Complaints about dicamba herbicide injury to soybeans picked up in Iowa during the last two weeks of July and in early August. However, the number of official complaints in Iowa this summer is still well below other states. The number of complaints in Missouri, Illinois, Indiana, Arkansas and Tennessee were greater.

Iowa State University Extension weed scientist Bob Hartzler said he expected the number of cases in Iowa to increase. “Official reports to regulatory agencies are the tip of the iceberg, as many farmers and applicators try to settle these issues among themselves rather than involve regulatory agencies,” notes Hartzler. Most of the cases of drift injury were related to applications on dicamba-resistant Roundup Ready 2 Xtend soybeans, as drift from those dicamba herbicide applications made its way to fields where non-dicamba resistant beans were planted.

New formulations haven’t solved the problem
The reports of drift were nearly evenly split between Monsanto’s Xtendimax and BASF’s Engenia dicamba herbicide formulations, says Hartzler. While the extent of damage has varied from state to state this year, the causes of off-target injury were similar. And while many problems could be attributed to the applicator failing to follow label restrictions, weed scientists reported that many applicators experienced problems while doing everything right.

“In my opinion, the biggest concern is that the new formulations don’t appear to have solved the age-old problem of dicamba volatility,” says Hartzler. “While Xtendimax and Engenia have reduced volatilization compared to older formulations, the combination of relatively high dicamba application rates and applications during high temperatures can result in significant dicamba concentrations leaving treated fields.”

Factors contributing to volatility
Current research is showing that the new formulations of dicamba (Xtendimax with Vapor Grip Technology and Engenia) haven’t solved the volatility problem associated with dicamba. Since the fundamentals of chemistry have not changed, the research done with older formulations can provide insight in explaining problems experienced during 2017, says Hartzler.

The older research looked at the effect of dicamba application rate, rainfall, evaporation surface on volatilization, formulation on volatilization and application timing. The article Factors influencing dicamba volatility originally appeared on the Iowa State University Weed Science website in 2001.

As you review this older research, keep in mind that the newer formulations of dicamba have made definite improvements over the dicamba salts studied in 1979. But reductions in volatility and no volatility are not the same thing, says Hartzler, especially with a crop as sensitive to dicamba as the soybean plant.

Drift symptoms and yield
For soybeans with dicamba drift injury, past research shows yield decreased. But in other cases where dicamba drift occurred, the bean yield wasn’t affected.

At harvest this fall, agronomists are encouraging soybean growers to document areas of the field that appear to have low, medium and high foliar injury symptoms. Yield monitors on combines will show a possible answer to the question of yield impact.