While off-target movement of dicamba herbicide has widely occurred in Arkansas, Missouri, Tennessee and southern Illinois this summer, significantly fewer reports related to Monsanto’s new Xtend dicamba-tolerant soybean system have occurred in Iowa.

As of July 20, the Iowa Department of Agriculture’s Pesticide Bureau had received 142 total pesticide Misuse Incident Reports for the 2017 crop season. Of the total, 46 reports alleged growth regulator herbicide (includes dicamba) exposure.

Dicamba has been around awhile. With over 50 years’ experience using the old type of dicamba in corn, why the concern about the new dicamba that’s registered for use in soybeans? Why is dicamba use in dicamba-resistant soybeans different than in corn? Bob Hartzler, Iowa State University Extension weed management specialist, sees two primary differences.

• Later application dates. The majority of dicamba applications in corn are made early in the growing season. Soybeans often aren’t emerged, or are in early vegetative stages at this time. The majority of dicamba applications in dicamba-resistant beans are made as the crop approaches reproductive stages. Susceptible soybean varieties will be at greater risk with applications made to dicamba-resistant beans than to corn since they are at a stage where yield impact is more likely.

With later application dates, temperatures will likely be higher during application in dicamba-resistant soybeans than in corn. While new dicamba formulations have reduced volatilization, they have not eliminated it. The higher the temperature, the greater the likelihood of vapor drift.

• Dicamba use rates. The postemergence application rate for dicamba is 0.5 pound per acre in both corn and dicamba-resistant soybeans. However, dicamba rates in corn are usually reduced significantly since it is often tank-mixed with other herbicides or due to crop injury concerns. Labels of products registered for use in dicamba-resistant soybeans do not allow reduced rates.

The use patterns of dicamba in soybeans greatly increase injury risk compared to how it has been used in corn. “While the magnitude of problems in Iowa during the 2017 growing season is much less than occurred in states south of Iowa, the number of injured fields is significant,” says Hartzler. “My concern is problems with off-target movement will increase in the future as use increases.”

Several factors contribute to problems
Why have Midsouth states had more off-target problems than the Corn Belt? Several factors likely contributed to this, notes Hartzler. Problems during the 2016 growing season were the result of illegal application of older dicamba formulations to dicamba-resistant soybeans or cotton, as neither of the reduced-volatility dicamba formulations were registered for use until 2017. While there is evidence that non-registered formulations continued to be applied in 2017, problems have also been seen with the new products.

The longer growing season in the Midsouth creates an extended planting period compared to Iowa. Prolonged planting results in dicamba applied over a longer period in states south of Iowa. Many fields were exposed to dicamba drift two or three times, greatly increasing the severity of injury.

The states with severe dicamba damage have different cropping systems than Iowa. Acreage potentially planted to dicamba-resistant varieties (cotton, soybean) is much greater in the South. In a southern Missouri county, for example, 80% of the acres could be planted to dicamba-resistant crops. In contrast, approximately a third of the row crop acres in typical Iowa counties could be planted to dicamba-resistant soybeans. While corn acres could be treated with dicamba, they pose less of a risk.

More acres treated with dicamba in South
Because of serious problems with Palmer amaranth, the percentage of dicamba-resistant crop acres treated with dicamba in the Midsouth is much higher than in Iowa. Kevin Bradley, University of Missouri Extension weed scientist, estimates 65% of soybeans planted in the Missouri boot heel area are dicamba-resistant varieties, and nearly all are sprayed with dicamba.  

“I don’t know the percentage of soybean varieties that were dicamba-resistant in Iowa in 2017, nor the percentage of these beans treated with dicamba,” says Hartzler. “Informal discussions with farmers suggest less than 50% of the dicamba-resistant soybeans were treated postemergence with dicamba. On a landscape basis there would be much more dicamba applied per square mile in the Midsouth than in Iowa, greatly increasing the risk to sensitive plants.”

Other factors also likely contributed to widespread off-target injury in the Midsouth. The flat landscape in the Delta region probably allows greater movement of dicamba from treated fields than in Iowa. Temperatures also tend to be higher in the Midsouth during the dicamba application season.

Moving forward
“Agriculture is in a difficult position in determining how best to move forward regarding the expanded use of dicamba and the risks posed by this technology,” sums up Hartzler. “There is no denying that new tools are needed to aid in weed management as problems with herbicide-resistant weeds continue to spread. We have to try to provide an objective evaluation of the risks associated with dicamba use on dicamba-resistant soybeans. The benefits dicamba provides in terms of weed management are well-documented.”