MINNEAPOLIS — A new herbicide, long awaited by farmers as a tool to fight “superweeds” but long opposed by environmental groups, has received final federal approval and is likely headed for Midwestern fields next year.

Enlist Duo, manufactured by Dow AgroSciences, could become an important competitor to Monsanto’s popular Roundup Ready system, which has dominated the corn and soybean market since the late 1990s.

The Environmental Protection Agency gave the green light last week for Dow AgroSciences to register Enlist Duo, clearing the way for the subsidiary of Dow Chemical to sell it in combination with new genetically engineered corn and soybean seeds.

The product initially will be allowed in six states: Wisconsin, Iowa, South Dakota, Illinois, Ohio and Indiana. Approval is pending in Minnesota and nine other states where a public comment period is now underway.

Minnesota Soybean Growers Association President George Goblish said that soybean farmers are excited about the decision, even though they won’t be able to take advantage of the product yet. “The Enlist Duo registration unties farmers’ hands and gives them another option to combat weed issues,” he said.

Enlist Duo combines two older weedkillers into one product: an herbicide component known as 2,4-D, and glyphosate, the chief ingredient in Roundup.

The herbicide would be sprayed early in the growing season, ideally when weeds and crops are 2 to 4 inches tall. The spraying would not affect Enlist corn and soybean plants because their seeds have genetically-engineered traits to tolerate the new herbicide formulation. The U.S. Department of Agriculture approved the seeds last month.

The decision to approve the herbicide “is protective of everyone and the environment,” said Jim Jones, assistant EPA administrator for the Office of Chemical Safety and Pollution Prevention. The decision was based on “sound science,” he said.