Out in Callaway County, Dennis Shryock has been working on the family farm for more than 40 years. This summer, he noticed that the leaves on his soybean plants had begun to curl.

Soybean plant leaves are typically flat, he said, but the leaves on his beans were “cupping,” as it is known in the industry. This is a tell-tale sign that the herbicide known as dicamba has damaged the plants.

“It will cause beans that are susceptible to dicamba to lose vigor, plant health and, possibly, yield,” he said.

Shryock is one of thousands of farmers throughout Missouri and the rest of the country who have been dealing with consequences of dicamba drift this year, where the chemical comes in contact with nearby fields during spraying.

It is a wave of damage that has affected millions of acres of crops. In Missouri alone, an estimated 325,000 acres of farmland have suffered this summer.

The Missouri Department of Agriculture has received more than 310 dicamba complaints this year, the highest in years. Most of the complaints came from farmers, particularly those in southeastern Missouri, whose crops were susceptible to chemical drift from other soybean fields. Plants not bred to be resistant to dicamba have been at serious risk of harm.

Kevin Bradley, a plant science professor at MU, said he has never seen dicamba damage on this scale before. It’s estimated that 3.1 million acres of farmland in the United States have been harmed this summer. Nationwide, more than 2,000 complaints have been made by farmers about the damage so far this season.

Most of the damage from dicamba in Missouri has occurred in the Bootheel, Bradley said, although dicamba complaints have been submitted throughout the state.

To address the problem, Missouri placed a temporary ban on dicamba in July. The ban was lifted within a week with restrictions on its use, including spraying during certain times of day and under a specific wind speed.

Yet, once the ban was lifted, more complaints began to arrive from farmers that the problem had reappeared.

Since then, farmers have been filing lawsuits against dicamba producers, and several states, along with the Environmental Protection Agency, are considering new rules for its use.

What is dicamba?

Dicamba is an herbicide developed for farmers to use on plants to eradicate troublesome weeds. According to the Mizzou Integrated Pest Management blog, it is one of many tools farmers can use when trying to deal with the weeds that damage their crops.

According to Scott Partridge, a representative of Monsanto, which produces the dicamba product Xtendimax, it is a highly volatile chemical that farmers have been using for 50 years. Other dicamba products include Banvel, Diablo, Oracle, Vanquish and Engenia.

About a decade ago, Monsanto began to adjust the formula for dicamba to give farmers “multiple modes of action” when dealing with difficult-to-control weeds, Partridge said. At the same time, Monsanto and others began to develop dicamba-resistant crops, including soybeans and cotton, so farmers could safely spray the chemical over the tops of those fields to kill weeds.

The chemical was designed to kill broadleaf weeds with the intent of increasing farmer productivity, but dicamba runs the risk of drifting into other farmer’s crops that are not dicamba-resistant.

Although it has frequently been used by farmers on dicamba-resistant corn, this summer was the first season farmers used it on soybeans.

Unlike other herbicides like Round-Up, which moves through air the same way water does, dicamba has volatility or a tendency to vaporize. This causes it to move through the air and drift onto other fields.

There is no way to completely eliminate volatility in dicamba, but Xtendimax eliminates the volatility of dicamba by up to 90 percent, Partridge said.

Bradley said dicamba drift can be caused by improper spraying, temperatures and tank contamination, among other variables. According to the Missouri Department of Agriculture, farmers are required to spray dicamba in accordance with certain restrictions that come with each dicamba product.

Product labels specify that farmers only spray dicamba at 10 mph and between the hours of 9 a.m. and 3 p.m. Operators must also be certified and keep detailed records of their use of the chemical.

Causes of drift

According to Partridge, Monsanto conducted 1,000 field visits to determine that 77 to 80 percent of the off-target movement of dicamba results from three primary causes.

The most common cause, he said, is the lack of buffer zones by farmers. The second is use of an improper nozzle. Partridge said Monsanto stipulates that the chemical be applied with a fan-type spray from a cone-shaped nozzle to produce a stream of larger droplets.

The third largest cause of off-target movement is improper boom height for a spray rig. Monsanto specifies a boom height of 24 inches, and Partridge said the company saw many spray rigs with booms much higher than the recommended height. As a result, he said, more material was sprayed into the air, which increased the off-target movement.

Bradley, the MU plant science professor, provided his list of the ways dicamba can drift off target — improper spraying techniques, tank contamination, temperature inversion and volatilization.

Improper spraying techniques include using a sprayer when the wind speed is too high or using an improper nozzle. Tank contamination occurs either by not cleaning out the tank properly and leaving some dicamba behind, or through contamination of another herbicide product.

Temperature inversion usually occurs on June and July evenings when the air closest to the surface is cooler than the air above, which moves the herbicide off-target through the air until it dissipates.

Finally, dicamba, Bradley said, can move off target through volatilization, when the chemical turns into vapor and moves through the air toward other plants.

Before dicamba was placed on the market, there were reports that universities were prevented from doing volatility testing. Bradley said he was aware of universities that were not allowed to test for the volatility of the Monsanto dicamba product, Xtendimax. When asked why, he said he was not certain.

According to Partridge, universities were not allowed to do volatility testing because there wasn’t enough time before an EPA registration process, although they can do them now. He said Monsanto conducted 1,200 studies of its dicamba product in 25 different geographies.

Volatility tests can’t be done in a laboratory, he added. Researchers need real-world locations for proper testing. It requires multiple geographies and target fields with the vegetation to be studied. There also must be an off-target down wind and surrounding fields to test for volatility.

That requires a lot of land to be tested during the growing season, he said, and there wasn’t enough time before the EPA registration process.

What’s next

Researchers are now trying to determine how the various causes of dicamba injury can be traced to environmental factors such as air and soil temperatures, humidity and moisture.

Bradley has recommended that farmers who want to plant resistant soybeans in 2018 and spray dicamba should do so on weeds that appear earlier in the season. They should avoid using dicamba on weeds that sprout later.

Apparently, farmers who sprayed dicamba in the months of April and May had fewer issues than farmers who sprayed dicamba in June, July and August.

Partridge said that giving farmers proper education could help prevent the off-target movement of dicamba. Monsanto trained around 50,000 applicators, which Partridge credits with slowing down the drift of the herbicide.

He cited Georgia as an example of proper herbicide use. There, he said, training in the application of dicamba is mandatory, and not a single complaint about off-target drifting was reported.

In Arkansas, which has seen more dicamba damage than any state, the government is moving toward a ban on dicamba use next summer, according to Reuters.

This summer, the Arkansas State Board imposed a 120-day ban and increased the penalty for misuse to as much as $25,000. Last week, the board gave initial approval to a ban on dicamba use from April 16 to Oct. 31.

In late September, the Missouri Department of Agriculture took a different tack. It announced that the current restrictions on dicamba products would remain until Dec. 1. That same week, Monsanto invited weed scientists to attend a summit over dicamba use.

Many declined to attend. They said the only reason for the summit was to win backing for a Monsanto product.

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Supervising editor is Jeanne Abbott: abbottjm@missouri.edu, 882-5741.

  • I'm a Columbian native. I'm currently doing general assignment for the Missourian. I can be reached at 573-808-3914 or jrrv23@mail.missouri.edu.

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