New formulations for dicamba are volatile; decision on 2018 use for crops expected soon

New dicamba formulations are not as volatile as old formulations, but still can cause damage, and the herbicide can move from its target sites, affecting other crops, gardens, and other plants, according to the University of Arkansas Division of Agriculture.

A 19-member Dicamba Task Force will examine the issue from every angle, will write a comprehensive report, and give a recommendation to the Arkansas State Plant Board in the coming weeks, Arkansas Agriculture Department Director of Communications Adriane Barnes told Talk Business & Politics.

Dicamba, used to kill weeds such as pig weed, is under a 120-day ban statewide after hundreds of damage reports were lodged with the state. ASPB will have to decide by October if the herbicide will be banned or how it might be limited during the 2018 growing season, Barnes said. Farmers will need to know by then if they will be able to use it. The Task Force is slated to meet at the Winthrop Rockefeller Institute on Petit Jean Mountain on Thursday (Aug. 17). The Task Force was formed by Gov. Asa Hutchinson, Arkansas Agriculture Secretary Wes Ward, and ASPB Director Terry Walker.

“It will be included,” Barnes said of the report. “The Task Force will have to come to a consensus about what to do going forward.”

Arkansas is among several states to ban dicamba. ASPB has received at least 876 complaints this year, and the number is expected to rise. It’s an effect weed control herbicide, but it’s volatile and can damage other crops.

Dicamba has been used as an herbicide for more than 50 years to manage 200 broad leaf weeds. It is a Weed Science Society of America Group 4 synthetic auxin – a plant hormone that causes plants to exhibit uncontrolled growth, according to the U of A. Dicamba works by mimicking auxin, a plant growth hormone, disrupting cell division. It is more volatile in warmer climates.

ASPB decided earlier this year to allow one formulation, Engenia dicamba, to be used in the state to fight pig weed, an aggressive weed that has plagued farmers in recent years. About 35% of the state’s 3.5 million soybean acres were planted with genetically-altered, dicamba tolerant seeds. About 75% (300,000 acres) of the state’s cotton crop was planted with dicamba resistant seeds. Complaints began in mid-May, and by June it had turned into a torrent. A vast majority of the complaints came from Northeast Arkansas, a prime soybean and cotton growing area in the state.

Scientists theorized dicamba was drifting into adjacent crop fields, gardens, and others. Misapplications, weather conditions, or some other unknown factors may have caused the drift. Tests proved the new formulations were less volatile than older ones, but there was still volatility, and it could last up to 36 hours after it was sprayed, weed scientist Jason Norsworthy said. Dicamba can attach to dust particles, meaning it can travel much further from target sites than previously thought.

“Although the new formulations are reduced in the amount of volatility that you can see, they’re not zero,” weed scientist Tom Barber said. “And we don’t know the level of volatility that’s required to injure soybeans. Soybeans are so sensitive, very, very low levels of volatility can cause injury.”

Some additives enhanced the volatility. Ammonium sulfate and glufosinate increase the damage capabilities of dicamba. Researchers found damage could spread up to 220 feet away from an application site, nearly double the buffer distance the Environmental Protection Agency requires between dicamba and non-dicamba fields. Tests showed that if the wind blew in one direction for several hours drift could cause damage on adjacent fields, and then if the wind changed hours later, the same amount of damage would be caused there.

How this will impact yields is still unknown. Rate of exposure, how many times a plant has been exposed, the growing stage of the plant at the time of the exposure, environmental conditions, and others will have to be factored. Some studies have shown a late-season replant can cause lower yields than allowing a damaged plant.

The task force report should be completed and made available to the public within the next three weeks, Barnes said. It will be presented to ASPB, Hutchinson, and the Arkansas Agriculture Department.

Future weed control options are on the horizon. Weed scientist Bob Scott said the HT3 soybean from Monsanto will be able to tolerate glyphosate, glufosinate and dicamba. Glyphosate is known commercially as Roundup. Glufosinate is known as Liberty. Scott said these beans may be available as early as 2019, but in limited amounts. Also on the horizon, possibly as early as 2018, are the Enlist soybeans. Scott said the beans are only awaiting Chinese approval for their being legal to use in the U.S.

“We observed very good pigweed control with these technologies,” he said.

Link here for a PDF dicamba fact sheet.