(Reuters) - The U.S. environmental agency is considering banning sprayings of the agricultural herbicide dicamba after a set deadline next year, according to state officials advising the agency on its response to crop damage linked to the weed killer.

John Weiss looks over his crop of soybeans, which he had reported to the state board for showing signs of damage due to the drifting of Dicamba, at his farm in Dell, Arkansas, July 25, 2017. REUTERS/Karen Pulfer Focht/File Photo

Setting a cut-off date, possibly sometime in the first half of 2018, would aim to protect plants vulnerable to dicamba, after growers across the U.S. farm belt reported the chemical drifted from where it was sprayed this summer, damaging millions of acres of soybeans and other crops.

A ban could hurt sales by Monsanto Co MON.N and DuPont which sell dicamba weed killers and soybean seeds with Monsanto's dicamba-tolerant Xtend trait. BASF BASFn.DE also sells a dicamba herbicide.

It is not yet known how damage attributed to the herbicides, used on Xtend soybeans and cotton, will affect yields of soybeans unable to withstand dicamba because the crops have not been harvested.

The Environmental Protection Agency (EPA) discussed a deadline for next year’s sprayings on a call with state officials last month that addressed steps the agency could take to prevent a repeat of the damage, four participants on the call told Reuters.

It was the latest of at least three conference calls the EPA has held with state regulators and experts since late July dedicated to dicamba-related crop damage and the first to focus on how to respond to the problem, participants said.

A cut-off date for usage in spring or early summer could protect vulnerable plants by only allowing farmers to spray fields before soybeans emerge from the ground, according to weed and pesticide specialists.

Monsanto spokeswoman Christi Dixon told Reuters on Aug. 23, the day of the last EPA call, that the agency had not indicated it planned to prohibit sprayings of dicamba herbicides on soybeans that had emerged. That action “would not be warranted,” she said.

The EPA had no immediate comment.

EPA officials on the last call made clear that it would be unacceptable to see the same extent of crop damage again next year, according to Andrew Thostenson, a pesticide specialist for North Dakota State University who participated in the call.

They said “there needed to be some significant changes for the use rules if we’re going to maintain it in 2018,” he said about dicamba usage.

State regulators and university specialists from Arkansas, Missouri, Illinois, Iowa and North Dakota are pressuring the EPA to decide soon on rules guiding usage because farmers will make planting decisions for next spring over the next several months.

Tighter usage limits could discourage cash-strapped growers from buying Monsanto’s more expensive dicamba-resistant Xtend soybean seeds. Dicamba-tolerant soybeans cost about $64 a bag, compared with about $28 a bag for Monsanto’s Roundup Ready soybeans and about $50 a bag for soybeans resistant to Bayer’s Liberty herbicide.

Already, a task force in Arkansas has advised the state to bar dicamba sprayings after April 15 next year, which would prevent most farmers there from using dicamba on Xtend soybeans after they emerge.

Arkansas previously blocked sales of Monsanto’s dicamba herbicide, XtendiMax with VaporGrip, in the state.

“If the EPA imposed a April 15 cut-off date for dicamba spraying, that would be catastrophic for Xtend - it invalidates the entire point of planting it,” said Jonas Oxgaard, analyst for investment management firm Bernstein.

Monsanto has projected its Xtend crop system would return a $5 to $10 premium per acre over soybeans with glyphosate resistance alone, creating a $400-$800 million opportunity for the company once the seeds are planted on an expected 80 million acres in the United States, according to Oxgaard.

By 2019, Monsanto predicts U.S. farmers will plant Xtend soybeans on 55 million acres, or more than 60 percent of the total planted this year.

RISKY DRIFT

About 3.1 million acres of soybeans vulnerable to dicamba were hurt by sprayings this summer, accounting for 3.5 percent of U.S. plantings, according to the University of Missouri.

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Chemical companies have blamed the crop damage on farmers misusing the herbicides.

Specialists, though, say the weed killers are also risky because they have a tendency to vaporize and drift across fields, referred to as volatility. Summer can be a riskier time for sprayings, they said, because high temperatures can increase volatility.

Monsanto previously denied requests by university researchers to study its XtendiMax herbicide for volatility, as previously reported by Reuters. In the end, the EPA gave dicamba weed killers from Monsanto and BASF abridged two-year registrations, less than the five years experts say is more common.

To address the crop damage, the EPA has also asked state officials about enhanced training for dicamba users; tighter restrictions on when and how the herbicides can be sprayed; and the possibility of reclassifying the products so the general public could not buy them, according to participants on the call.

“Everything is an option,” said Jason Norsworthy, a University of Arkansas professor who was on the call.

Monsanto Chief Technology Officer Robb Fraley said in a statement that the company was communicating with the EPA, which is “evaluating potential actions to facilitate enhanced training and compliance for 2018.”

DuPont, too, is working with the EPA and state regulators on issues involving its dicamba herbicide, FeXapan, spokeswoman Laura Svec said.

Rival BASF “could see some label enhancements” to its dicamba herbicide, Engenia, if the EPA requires changes, spokeswoman Odessa Hines told Reuters. The company “will be as flexible as possible” so farmers can use the product, she said.

(This version of the story corrects “are” to “is” in the first paragraph)