Dicamba's dilemma is its great weed control results and volatility
Adam Martens of Inman, Kansas, has a predicament. He’s happy with the soybean weed control he gleans by applying a preemergence residual herbicide mix followed by a postemergence combination of glufosinate (Group 10), glyphosate (Group 9), and 2,4-D choline (Group 4) on Enlist E3 soybeans.
He’s unhappy, though, with an uninvited herbicide that he says damages his soybeans.
“We just can’t keep the [off-target] dicamba off our beans,” he says.
Off-target movement, particularly in later soybean reproductive phases, can clip yields.
University of Missouri (MU) research shows 1/200 of a labeled rate for dicamba caused 14% yield loss after an R2 (full flower) application, says Kevin Bradley, MU Extension weed specialist.
Harry Stine empathizes with Martens. “In the fall of 2017, we’d already had a lot of damage [from off-target dicamba] in our research plots,” says the founder of Stine Seed Co. The firm’s genetics make up a substantial portion of U.S. soybean varieties. Dicamba injury from off-target movement has increasingly occurred since then in Stine soybean plots, he says.
“These are new varieties that will come out over the next few years,” he says. “That’s tremendous damage to the industry.”
Dicamba has had a split personality since the Environmental Protection Agency (EPA) registered the Group 4 herbicides XtendiMax (Bayer) and Engenia (BASF) for use on dicamba-tolerant soybeans in 2017. The EPA later approved Syngenta’s Tavium that teams dicamba with S-metolachlor (Group 15) in 2019. The current 2021 to 2025 federal label places a June 30 application cutoff date for all three. However, Tavium has an earlier application window, as it cannot be applied after the early vegetative V4 stage.
“Dicamba is a great tool for managing troublesome herbicide-resistant weeds such as waterhemp and Palmer amaranth,” says Prashant Jha, an Iowa State University (ISU) Extension weed specialist. That’s conditional upon using it in combination with other tools, such as residual preemergence herbicides, he adds.
Volatility, though, is dicamba’s weakness, as even these formulations pegged as lower in volatility can volatize into a gas and move to nontarget vegetation, says Aaron Hager, University of Illinois Extension weed specialist.
“Volatility doesn’t follow the label restrictions,” he says “Volatility is driven by the herbicide molecule itself. You just never know at the beginning of the year what the scope and magnitude of it will be.”
Off-target damage is higher in areas of extensive dicamba use, such as the Bootheel in southeastern Missouri, says Bradley.
“This is the highest area for rate of adoption anywhere in the U.S. If U.S. agriculture moves the needle toward going all the way [with dicamba use], I believe we have an example in southeastern Missouri, northeastern Arkansas, and western Tennessee as to what it will look like,” says Bradley, referencing other areas with high dicamba injury.
Statewide, Missouri complaints to the Missouri Department of Agriculture have declined from 120 in 2020 to 53 in 2021. In other states, off-target dicamba complaints stayed steady to even slightly down.
The Minnesota Department of Agriculture investigated 127 dicamba damage complaints in 2021, up slightly from 2020’s 116 cases. Still, off-target dicamba injury and symptomatology is likely higher, say university specialists.
“Cupping of several trifoliates was common on about 40% of the soybean acres in mid-July, creating lots of discussion and concern,” says Joel DeJong, an ISU field agronomist based in northwestern Iowa. Symptomatology occurred in organic, conventional, and non-tolerant dicamba soybean fields, he adds.
Nor do official complaints always turn into action. Bradley believes the amount of Missouri dicamba injury is higher than that reflected in official complaints.
“Farmers don’t like to turn in other farmers,” he says.
Meanwhile, no significant changes have occurred due to turning complaints into the Missouri Department of Agriculture in the past.
That’s been Martens’ experience in Kansas. “There are no repercussions for not following the rules,” he says.
Still, declining numbers in some states coincide with the trend BASF sees with Engenia, says Nick Fassler, BASF technical marketing manager. “We consistently year in and year out get less and less complaints,” he says.
