“There’s been a love/hate relationship between farmers and dicamba for years,” says Iowa State University weed scientist Bob Hartzler. “The herbicide works, but it’s also known for its ability to move off-target through volatilization and particle drift, where it can cause serious damage.”
The potential for dicamba to volatilize and move off-target increases as temperatures rise, and about a third more dicamba volatilizes off of corn or soybean leaves than when it reaches the soil. The combination of potential damage and volatilization characteristics led Hartzler and ISU to recommend dicamba use be limited to a preemergence herbicide application on Xtend soybeans.
Estimated dicamba-injured soybean acreage
(Reported by state weed scientists, October 15, 2017)
Official dicamba-related injury investigations
(Reported by state agriculture departments, October 15, 2017)
Bob Hartzler is old enough to remember dicamba volatilization and drift in the early days of its use. The Iowa State University weed scientist told an audience at the ISU Integrated Crop Management Conference he’s one of the few active weed scientists older than dicamba.
Hartzler recalls the 1970s and early 1980s, when dicamba-based Banvel used in combination with 2,4-D was the most popular postemergence herbicide program for corn production in Iowa. But he also recalls the University of Illinois warning of the dangers of dicamba damage to soybeans in 1971.
Hartzler expects dicamba use to expand, but he’s among many weed scientists and farmers who have concerns about containing it to the field where it’s applied. After seeing the widespread damage from new products last year, Hartzler and ISU are recommending dicamba only as a preemergence herbicide on Xtend soybeans.
Hard to contain
Less than 0.1% of the labeled rate of dicamba (0.5 lb./acre) can cause significant damage to soybeans, Hartzler said. Dicamba’s relatively high vapor pressure makes it more prone to evaporate after application, and that combined with soybean plant sensitivity—soybeans are especially hypersensitive at the reproductive stage when plants are blooming—lead to potential damage.
“I believe from what we all saw last summer, these losses are biologically significant.Yields weren’t always affected when soybeans were damaged, but that’s not an acceptable measure. The threshold should be whether you can observe a biological response in an area where the herbicide was not applied,” Hartzler said.
The parent acid of dicamba is more volatile than most pesticides, Hartzler explained. Dicamba is formulated as various salts to reduce volatility and also make it more convenient to use. However, once the product is mixed and applied, the formulated salt may revert back to the volatile parent acid. “More recent versions of dicamba have tried to keep the parent acid locked up, and that has worked in the lab, but in the real world it can disassociate. There are just so many compounds in the real world that dicamba can interact with,” Hartzler said.Recent observation shows that significant quantities of dicamba can vaporize off treated areas for three days after application.
In this field, Hartzler noted an unusual case of damage caused by both physical drift and runoff from the field uphill from this damaged field.
Volatilization reduced but not eliminated
Volatilization is reduced with the new formulations of dicamba, but Hartzler isn’t convinced they’re reduced as much as claimed. “Three universities have compared Banvel, Engenia, and XtendiMax with Vapor Grip Technology and they have consistent results,” Hartzler says. “Yes, they have reduced volatility, but the reduction is only about 33 percent.”
Of a record total of 271 pesticide misuse complaints to the Iowa Department of Agriculture and Land Stewardship in 2017, 107 of them involved dicamba. That’s with estimated dicamba use on only 150,000 acres in the state, far less than states to the south and east. Arkansas estimated 900,000 acres treated with dicamba and reported 986 official complaints of off-target injury. Illinois, Tennessee, Missouri and the Dakotas had among the highest acres treated with dicamba, and highest number of acres of damage reported.
Retailers are concerned
The Illinois Fertilizer and Chemical Association included questions on dicamba in a survey of ag retail members last August. Of 116 member-retailers who said they had applied dicamba, 85% reported symptoms of dicamba damage to adjacent soybean fields. Many reported no problems with dicamba as a burndown, but numerous applications in June resulted in complaints. More than 60% of the retailers were extremely concerned about potential damage to orchards, vegetable crops, gardens and trees as more dicamba is used in the future.
“Most people who investigated dicamba complaints in Iowa say contaminated spray equipment, particle drift, and volatilization were all involved in off-target injury,” Hartzler said. “Better training and improved decision-making can help minimize damage going forward, but the risks associated with volatilization won’t be easily managed because vapor movement is determined by the environment following application rather than actions of the applicator.”
Early postemergence applications in V2-V3 soybeans have lower risks for off-target damage, but Hartzler pointed out rain and wind in May severely limit the times available for safely spraying dicamba. Higher temperatures in mid-June, when the spraying is more likely to be done, increase the risk of volatilization. And that has led ISU weed scientists to recommend only preemergence applications of dicamba in dicamba-tolerant soybeans.
2018 EPA Label Changes
The U.S. Environmental Protection Agency has classified Engenia, XtendiMax, and FeXapan as Restricted Use Products and issued several important label changes for their use on dicamba-resistant soybeans. The classification requires all applicators to be certified and to maintain detailed records of all applications. Among other EPA requirements:
- All applicators required to complete dicamba-specific training.
- Maximum wind speed allowed at application reduced from 15 mph to 10 mph.
- Application limited to hours between sunrise and sunset.
- Label language expanded regarding sprayer cleanout and susceptible crops.