The number of drift and off-target herbicide complaints reported to the Iowa Department of Agriculture and Land Stewardship has increased in recent weeks. But it’s nowhere near the number of complaints that have been registered this summer in states to the south of Iowa — Missouri, Arkansas, Tennessee, southern Illinois and southern Indiana, etc.

As of Aug. 3, the Iowa Department of Agriculture’s Pesticide Bureau had received 178 Misuse Incident Reports for the 2017 crop season. Of the total, 74 reports alleged exposure to growth-regulator-type herbicides, such as dicamba.

Problems associated with dicamba have escalated nationwide, especially in the South, as farmers started to use the new dicamba system to combat herbicide-resistant weeds. This spring marked the official launch of the Roundup Ready to Xtend system, which includes the new dicamba herbicide formulations (XtendiMax, Engenia and FeXapan). Engenia, XtendiMax and FeXapan are labeled for use in Iowa. These herbicides are for use on dicamba-resistant soybean varieties only. If dicamba drifts onto fields of non-Xtend soybeans, you see the common symptom — cupping of soybean leaves.

Dicamba injury symptoms concern farmers
“We are seeing those symptoms on some fields in Iowa,” says Paul Kassel, an Iowa State University Extension field agronomist at Spencer in northwest Iowa.  “Fields affected by dicamba movement have been very slow to recover. This is likely due in part to the continued dry weather. Farmers who have soybean fields affected by dicamba movement are concerned about the yield impact from this drift.”

Joel DeJong, ISU Extension agronomist at Le Mars, says he’s been keeping track of a soybean field injured by dicamba drift on June 20 that is still not showing a significant return to normal leaf shape after 40 days. And the brown gaps in the soybean rows are wider the closer they are to the neighboring field where the herbicide drift originated. “Phone calls about off-target dicamba injury continue to come in, and last week might have been the most calls I’ve received. Crop recovery from the cupping is very slow in the areas where moisture is limited,” says DeJong.

Number of cases in Iowa higher than usual
Bob Hartzler, ISU Extension weed management specialist at Ames, is keeping tabs on the dicamba situation in Iowa. As of July 20, IDALS had received 142 reports of pesticide misuse, and historically this number has ranged from 58 to 85 at that time of year. Dicamba was involved in 46 of these 142 cases, says Hartzler. Two of the dicamba cases involved use in non-crop areas, 10 involved applications on corn and 36 were related to applications to dicamba-resistant soybeans (Xtend soybean varieties). 

The soybean cases were nearly evenly split between Monsanto’s dicamba herbicide (XtendiMax with Vapor Grip Technology) and BASF’s Engenia dicamba herbicide. There were no reports involving products not registered for use on soybeans, says Hartzler. Cases also were evenly split between commercial and private applicators. 

The total number of dicamba cases reported to IDALS increased to 74 on Aug. 3, notes Hartzler, “and I expect the number of cases will increase. Official reports to regulatory agencies are the tip of the iceberg since many farmers and applicators attempt to settle drift complaints and related application issues among themselves rather than involve regulatory agencies.”

EPA teleconference July 28
EPA officials held a teleconference on July 28 with university weed scientists from states experiencing off-target injury from dicamba applications on dicamba-resistant soybeans. The weed scientists described the magnitude of the problem in their respective states, and likely sources of off-target movement. While the extent of damage varied from state to state, the causes of off-target injury were similar, says Hartzler, who participated in the EPA teleconference.

Sources of problems the university weed scientists cited included:
• particle drift
• contaminated sprayers
• spraying during temperature inversions
• volatilization
• movement out of treated fields in runoff water

LEAF CUPPING: Field reports indicate very small amounts of dicamba as drift or contamination from spray tanks can produce soybean injury symptoms such as leaf cupping.

While many dicamba drift problems could be attributed to the farmer or commercial applicator, failing to follow label restrictions and instructions, the university weed scientists reported that many applicators experienced problems while doing everything right.

Do new dicamba formulations over-promise?
“In my opinion, the biggest concern is that the new formulations do not appear to have solved the age-old problem of dicamba volatility,” says Hartzler. “Researchers from three universities presented data from current research investigating vapor loss of the new products. While XtendiMax and Engenia have reduced volatilization compared to older formulations, the combination of relatively high dicamba application rates and applications made during high temperatures can result in significant dicamba concentrations leaving treated fields.”

EPA officials used the teleconference as a means of gathering information on the scope of the problem, and this knowledge will be used to chart the path forward. Both groups (academics and regulatory) acknowledged the need for new tools to help manage the herbicide-resistant weed problem, but recognize that the extent of off-target injury observed in 2017 (and in some states in 2016) is unacceptable. “Difficult decisions will need to be made on how dicamba is used in the future that will preserve the value of this herbicide tool while protecting sensitive plants in the landscape,” says Hartzler.

Read Monsanto’s open letter to growers
Also last week, Monsanto posted an online letter to growers at Monsanto.com. With more than 1,000 pesticide drift complaints in total in the Midwest and Southern states in 2017, in which dicamba herbicide is named as the suspect, Monsanto has come up with a plan to deal with the situation.

The three-step approach outlined in the company’s letter to growers involves providing support to farmers who have dicamba damage symptoms, performing more research on weather and other environmental factors affecting dicamba movement, and continuing with more herbicide applicator training and education.