By Jonathan Kaminsky

(Reuters) - New Mexico's attorney general sued on Thursday to block a horse slaughter plant scheduled to open next month from becoming the first facility of its kind to operate in the United States in more than five years.

The move is the latest in an ongoing legal battle that has pitted animal protection groups and their allies against an industry fighting to regain a foothold in the United States.

"Commercial horse slaughter is completely at odds with our traditions and our values as New Mexicans," New Mexico Attorney General Gary King said in a written statement. "It also poses a tangible risk to consumers and to our environment."

King's lawsuit and other U.S. efforts in recent months and years to restrict the slaughtering of horses underscores the discomfort many Americans feel toward the practice.

The lawsuit and an accompanying request for a temporary restraining order target Valley Meat Co., which plans to convert its cattle slaughterhouse in Roswell, New Mexico, to one processing horse meat starting on January 1.

According to King's lawsuit, Valley Meat has a long history of violating state environmental rules, including leaving large piles of cattle carcasses to rot and be eaten by maggots.

The suit further alleges that because many horses are administered scores of drugs while alive, their meat is likely adulterated, unsuitable for human consumption and in violation of state food safety laws.

An attorney for Valley Meat called the lawsuit an inaccurate portrayal of the company's environmental history and an act of political grandstanding by King, a Democrat who has announced plans to run for governor of New Mexico in 2014.

"The idea of suing someone for an anticipated violation of the (New Mexico) Food Act is crazy," said Blair Dunn, an attorney for the company. "This amounts to nothing more than a frivolous and harassing lawsuit going after a lawful business."

The horses would be taken off any drugs before slaughter and given time to flush the substances from their system, Dunn said.

The piles of rotting cattle described in King's lawsuit consisted of at least 80 percent manure and were in fact compost piles, Dunn said.

Plans call for the Valley Meat facility to be equipped to slaughter 120 horses a day, he said. The company would buy horses at auction from owners who no longer want them and from the stock of free-roaming horses of domestic ancestry on Native American tribal lands.

Horse meat cannot be sold as food in the United States but Valley Meat hopes to sell it as feed for U.S. zoo animals and for human consumption in Europe, China and Japan.

A federal appeals court in Colorado last week vacated a temporary ban on U.S. Department of Agriculture inspections at slaughterhouse facilities, ruling against animal protection groups seeking to block Valley Meat and other aspiring horse slaughter facilities from opening.

Congress effectively banned horse slaughter in 2006 by saying the USDA could not spend any money to inspect the plants. The ban had been extended a year at a time as part of USDA funding bills, but the language was omitted in 2011.

(Editing by Alex Dobuzinskis and Andrew Hay)