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Dicamba Questions
Monday, September 18, 2017 12:15PM CDT

By Emily Unglesbee
DTN Staff Reporter

ROCKVILLE, Md. (DTN) -- Among all the buzz and blame surrounding the dicamba injury crisis of 2017, Purdue University weed scientists Bill Johnson and Joe Ikley have tried to answer a simple question for farmers.

How often could an applicator legally spray dicamba this summer?

The answers they found are important, especially for growers weighing seed and herbicide decisions this fall.

In the 2017 summer, under the current federal dicamba herbicide labels, the researchers found that growers had 334 hours to spray in June (46% of the month) and 267 hours in July (36% of the month) at their test location near West Lafayette, Indiana.

But under more restrictive rules, namely the ones Missouri passed this summer in response to injury complaints, that window shrank considerably. When spraying was limited to 9 a.m. to 3 p.m. and within wind speeds of 3 to 10 mph, applicators had only 49 hours in June (7% of the month) and 101 hours in July (14% of the month) to legally spray dicamba.

Dicamba use restrictions are already under consideration at the state level for 2018, and the EPA is expected to add its own changes soon. With Monsanto aiming for 40 million to 50 million Xtend soybean acres next year, that leaves growers with a dire reality, Johnson said.

"Growers need to understand how very hard it is to use this technology safely," he said. "We do not have the sprayer or sprayer operator capabilities in any of these states to spray all the necessary acres within these spray windows."


Johnson and Ikley combed through 2017 weather data from the Purdue Automatic Weather Station (PAAWS) at the Agronomy Farm for Research and Education near Lafayette, Indiana.

They used wind speeds taken every two seconds along with rainfall data to figure out which days this summer were safe to spray under the XtendiMax label. That meant no sustained wind speeds over 15 mph -- including gusts.

"One of the big things we learned is applicators are looking at weather forecasts and looking at sustained wind speeds rather than gusts," which are a dangerous source of physical drift, Johnson pointed out. His analysis included wind gusts, which reduced the number of safe spray hours.

The researchers also ruled out wind speeds below 3 mph, as the XtendiMax label requires. Data from the University of Missouri has shown that 90% of temperature inversions in June and July of 2015 and 2016 occurred when wind speeds were below 3 mph, said MU weed scientist Mandy Bish.

"It seems like a pretty reliable indicator of inversions," which have also been fingered as a source of dicamba drift this year, she said.

Johnson and Ikley also eliminated days with rain in the 24-hour forecast, as the label specifies, as well as days when the ground was too wet to hold a sprayer.

The result was 13 "ideal" days to spray in June (a day with more than eight safe spraying hours), 13 days that were unsafe to spray and four with a scattered amount of safe spray hours. In July, that Indiana location had 18 ideal days and 13 no-spray days, but that number did not account for the R2-growth-stage cutoff on the XtendiMax label, Johnson noted.

"Around here, a lot of beans were planted in late April, so my guess is most fields were into R2 by the Fourth of July," he said.

But Johnson is confident that growers sprayed beyond the R2 cutoff in Indiana this year. "We had spraying even into August," he said.


No one knows exactly what the EPA will release regarding dicamba use in 2018. But recent state actions give us good insight into the possibilities, Johnson noted.

Indiana is in the process of making dicamba a restricted use pesticide, and a state task force is weighing additional label requirements for next year.

Missouri instituted an emergency set of rules this summer that limited spraying to the hours of 9 a.m. to 3 p.m. and narrowed the wind speed window to 3 to 10 mph.

These restrictions are well within the realm of possible EPA and state restrictions for 2018, so Johnson and Ikley also crunched the weather data with them in mind.

The results -- just five safe days to spray in June and not a single June day with eight consecutive safe spraying hours -- were alarming, Johnson said.

"June is the ideal time to spray weeds," he noted. Once aggressive weeds like marestail and waterhemp grow past 4 inches high, they are far harder to kill, and spraying them is technically an off-label action.

With 11 safe spray days, July fared better under the Missouri rules, but at that point in the season, weeds are often too large to control, and the R2-growth-stage cutoff looms, Johnson pointed out.

Growers should carefully consider these narrow spray windows under restrictive dicamba use rules as they purchase seed for next year, Johnson warned.

Consult carefully with your seed dealer over any purchases, he said.

"Hopefully, their seed suppliers offer them flexibility if new label restrictions render these products unusable in some specific situations," he said. "The other reality is potential cut-off dates, meaning some states may say growers can't spray this in June or July at all."

Arkansas, for example, is currently weighing an in-season ban on dicamba spraying that would only permit applications from Jan. 1 to April 15 in 2018. See the DTN story here: http://bit.ly/….

See Johnson and Ikley's spray day calculations here: http://bit.ly/….

Emily Unglesbee can be reached at Emily.unglesbee@dtn.com

Follow Emily Unglesbee on Twitter @Emily_Unglesbee


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