CHICAGO (Reuters) - America’s two biggest independent seed sellers, Beck’s Hybrids and Stine Seed, told Reuters they are pushing U.S. environmental regulators to bar farmers from spraying dicamba weed killer during upcoming summers in a potential blow to Bayer AG’s Monsanto Co.

Rodrigo Werle, a weed scientist at the University of Wisconsin inspects soybean fields as part of the university's research into whether the weed killer Dicamba drifted away from where it was sprayed in Arlington, Wisconsin, August 2, 2018. REUTERS/Tom Polansek

Limiting spraying of the chemical to the spring season, before crops are planted, would prevent farmers from using the herbicide on dicamba-resistant soybeans that Monsanto engineered. The seeds are sold by companies including Beck’s and Stine.

Last summer, after farmers planted Monsanto’s dicamba-resistant soy seeds en masse, the herbicide drifted onto nearby farms and damaged an estimated 3.6 million acres of non-resistant soybeans, or 4 percent of all U.S. plantings.

Problems have not gone away. As of July 15, the University of Missouri estimated that more than a million acres of non-resistant soybeans were hurt by dicamba. Homeowners who live near farms have also complained of damage to their trees and flowers.

The U.S. Environmental Protection Agency (EPA) is now weighing such complaints as part of a high-stakes decision on the herbicide’s future.

Bayer bought Monsanto and its portfolio of dicamba-resistant Xtend brand soy seeds for $63 billion this year in a deal that created the world’s largest seed and pesticides maker.

St. Louis-based Monsanto sells dicamba herbicide, along with rivals BASF SE and DowDuPont Inc. Monsanto and BASF said farmers need dicamba to kill tough weeds and that the chemical can be used safely. DowDuPont declined to comment.

Monsanto is banking on Xtend soybean seeds to dominate soy production in the United States, the world’s biggest producer. They are seen as a replacement for the company’s Roundup Ready line of seeds, engineered to tolerate the weed killer glyphosate, which has lost effectiveness as weeds develop their own tolerance to the chemical.

EPA approval for dicamba to be sprayed on resistant crops expires this autumn. The agency could extend its approval, with or without new restrictions on use, or take dicamba off the market. Seed companies expect a decision in the coming weeks.

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Most complaints about dicamba drifting would stop if the EPA restricted its use to killing weeds in fields before crops are planted, Beck’s Hybrids told the agency in a July 27 letter seen by Reuters.

“Anybody that sprays it, you have issues with the volatilization,” CEO Sonny Beck said in an interview on Wednesday, referring to the chemical vaporizing and drifting.

Though his company profited from selling more than a million bags of Xtend soybean seeds this year, Beck said he worried that continued problems with the chemical could give the agriculture sector a bad reputation among consumers.

Restricting use would also help prevent weeds from developing resistance to dicamba, he said.

New limits would be another headache for Bayer, following its acquisition of Monsanto.

Last week a California jury ruled Monsanto must pay $289 million in damages in the first U.S. lawsuit over alleged links between glyphosate and cancer. Monsanto denies glyphosate causes cancer.

Earlier this month, a Brazilian judge suspended the use of products containing glyphosate.

MONSANTO EXPECTS EPA NOD

Monsanto has blamed U.S. field damage from dicamba largely on improper applications by farmers and says mandatory training helped this year.

Inquiries to the company about dicamba problems dropped to about nine per million acres of dicamba-resistant crops planted, down from about 40 inquiries per million acres last year, said Ryan Rubischko, who heads the company’s dicamba portfolio. He said Monsanto expects the EPA to extend its approval for dicamba.

In a sign the company is concerned, however, Monsanto has asked seed sellers to contact the agency to express support for the product, according to an email the company sent this week that was seen by Reuters. The email noted others had encouraged the EPA to add restrictions on dicamba or prevent sales.

Monsanto likened those efforts to an “uninformed vocal minority” in the email. Rubischko confirmed the company had asked dicamba users to give positive feedback to regulators.

The EPA did not respond to requests for comment.

The agency has held weekly phone calls with agriculture officials in farm states this summer to assess dicamba damage. Agency officials also visited farms in Tennessee, Missouri and Arkansas to see damaged crops first-hand, according to tour participants.

Farther north, Monsanto funded a study by University of Wisconsin researchers that showed dicamba hurt non-resistant soybeans that were covered with plastic when the chemical was sprayed on nearby Xtend soybeans after planting.

Stine Seed has told the EPA in writing and conversations that dicamba should not be sprayed on top of growing soybeans to control weeds, CEO Harry Stine said in an interview on Tuesday. The herbicide has damaged fields of Stine soy seeds by drifting, he said.

Stine Seed is preparing to launch products that will compete with Xtend soy and also works with Monsanto on seed technology.

“I’ve been doing this for 50 years and we’ve never had anything be as damaging as this dicamba situation,” Harry Stine said. “In this case, Monsanto made an error.”