COLUMBIA, Mo. — With the definition of farm-raised deer in question, Gov. Jay Nixon declared Tuesday that they are "wildlife" not "livestock" and vetoed legislation that would have shifted their regulation to state agriculture officials instead of the agency that oversees hunting.

Nixon cast his veto as a bold defense of the constitutionally independent Missouri Department of Conservation, which is pursuing tougher regulations on deer breeders and ranchers who raise animals for fenced-in hunting preserves. The wildlife agency says the restrictions are necessary to prevent the spread of disease through Missouri's 1.3 million wild deer.

The captive deer industry and legislative sponsors of the bill plan to pursue a veto override attempt when the Legislature convenes in September.

Nixon described Missouri's roughly 200 deer breeding operations and 45 hunting preserves as a "narrow commercial interest" that was trying to "mess with 80 years of success" in regulating wildlife by the Department of Conservation. He announced the veto at a special meeting of the Missouri Conservation Commission to the applause of about 150 agency supporters gathered at a Columbia hotel.

"This destructive measure would violate our state constitution, would violate the will of the voters and undermine longstanding successful conservation practices," Nixon said.

Deer breeders and hunting preserves must get operating permits from the Conservation Department, but the agriculture agency has a role in regulating health issues and the movement of the animals among farms, said Sam James, president of the Missouri Deer Association, which represents breeding and hunting ranches.

The legislation would have defined captive deer as "livestock" similar to cattle, hogs and sheep. That could have shifted oversight entirely to the Agriculture Department while making deer ranchers eligible for state sales tax exemptions and enabling them to sell deer meat as food.

The legislation also could have effectively negated newly proposed Conservation Department regulations, which would ban the importation of deer from other states and require double fencing for new deer ranching permits.

Nixon mocked the attempt to categorize deer with traditional farm animals.

"The bottom line is a deer is not a cow — just try to milk one," he said.

But supporters of the bill said the definition of livestock should depend on how animals are raised, not their species. Like other livestock, captive deer remain behind a fence and are bought and cared for by their owners, James said.

"They consider our animals to be wildlife, and we don't," said James, who has more than 200 deer at a central Missouri breeding facility and 225 more — some imported from Ohio — at two hunting preserves.