Large-scale dicamba field trials

Preliminary University of Arkansas research results shed light on potential drift and volatility of three new formulations.

dicamba-damaged soybeans
These soybeans were damaged by off-target movement of dicamba. The cupped leaves are a telltale symptom of dicamba injury.

By Vicky Boyd
Editor

Even if you follow all of the label recommendations when applying the new dicamba formulations, the herbicides can still volatilize and move off site if environmental conditions are right.

Those are the preliminary findings of several field trials conducted by University of Arkansas researchers at the Northeast Research and Extension Center in Keiser as well as other locations within the state.

University of Tennessee and University of Missouri researchers are conducting many similar experiments in their respective states.

UA researchers shared some of their results at a recent field day at Keiser.

The trials were prompted by the 2017 registrations of three dicamba products, all of which are marketed as having reduced volatility. They are XtendiMax with VaporGrip technology from Monsanto, FeXipan with VaporGrip technology from DuPont Crop Protection and Engenia from BASF.

XtendiMax and FeXipan are both DGA dicamba products with a proprietary technology to reduce volatility. Engenia is a BAPMA dicamba formulation that inherently has lower volatility.

The dicamba formulations are paired with Xtend soybeans and XtendFlex cotton genetically engineered to the herbicide. Soybeans without the trait are extremely sensitive to dicamba.

Drift occurs when small droplets, referred to as fines, are blown off target, landing on a sensitive crop. Even the recommended nozzles for dicamba that produce extremely course and ultra course droplets also yield at least 1 percent fines, says Dr. Tom Barber, UA Extension weed specialist.

Volatility, on the other hand, occurs when dicamba is applied to plants or the soil. It may dry, but when environmental conditions are right, the material will vaporize and move to nearby fields or crops.

Of the three new dicamba formulations, Dr. Jason Norsworthy, UA professor of weed science in Fayetteville, says university researchers had only been able to conduct field trials with Engenia before 2017.

Why the problem now?

tom barber university of arkansas weed extension specialist
University of Arkansas Extension weed specialist Tom Barber discusses the results of some of his dicamba drift trials.

Dicamba itself is nothing new, having been used on pastures and as a burndown product for years. Why has the herbicide become an issue this year? Barber asks.

The other products typically are used earlier in the spring when temperatures are cooler.

“Volatization occurs when those temperatures increase, especially the temperatures of the surfaces where that dicamba is hitting,” he says.

As of Aug. 8, 864 complaints of suspected dicamba misuse had been filed with the Arkansas Plant Board, according to plant board figures.

As of Aug. 2, 240 dicamba-related complaints had been filed with the Missouri Department of Agriculture since Jan. 1, according to department figures.

Off-target dicamba injury to soybeans isn’t limited to the Mid-South either, says Dr. Larry Steckel, a University of Tennessee Extension weed specialist.

“It’s been a huge issue from Minnesota to Mississippi and from Kansas to the east,” he says.

Boom height, nozzles affect drift

Barber, Norsworthy and other UA researchers have conducted several trials to help answer questions producers and applicators have about dicamba movement and damage to soybeans.

In one trial, Barber examined how spray boom height and nozzle selection could significantly affect drift. XtendiMax was applied at the labeled rate of 22 ounces per acre for 15 seconds through a stationary boom held 24 and 48 inches above the crop. The treatments also involved three nozzle types: medium flat fan, course TT and ultra-course TTI.

The wind speed varied during the two reps but was never more than 4 mph. That is still significantly less than the 10 mph on many state labels or 15 mph on the federal label.

Both boom heights also were significantly lower than the 6-foot average seen with Arkansas applicators.

Fourteen days after treatment, Barber and a crew returned and measured the average distance from the treated area that they found dead soybeans. The XtendiMax treatment with medium nozzles and a 48-inch boom height produced the largest dead swath of soybeans extending more than 7 meters (23 feet) from the treated area.

On the other end of the spectrum, XtendiMax applied through ultra-course TTI nozzles at 24 inches above the crop only produced a dead zone of about 2.5 meters (8 feet). Barber says the results are preliminary and he has not had time yet to determine whether the treatments were significantly different from one another.

Dicamba gets up and moves

jason norsworthy university of arkansas weed science professor
Jason Norsworthy, a University of Arkansas weed science professor, talks about dicamba volatilization. The red flags denote soybeans injured from volatilization trial.

Both Barber and Norsworthy also conducted several trials examining dicamba volatilization. In a large-scale trial, Norsworthy treated two 3.5-acre fields — one with XtendiMax and one with Engenia. The applications were made using a Mudmaster sprayer with TTI1103 nozzles, a boom height of 30 inches (24 inches above 6-inch-tall plants), a rate of 10 gallons per acre and a ground speed of 9 mph. The application started at 11:56 a.m. and finished at 12:19 p.m.

He also had weather stations to record environmental conditions. During treatment, the wind averaged 2.9 mph with a maximum of 7 mph.

Twelve days after application, Norsworthy and a crew returned to visually survey for damaged soybeans.

They found visibly injured soybeans as far away as 303 feet on the north and east sides of the XtendiMax-treated plot and as far away as 302 feet on the north and east sides of the Engenia-treated plot. He says this makes sense considering the winds were from the west or south during the 72-hour period after application.

Currently, the federal label for the three dicamba products calls for a 110-foot buffer only on the downwind side if sensitive crops are present. Before the Arkansas Plant Board banned all uses of dicamba, it had required a 100-foot buffer on all sides of a dicamba-treated field.

Norsworthy also took soybean plants raised in the university’s Fayetteville greenhouse and randomly placed them in the treated area 30 minutes after spraying was completed. Another set of 20 plants was put out in the treated field 24 hours after dicamba application.

All plants were kept in the field for 36 hours after application, then returned to the greenhouse for 21 days before rating them. Depending on the environment, symptoms of dicamba injury may take two to three weeks to develop.

“Whether a half hour or whether at 24 hours after spraying, the amount of damage we saw in those plants was pretty comparable,” he says. “We still had a tremendous amount of material that was coming off of those fields based on what we’re seeing with these plants.”

The results are comparable to work by Drs. Tom Mueller, University of Tennessee, and Kevin Bradley, University Missouri, who are using detectors to measure dicamba concentrations in the air after treatments.

Mueller still found measurable amounts 36 hours after treatment. Bradley actually continued to monitor the air for 72 hours after treatment. He only has analyzed data for the first 36-hour period so far, but found measurable amounts during that time.

“We know at least out to 36 hours that we have material that’s coming off of these treated areas,” Norsworthy says. “Now what does that mean? You can spray the field correctly but you can also have an inversion the next day or the following day that has the potential to move and move over vast acreage. And the amount of damage that’s then caused is the result of the amount of material that’s used and the amount of acreage treated.”

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