Something unusual happened a couple of days before Thanksgiving.

Federal environment officials filed court documents to pull the plug on a new herbicide combination that they had approved just a year ago and had been distributed on a trial basis to some Minnesota farms. The Environmental Protection Agency (EPA) had studied the product, Enlist Duo, for several years before registering it, so the agency’s sudden change of heart came as a surprise to many.

Dow AgroSciences created the herbicide and promoted it as a long-awaited solution for farmers battling herbicide-resistant “superweeds.” The EPA now wants to cancel the herbicide’s registration and study it again because the mixture could be more toxic than earlier believed, especially to endangered plants.

But Dow, in court documents filed last week, said not so fast. The agency can review the product, the company contends, but EPA has no right to cancel the registration without due process. Dow is telling customers that it still expects Enlist Duo to be available for the 2016 growing season.

Watching the case closely are environmentalists who oppose the registration and other chemical companies with similar herbicides under development.

Paul Meints, research director for the Minnesota Corn Growers Association, said his group has not been involved with the lawsuit but found the EPA’s actions “very curious.”

“It’s not very common that EPA approves something and then asks for a pullback,” Meints said.

Because Enlist Duo is not on the market yet, Meints said the delay won’t drastically affect Minnesota producers. But corn growers look forward to having new herbicide options eventually, he said, especially to kill giant ragweed and water hemp — weed species that in many fields have become resistant to widely used glyphosate, more commonly known as Roundup.

“Having an additional material to help with herbicide-resistant weeds is always positive in terms of preventing that problem from becoming worse,” Meints said.

The EPA made the final decision to approve Enlist Duo for use in six states in October 2014 and last March added nine more states, including Minnesota. It included label requirements that farmers maintain a 30-foot buffer around fields to be sprayed to protect the surrounding environment.

Two environmental groups — the Natural Resources Defense Council and the Center for Food Safety — challenged the decision in the U.S. Court of Appeals for the Ninth Circuit in San Francisco.

It was in that court that the EPA filed a motion on Nov. 24 to revoke its approval of Enlist Duo because the agency said it “did not have all relevant information at the time it made its registration decision.”

Specifically, EPA said that Dow had not provided information about the potential synergistic effects of the two active ingredients in Enlist Duo, glyphosate and 2,4-D, meaning that the mixture of the two might be more toxic than the sum of their ingredients individually.

Dow created the product because overuse of glyphosate has caused weeds “on tens of millions of acres of American farmland” to become resistant to that herbicide, according to the company. So Dow genetically changed corn and soybean seeds to tolerate a more powerful mix of glyphosate and 2,4-D, and created Enlist Duo herbicide so that the combo could kill weeds that glyphosate alone could not.

Both glyphosate and 2,4-D have been used for decades. Although 2,4-D was an ingredient of Agent Orange used in Vietnam, it was a different herbicide in that product, 2,4,5-T, that contained cancer-causing dioxin and other contaminants and was canceled in 1985.

The EPA studied the two chemicals in Dow’s product, and initially concluded that the combination was not synergistic, meaning that the toxicity of the two ingredients combined was not greater than the effects of the individual compounds.

But Dow AgroSciences had told a different federal agency — the U.S. Patent Office — that the combo would be synergistic, in a patent claim the company has since abandoned.

EPA discovered that filing and demanded Dow’s data about the herbicide’s synergy in October. After a preliminary review of the information, the agency said the product’s registration approval needed to be reversed and reanalyzed because its toxicity may have been “understated” and its protective buffer restriction might be insufficient.

“EPA can no longer be confident that Enlist Duo will not cause risks of concern to non-target organisms, including those listed as endangered, when used according to the approved label,” the agency said.

The environmental groups were pleased that the EPA filed the motion, but said they had repeatedly raised the issue of synergy and safety since they sued the agency more than a year ago. They also have claimed that it was unlawful for EPA not to consult with the U.S. Fish and Wildlife Service about the potential harm to endangered species that could be exposed to Enlist Duo.

“We don’t think it’s the right solution to the superweeds epidemic that the first generation of genetically engineered crops has caused,” said George Kimbrell, an attorney for the Center for Food Safety, in an interview. Approval and use of Enlist Duo will simply enable weeds to also become resistant to 2,4-D eventually, he said. “We need a more sustainable solution than doubling down on more pesticides.”

The impact of the case goes well beyond one product, Kimbrell said. Monsanto and other companies also have new herbicides in the pipeline that essentially combine old herbicides, so how EPA deals with analyzing the synergistic effects of Enlist Duo’s ingredients will set a precedent for other products that follow.

“This is a critical juncture,” Kimbrell said. “It represents the next generation of these genetically engineered crops, and what decision we’re going to make as a society about how we produce our food, going forward.”

Court documents show that preliminary to commercial sale in 2016, Dow distributed the new herbicide early in 2015 on a trial basis to 25 selected corn farmers in eight states, including Minnesota, and to seven soybean growers in four states, including Minnesota.

Dow opposed the EPA’s request to revoke registration of the herbicide last week in a court filing. Allowing the agency to review additional data is not controversial, the company said, but canceling the registration is “entirely novel and unlawful.”

Dow has a legal property interest in the pesticide’s registration, attorneys argued, and that cannot be revoked without due process that requires public notice, analysis of the impact, a public administrative hearing and other procedures.

The company declined interview requests but issued a statement last week that it expects EPA to address questions about synergy “so growers will have access to this much needed technology to combat resistant and hard-to-control weeds in time for the 2016 U.S. growing season.”

The company’s parent, Dow Chemical, was in advanced talks last week about a merger with DuPont Co.

It’s unclear when the three-judge panel might decide on the EPA motion, but it likely could be several weeks. If granted, it is also unclear how long the EPA would take to reconsider the herbicide.

Meints said that Minnesota corn growers will be watching, but they do not expect quick action. “Our guys seem content to just see what happens, and until then will continue to use the [herbicide] systems we have in play,” he said.