Nearly 20 years ago, Tom Wahl read about companies researching how to genetically modify soybeans to tolerate growth regulator (Group 4) herbicides, such as dicamba and 2,4-D.

This concerned Wahl, who grows chestnut and other trees with his wife, Kathy Dice, near Wapello, Iowa. Volatility and growth regulator herbicides often worked in tandem, akin to a hot dog and mustard. Volatility occurs when a herbicide converts to a gas that leaves an application site and damages plants where it lands, sometimes miles away.

Corn farmers had used dicamba for decades. However, an Iowa State University analysis showed it was often applied at low rates and early in the season to avoid off-target damage to nearby plants. Wahl feared spraying soybeans tolerant to such Group 4 herbicides over widespread areas could increase tree damage potential.

He warned others. “But nobody paid much attention until they actually started being used,” he says.

Well, yes.

Off-target movement via volatility combined with litigation has plagued dicamba used in dicamba-tolerant soybeans from the start. The latest chapter came last February, when a U.S. District Court in Arizona vacated registrations for three dicamba formulations:

As it stands now, these formulations cannot be applied to dicamba-tolerant soybeans in 2025.

Proposed labels

Bayer has submitted a proposed 2025 label to the Environmental Protection Agency for the dicamba formation KHNP0090, formerly known as XtendiMax. This label allows two applications prior to soybean emergence up to June 12. Unlike past labels, it allows no postemergence applications.

“Bayer, grower groups and others have expressed to the EPA the importance of prioritizing review so growers can have access to the technology as soon as possible,” wrote a Bayer official in an email. “We hope the EPA will continue to move swiftly so growers have access to the technology as soon as possible.”

BASF also submitted a dicamba label to the EPA for Engenia. It allows for preemergence and/or postemergence applications up to the V2 stage or June 12, whichever comes first. (The V2 stage is when two trifoliate leaves completely unroll at around a 6- to 8-inch height.)

“ASA [American Soybean Association] fought to keep over-the-top [postemergence] applications, because they’re a crucial part of the technology,” says Alan Meadows, a farmer and ASA director from Halls, Tennessee. “BASF did give us a little bit more room to operate after soybeans come up. With the Bayer label, it’s strictly preemergence.”

It’s important to note these labels are proposals, not final ones, say officials for both companies.

If these label proposals stand, though, preemergence dicamba will most likely require tank-mixing with other herbicides that have extended soil residual activity, says Aaron Hager, University of Illinois Extension weeds specialist.

“One factor that has been terribly overlooked is the fact that dicamba has soil residual activity, but most often only until the first rainfall event,” he says. “After that, it’s gone. For this, you will need an effective post-application.”

Company officials say any tankmix herbicides would not be known until after label approval.

These proposals face an EPA approval process to ensure compliance with the Federal Insecticide, Fungicide, and Rodenticide Act and the Endangered Species Act. Bayer’s label was submitted as a R170 label, which is an additional food use label, says Brigit Rollins, staff attorney for the National Agricultural Law Center.

“Under FIFRA, an R170 label has a mandatory review period of 17 months,” she says. “Just doing the math, it’s hard to see how this label would be approved before the 2025 growing season.”

How we got here

The dicamba-tolerant soybean system has faced a rocky road over the past eight years. Citing farmer demand for high-yielding soybeans, Monsanto (now owned by Bayer) released dicamba-tolerant soybeans in 2016 without matching dicamba herbicides.

“What happened is some of those 2016 acres were sprayed with [illegal] dicamba,” Hager says. Off-target movement resulted, as the generic formulations had no volatility reducing agents nor rigid application guidelines to reduce volatility.

In 2017, dicamba-tolerant soybeans were matched to EPA-approved dicamba formulations in the Roundup Ready 2 Xtend System. When properly applied, good to excellent weed control results from these formulations, particularly on the soybean scourge of waterhemp, Hager says.

However, Wahl’s premonition regarding volatility came true after the debut of dicamba-tolerant soybeans. “State departments of agriculture soon reported an uptick in off-target dicamba complaints,”Rollins says.

In Iowa, complaints about growth regulator herbicides (which includes dicamba) jumped from 43 in 2016 to 171 in 2017. An analysis by the University of Missouri showed dicamba injured 3.6 million acres out of 89.5 million soybean acres planted nationwide in 2017.

