrip) formulation addresses volatility,” he said. “That’s why we do these kinds of things – to insure that our product does what we think it does, and this is a pretty good demonstration that on this day and in this field it worked as expected.”
The Montgomery’s said they plan to repeat the test in other areas beyond the two locations – here near East Prairie and in Tennessee where the trials were conducted in 2017 – but that doing so is a difficult proposition.
“It’s asking a lot of a grower to give up one of his soybean fields for a research project like this,” said Bob. “And then you tell him ‘by the way, we’re going to kill five or six acres right in the middle of the field. Not many growers will do that, but the farmer who has this field has been great to work with.”
Another potential obstacle is that the grower cannot spray any other postemergence herbicides on the field because it might effect the results of the trial. In the East Prairie field, the grower brought in a crew of choppers to clean up the Palmer amaranth or pigweed that grew up in non-sprayed area.