Dicamba Details

Why Dicamba Drifted Last Year

Some fields last year showed classic dicamba injury symptoms, but those that had the least visual symptoms sometimes had the most yield losses. (DTN file photo)

DECATUR, Ill. (DTN) -- Kevin Bradley wants to clear up a few misunderstandings about last summer's dicamba debacles.

"Some farmers in the Midsouth misused dicamba last year. There's no question about that," said Bradley. "But comments that the recently approved formulations are going to magically prevent drift and injury are misleading.

"My hope now is that farmers in other parts of the country learn from what happened, so nothing like this will happen again," he said. Dicamba injury complaints were concentrated in the Missouri, Tennessee and Arkansas, but there were some incidents reported in the Midwest.

Bradley, a University of Missouri weed scientist, walked a lot of fields connected with the 124 dicamba-related complaints filed with the Missouri Department of Agriculture in 2016. His job isn't to investigate drift complaints, but it is to advise farmers on weed control best management practices.

"Growers that sprayed weren't anxious to tell us what they did, so we are still sorting some of this out. But seeing it firsthand allowed me to draw some general conclusions about the situation," he added.

There are four general ways off-target movement happens -- physical drift, temperature inversions, volatility and sprayer contamination -- and all are very different, Bradley noted. The first dicamba injury case in Missouri was reported toward the end of June and these cases continued throughout the month of July. Generally, it takes about 10 to 14 days to see visual symptoms of dicamba. Here are 10 lessons he'd like spray applicators to consider as they prepare for next year:

1. Soybeans (non-tolerant to dicamba) are super sensitive to dicamba. "This is unlike anything we've ever dealt with before," Bradley said. "You get away with drifting a little bit of glyphosate or Liberty [glufosinate] or probably a lot of other herbicides -- it happens all the time. With dicamba, you're going to think everything went OK; then, you're going to start seeing places where it moved."

2. Physical particle drift caused the most complaints. Bradley estimated that 60% of the fields he examined had physical drift -- which usually exhibits itself in distinct patterns of damage along the edges of fields. Some fields were hit multiple times. Since there was no labeled herbicide available, growers were guessing at nozzles, droplet size and likely had a high degree of driftable fines in their spray.

3. Temperature inversions likely accounted for the majority of the remaining injury, Bradley estimated. When whole fields are damaged consistently from one end to the other, he thinks temperature inversions are most likely involved. This is when cool air gets trapped below warm air late in the day when the wind is still. As air starts to mix again, herbicide that has volatilized, or turned to vapor and mixed with air particles, can move. Less volatile dicamba formulations could presumably reduce particles that get carried in a temperature inversion, but Bradley said independent data backing this up is lacking.

University of Missouri weed scientist Mandy Bish has sensors placed around Missouri to monitor temperature inversions. There were 24 inversions recorded in southeast Missouri (where most of the damage happened) during June 2016 that generally started at 5 p.m. and lasted for eight to 10 hours. "We need more information, but that's almost one temperature inversion per day, and that has huge implications for anyone spraying in 2017," Bradley said. "We do know some of the damage came from growers spraying at night."

4. Volatility was a final factor in the Missouri drift issues. Since last season, EPA has approved two new lower-volatility dicamba formulations -- Monsanto's XtendiMax with VaporGrip and BASF's Engenia. The companies claim the formulations lower volatility by approximately 90% compared to DGA (Clarity) dicamba formulations. However, adding ammonia sulfate (AMS) and other unapproved tank components can also wipe out the advantages of the new lower-volatility formulations.

5. Both DMA (Banvel) and DGA (Clarity) formulations were used to spray in Missouri last summer. That's important, Bradley said, because the rumor is it was only old, highly volatile DMA formulations that caused problems. The new XtendiMax formulation is a DGA-based salt with a reduced volatility component added. The new formulations are just about volatility reduction, Bradley said. The new formulations do not change the potential for particle drift or what happens if dicamba touches a sensitive plant.

6. High winds weren't the issue. Bradley said wind speed records (using 5-minute averages) indicated southeast Missouri wasn't all that windy in June and July last year. "Everything we saw would have met the 15 mph restriction stipulated on federal labels for the new formulations," he said. He's recommending applicators use 10 mph as the upper limit.

7. Visual damage is not always evident. Bradley said soybeans damaged earlier in the season will show typical leaf cupping and other symptoms. However, his research and last summer's experience showed visual responses are harder to see in soybeans that are at flowering stages or beyond. Drifting on to more mature soybeans causes more yield loss, though. At R-2 growth stage, he's documented a 9-bushel-per-acre yield loss with a 1/200th of a 1X rate of dicamba.

8. Tissue testing doesn't necessarily give final answers. Many of the Missouri samples taken last year came back with a non-detect for dicamba, although they had symptoms. "There was no doubt in my mind that it was dicamba, but too much time had passed between the spray incident and the test," he said.

9. Tank and sprayer contamination is real. "Our research shows 8 ounces of a dicamba solution (not the pure product) left in the tank will cause foliar injury. One gallon of solution left in the tank will cause significant yield loss," Bradley said.

10. Sensitive crops are everywhere. Beyond soybeans, Bradley said trees such as red oak, white oak and pecan have shown extreme sensitivity, as well as tomatoes.

The Missouri Department of Agriculture officially pegged the number of Missouri soybean acres damaged by dicamba at 45,000 acres. Cantaloupe, tomato, purple hull peas, watermelon and peaches were also reported injured.

"Farmers don't like to turn in farmers and, unofficially, my conservative estimate is 100,000 acres of soybean damaged in this state last year," Bradley said. Monsanto, DuPont Pioneer, Syngenta and many licensees are offering Xtend soybeans and cotton this year, so acreage figures are expected to jump.

"I don't think there was any malicious intent by those that sprayed in 2016, and I know many had regrets and wished they could have taken back the action," he said.

"The social aspect of what happened in 2016 cannot be understated," Bradley added. "There's no training in the world that prepared me for what I dealt with this summer. There are neighbors that have known each other for decades and go to church together that no longer speak. We've seen violence happen because of an herbicide.

"The stories of those that wouldn't turn in their neighbor, but are looking at $10,000 and more in losses is a sad thing to listen to all summer. Get yourself to a meeting and learn how to do this right," he recommended. "Keep records and know what your neighbor is growing and where."

Pamela Smith can be reached at Pamela.smith@dtn.com

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