Dicamba herbicide drift injury
COMMON SYMPTOM: Dicamba herbicide drift injury to non-resistant soybean varieties is expressed as cupping and crinkling of soybean leaves. Pesticide complaints remain high in Iowa for the 2018 growing season.

Dicamba drift issues continue to haunt Iowa

Number of dicamba off-target complaints remains high in state.

The number of complaints of pesticide misuse continues to be significant in Iowa. From spring through mid-August, the Iowa Department of Agriculture and Land Stewardship received 238 ag-related pesticide misuse reports, compared to 248 total pesticide misuse complaints last year.

About 47% of the ag complaints this year involve a growth regulator herbicide, and most of those complaints involve herbicide drift. Dicamba and 2,4-D are the most widely used growth regulator herbicides in Iowa, but several other growth regulator herbicides are used.

Last year saw a big increase in drift complaints as new formulations of dicamba came on the market for use on dicamba-resistant soybean varieties. Problems arise when the dicamba herbicide applications drift to nearby fields where nonresistant soybeans are planted, damaging the nonresistant soybeans.
risk higher with postemergent

Challenging weather conditions for application this spring and summer contributed to more investigations of off-target herbicide damage by the IDALS Pesticide Bureau. However, weather isn’t the only factor causing this problem.

Because of the extreme sensitivity of soybeans and many other plants to dicamba, it takes a much higher level of management to minimize the risk of off-target injury when using dicamba, especially in June and July when temperatures are higher.

Thus, the risk of drift occurring with a postemergence application of dicamba is greater than with dicamba applied preplant. To reduce the potential for dicamba drift and related problems, Iowa State University Extension weed management specialists Bob Hartzler and Mike Owen recommended that dicamba be applied only preplant in Iowa in 2018, and not applied postemergence. In addition to increased drift potential, there’s also the volatilization issue with dicamba.

Evaluating precautions
Off-target injury associated with dicamba application on dicamba-resistant beans led to a record number of pesticide misuse investigations by IDALS last year. Nationwide, estimates were 3.6 million acres of soybeans injured by off-site movement of dicamba during 2017, including 150,000 acres in Iowa.

Off-target dicamba damage shows up in several ways, says ISU’s Hartzler. Those include physical drift from application in windy conditions, volatilization of the product after it is applied, runoff from fields when heavy rains occur following application, and improper clean-out of sprayer tanks.

Prompted by last year’s troubles, the U.S. Environmental Protection Agency implemented new labeling requirements and increased separation distances for spraying dicamba over the top on soybeans, hoping to curb the problems.

EPA also required farmers and commercial applicators who planned to apply dicamba in 2018 to attend training sessions last winter. EPA is now evaluating how successful the label changes and the required training have been in reducing off-target movement from applying dicamba.
acres damaged in 2018

Soybean varieties with the Xtend trait are resistant to dicamba herbicide; those without the trait are susceptible to injury from dicamba. Hartzler has been reluctant to provide estimates of the number of Iowa soybean acres damaged in 2018 from dicamba applied to Xtend soybeans due to the difficulty in developing a realistic number of affected acres. 

“While there has been a significant number of acres damaged by dicamba this year, I’m sure it is less than 5% of Iowa’s nearly 10 million soybean acres,” Hartzler says. 

“Due to this relatively small number of acres affected, in relation to total soybean acres in Iowa, dicamba injury will not significantly impact Iowa’s productivity in 2018,” he says. “However, if you are a farmer whose crop has been damaged by dicamba, the fact that the majority of soybean acres in the state were not affected is of little consolation.”

To get a better handle on the extent of dicamba injury across Iowa, Hartzler in mid-August asked ISU Extension field agronomists located around the state to complete a brief online survey. 

Half of the agronomists said the number of soybean acres damaged by dicamba was similar to 2017, whereas the remainder were split between fewer acres and more acres damaged in 2018 than 2017. 

“When I’ve asked commercial agronomists the same question, the range of responses is similar to those of my Extension colleagues,” Hartzler says.

Volatility involved in some cases
More than 75% of the ISU field agronomists polled felt volatility was involved in at least 25% of the drift cases they investigated, while 25% of the agronomists thought movement following application played a role in over 50% of the incidences they investigated.

Complaints to state regulatory agencies is one measure EPA will consider in its upcoming decision regarding future use of dicamba on Xtend soybean varieties. 

“We know the reported incidences represent a very small fraction of total drift cases, as farmers are reluctant to involve regulatory agencies,” Hartzler says. “Many farmers just don’t report that their soybeans were injured.”

Most of the ISU Extension agronomists in Hartzler’s survey said IDALS was contacted in less than 25% of the dicamba cases, and no one in the survey said IDALS was contacted in the majority of cases.

Off-target movement still problem
Most growers using the Xtend system are happy with the increased performance in weed control obtained with dicamba compared to alternatives, the survey shows. However, one ISU Extension agronomist said farmers planting non-dicamba-resistant soybeans in his area “are really upset with the continued off-target movement of dicamba.” 

Based on what Hartzler has observed and heard from talking to farmers, commercial applicators and others, he says,

“It is my opinion that the new label restrictions put into place by EPA following the 2017 growing season, and the training required for applicators of the new dicamba products, have failed to reduce off-target problems to an acceptable level,” Hartzler says.

EPA label revisions
EPA officials recently held two teleconferences with academic weed scientists from states where the new dicamba herbicide products are registered. In those conversations, there was near unanimous agreement that the level of off-target injury observed in 2018 is unacceptable.

EPA officials asked for suggestions on label modifications that could reduce problems in the future. Hartzler says the following ideas were put forward:

• All herbicide products containing dicamba should be labeled as restricted-use products.

• Volatility is viewed as a contributing factor to off-target damage, thus some sort of temperature restriction should be implemented.

• Date restrictions for application are viewed as more effective than the current growth stage restriction, but they would need to be state-specific.

• Better clarification is needed between sensitive and susceptible crops.

• Buffers need to be established 360 degrees around rather than downwind.

EPA officials said they plan to announce their decision in the near future so farmers and others in the ag industry will know the status of the technology before making 2019 seed purchases. 

“Off-target movement of dicamba is complex. There is no simple solution. And whatever action EPA takes will not make everyone happy,” Hartzler says. “Agriculture must do a better job managing pesticide applications, so we can continue to have these valuable crop protection tools available to use.”

TAGS: Weeds
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