DECATUR, ILL. — Archer Daniels Midland Co. said it will close its ethanol plant in Walhalla, N.D., in April as part of an effort to optimize its U.S. corn processing operations.

According to ADM, the Walhalla facility “was not delivering sufficient returns because its geographic location and scale made it difficult to compete in the marketplace.”

The facility opened in 1985 as Dawn Enterprises. It has the capacity to make about 30 million gallons of ethanol fuel per year. The plant employs 61 people.

ADM said the closing will not affect its ability to supply customers with ethanol and feed products, and the company will serve customers through its operations in Cedar Rapids and Clinton, Iowa; Decatur and Peoria, Ill.; Columbus, Neb.; and Marshall, Minn.

ADM did not disclose what it plans to do with the facility once it closes, but Sen. John Hoeven of North Dakota urged company officials to work with private investors and state and local developers to help keep the plant operating under new ownership if possible.

“Our primary concerns are first to make sure that employees are taken care of and second, that the company work with us to try and find another company to resume production at the facility,” Mr. Hoeven said. “We’ve offered to work with ADM, as well as local developers and other private-sector investors, to keep the plant operating under new ownership.”