In 2021, Harry Stine, CEO of Stine Seed Co., called for an end to dicamba use on dicamba-tolerant soybeans, due to off-target damage to thousands of Stine soybean plots.

“Dicamba is a great herbicide, but it has off-target movement and you can’t eliminate that,” says Myron Stine, Stine president.

Wahl was affected, too. The trees he and Dice raise had been previously hit by alachlor (Lasso) and metolachlor (Dual Magnum). Still, more extensive damage from dicamba resulted in 2017, when affected chestnut tree leaves were tightly curled and 10% to 15% smaller than affected ones.

Commercial chestnut tree damage was not permanent, Wahl says. They’re also protected by a one-fourth to one-half mile buffer of pasture, forest and Conservation Reserve Program acres on the south and west side of the orchard. Still, unprotected non-commercial chestnut trees continue to be damaged, Wahl says.

“We had a lot of damage to our pear trees that were adjacent to a crop field to the north [minus a buffer], and they still haven’t set fruit like before,” he adds.

Restrictions — accompanied by myriad paperwork — were added to reduce off-target potential in 2018 and 2020 registrations. In turn, this made it more difficult to use dicamba as a postemergence herbicide.Data in 2020 compiled by University of Minnesota Extension educators Jared Goplen and Dave Nicolai showed central Minnesota farmers had fewer than 40 hours they could legally apply dicamba from June 1 to June 15, 2020.

These complications allowed Enlist E3 soybeans to grow in use, garnering 55% of the U.S. soybean market, Myron Stine says. He adds they are easier to manage, as the 2,4-D choline that’s the bedrock of the system has not been as prone to off-target movement and has not endured dicamba’s restrictions.

Damage to Stine seed plots has also decreased since 2021, mainly due to more Enlist E3 plantings, he says.

“The heart of our [varietal] testing is in areas where most of the market is using the Enlist E3 technology,” Stine says. “It’s still an issue, but less of an issue.”

What now?

“Dicamba still has utility in this [preemergence] time frame,” says Meaghan Anderson, an ISU Extension field agronomist. “It provides a lot of flexibility for burndown applications. More importantly, it is going to be safer to use [rather than postemergence] as long as it’s applied on-label. We’re also spraying residue, rather than green [crop and weed] tissue.”

Since warm temperatures fan volatility, applying dicamba earlier increases the chances of cooler temperatures and less volatility, she adds.

Industry officials say they have also made strides to reduce off-target movement.

“We’ve done a lot with engineering formulations, training and investments in our application technology,” says Paul Rea, BASF senior vice president of agricultural solutions, North America. “We can control drift by applying at the right time of day and using the right technology. The number of drift and off-target movement complaints has dropped substantially over the last number of years. And so that tells us that farmers have used all these tools.”

Hager’s skeptical.

“If the company line continues to be that everything is going to be fine and dandy if these products are applied according to label, why do they keep moving up the application dates and making other application restrictions?” asks Hager.

Rather than date, he says temperature is more of an important volatility factor.

“On average, temperatures should be cooler in late spring, but we’ve seen 95-degree [F] days over Memorial Day,” he says. “This can happen before the June 12 application deadline. And if it’s 95 degrees and you’re spraying 600,000 to 700,000 acres in the state [Illinois] guess what’s going to happen? It’s not going to make any difference [with off-target movement].”

Wahl doesn’t see earlier cutoffs helping the situation. “Whenever we would see damage [to trees], we would first see it in late April or early May,” he says.

Certainty, consistency and choice

Others are concerned nixing dicamba would rob farmers of a weed management tool.

“This would be catastrophic not to have any kind of dicamba label next year,” Meadows says. “It’s a technology that is relied upon heavily to control [herbicide-] resistant weeds.”

“This is tied to choice, to give farmers more options to farm in the most sustainable manner possible.” says Mark Reisinger, interim CEO of the Agribusiness Association of Iowa. The vacating of labels has created uncertainty, much to the detriment of farmers, applicators, retailers and others in the supply chain, he says.

“A number of our retailers used it [dicamba on dicamba-tolerant soybeans] when it initially came out, but have been hesitant since then because of the unavailability,” he adds. “Their customers don’t want the disruption in the technologies they’re using. What they are looking for is certainty, consistency and choice.”