A ‘perfect storm’

Deregulated herbicide trait and illegal herbicide use converge for historic drift injury to crops.

Nov-Dec 2016 Soybean SouthSoybean growers in several states are dealing with the aftermath of a “perfect storm” created by dicamba-resistant cotton and soybean varieties and illegal use of dicamba herbicide.

The result is thousands of acres of crop injury and hundreds of complaints filed with regulatory officials in 10 states about off-target movement of illegally applied dicamba.

Labels of currently registered dicamba products do not allow in-season over-the-top applications to Xtend crops, which have been genetically engineered to tolerate the herbicide.

The Arkansas Plant Board has received about two dozen complaints, with half of those from just two northeastern counties — Craighead and Mississippi. The Missouri Department of Agriculture has received more than 125 complaints representing about 45,000 acres, with about half from four Bootheel counties in the southeastern part of the state.

Lawmakers in Arkansas continue to grapple with how to address the problem, and the Plant Board has proposed several use restrictions on various dicamba formulations.

For example, it would ban DMA salt and acid formulations except on pastures and rangeland and then only if susceptible crops are at least 1 mile away.

The board also seeks to prohibit applications of DGA salt and sodium salt formulations from April 15-Sept. 15, except on pastures and rangeland with a 1-mile susceptible crop buffer.

In addition, it is proposing a 100-foot buffer in all directions with a 1/4-mile downwind buffer for Eugenia’s BAPMA dicamba formulation once the EPA registers it.

The proposal is undergoing a 30-day comment period. The board plans to hold a public hearing during a specially called meeting on the adoption of the proposed dicamba rules. It is scheduled for 1:30 p.m., Nov. 21.

If the board adopts the rule, it will be sent to the Legislative Council for review. If the council approves the rule, it will be filed as a final rule with an effective date.

Drift hits some regions harder than others

dicamba injury
Pegging yield loss from dicamba injury is difficult. It depends on the plant’s growth stage, cultivar and actual amount of herbicide, among other variables —photo courtesy University of Arkansas .

Tom Barber, a University of Arkansas Extension weed specialist based in Lonoke, says he suspects Northeast Arkansas was hard hit because Xtend soybean and Xtend cotton varieties were more widely planted there than in other parts of the state. And with that came a larger number of producers who may have illegally applied dicamba.

“(Damage) was all over – that’s just a relative term, but it wasn’t hard to find,” he says. “Soybeans are extremely sensitive to dicamba, so just a small amount will produce visual symptoms.”

Kevin Bradley, associate professor of weed science with the University of Missouri in Columbia, says it’s difficult to know exactly what caused drift injury in particular fields.

“You get information from people who were drifted on and you don’t get very much from people who have done it,” he says.
But he and his colleagues have some strong theories. The Bootheel, the northeastern part of Arkansas and eastern Tennessee — the three areas where the problem has been concentrated — all have similar cropping patterns that include cotton.

Cotton growers have had access to varieties with the dicamba-resistant trait for two seasons, whereas soybean growers have had access to it for just the 2016 season.

“It appears to me, based on everything I’ve looked at and visited, most of the (dicamba) applications were to cotton,” Bradley says. “There definitely were some soybeans that were sprayed too, but most of it was cotton.”

How much yield reduction growers with injured crops will experience is unknown. Researchers have conducted numerous trials that quantified injury from various amounts of dicamba drift on crops at specific growth stages.

But, as Bradley points out, “There’s no way to know how much drift actually got on Farmer John’s field on July 29.”

He and colleagues are working with a handful of growers who have drift injury and have yield mapping software in their combines. The researchers will try to correlate drift injury to yield reductions.

Most growers also have a good idea about their fields’ historical yield averages, and Bradley says they likely will be able to estimate yield reductions from drift injury.

dicamba injury
Soybeans are extremely sensitive to even small amounts of dicamba. The beans here display the telltale injury signs of cupped leaves—photo courtesy University of Arkansas

Mistimed approvals

Dicamba drift came to a head this summer after the Environmental Protection Agency in January 2015 deregulated Monsanto’s Xtend trait, which imparts resistance to dicamba herbicide in genetically engineered soybeans and cotton.

But the agency did not register the complementary Roundup Xtend and XtendiMax herbicides, which are based on diglycolamine (DGA) dicamba and that include Monsanto’s patented VaporGrip technology.

BASF has an EPA registration pending for Eugenia, a BAPMA dicamba product based on a slightly different active ingredient. It also has reduced-drift characteristics, according to BASF.

Dicamba is known for its volatility. Even when applied according to label, it can vaporize and move to neighboring crops under the right conditions. The VaporGrip technology is designed to significantly reduce that volatility and associated off-target movement, according to Monsanto.

Monsanto expects registration of its two dicamba products this fall, based on conversations with the EPA, says Boyd Carey, the company’s lead representative for crop-protection systems in North America.

Although several universities have looked at the efficacy of the dicamba-based herbicides on weeds, they have not tested the VaporGrip technology on potential drift or off-target movement, he says.

Barber says the Plant Board has historically relied on third-party research results — whether from the University of Arkansas or other universities — on which to base its decisions.

“When the Plant Board asked us to come in front of the Pesticide Committee and talk about our data, we didn’t have any data to talk about,” he says, referring to drift-related trials.

Monsanto already has developed an applicator training program to educate users about proper nozzle choices, droplet size, spray pressure, boom height, sprayer speed and weather conditions. If the EPA grants registrations, Carey says Monsanto will work with state pesticide regulators to ensure its training program addresses their concerns.

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