Off-target issues exist, says Bill Gordon, a Worthington, Minnesota, farmer and chairman of the American Soybean Association. For the most part, though, dicamba-tolerant technology is working as advertised on his farm, he says. “This year, I planted the [seed] technology, but the timing was not right to meet the label [cutoff date],” he says. “I used a different [herbicide] technology. I have used it [dicamba] in the past and had no issues.”
“There is that perception out there that dicamba is all over the place,” says Jeff Herrmann, Bayer crop protection engagement manager. Still, most 2021 off-target dicamba complaints Bayer investigated involved incidents where dicamba moved downwind through physical drift, he says.
Actors in romantic comedies often lament this line: “Women (or Men). Can’t live with them, can’t live without them.” That’s akin to the current dicamba situation.
“We believe that our customers will need all available technologies available to combat herbicide-resistant weeds,” says Carl Peterson, president of Peterson Farms Seed, Harwood, North Dakota. “Resistance is going to get worse. Having said that, off-target dicamba movement is the real thing.”
Like Stine Seed, Peterson Farms Seed soybean test plots have also suffered off-target dicamba damage, he says.
The specter of any negative impact on future soybean varieties from off-target dicamba concerns Stine. “If you get equal dicamba damage across plots, in theory, we’re probably still able to separate the good and bad lines,” he says.
Concerns are higher with greater blocks of land and thousands of soybean lines, where damage could be unequal and mask varietal differences.
“Bayer and Monsanto have done a lot of good for agriculture and a lot of good for us,” Stine says. “We signed a major collaborative agreement with Monsanto in 1997, and we’ve been producing products with our germplasm for their system ever since.”
However, he says a number of other high-yielding herbicide-tolerant options, such as Enlist E3 (tolerance to glufosinate, glyphosate, and 2,4-D choline) and LibertyLink GT27 (tolerance to glyphosate, glufosinate, and Alite 27, a Group 27 herbicide) and other herbicide-tolerant stacks control weeds well without the baggage of dicamba.
“There’s absolutely no need for dicamba,” he says. “We don’t like to derive revenue from something that’s harmful to agriculture and society.”
Herrmann counters that EPA’s five-year reregistration granted in October 2020 assured that dicamba-tolerant crops and accompanying dicamba herbicides are durable and sustainable.
“Cases that we have investigated are things we can fix, such as spraying when there is a susceptible crop downwind like Enlist [E3] soybeans,” he says.
BASF stresses continued education, such as proper tank mixing and tank hygiene, and other label guidelines are crucial in ensuring that optimal on-target applications occur, says Fassler.
“It is absolutely critical to follow the label for Tavium and any product,” adds Pete Eure, Syngenta technical product lead for herbicides.
What EPA Says
Ultimately, dicamba’s fate lies with the EPA, which issued this statement at press time: “EPA is extremely concerned about these reports and is taking steps to better understand the nature and severity of these incidents in order to assess the sufficiency of the mitigations in the 2020 [reregistration] decision and, as necessary, take appropriate regulatory action.”
The EPA has asked makers of dicamba formulations applied to dicamba-tolerant soybeans to submit unreasonable adverse impacts of the pesticide to EPA, according to an EPA spokesperson.
EPA also held dicamba listening sessions with several parties this fall.
“It’s not like we hate Bayer,” says Martens, “but either figure out how to get it not to float around, which everyone knows it does, or get rid of it.”
Weather Influences Symptoms
Weather helped determine whether herbicide injury symptomatology occurred in 2021, says Scott Kay, BASF vice president of U.S. crop protection.
Excellent weed control and minimal if any dicamba damage occurred in areas with adequate rainfall.
It was a different story in droughtstressed areas.
“There was the most dramatic show of symptomatology from everything [all herbicides applied] this year in those highly stressed areas,” says Kay.
Rainfall doesn’t necessarily make the symptoms of dicamba injury, such as cupped leaves, vanish.
However, it does allow plants to continue to grow, says Aaron Hager, University of Illinois Extension weeds specialist.
“A lot of times, you won’t see symptomatology on the next set of leaves,” he says.