* Animal welfare groups say they will sue to prevent slaughter
* Congress may vote soon on whether to reinstate slaughter ban
WASHINGTON, June 28 (Reuters) - A New Mexico meat plant received federal approval on Friday to slaughter horses for meat, a move that drew immediate opposition from animal rights group and will likely be opposed by the White House.
The U.S. Agriculture Department said it was required by law to issue a "grant of inspection" to Valley Meat Co, Roswell, New Mexico, because it had met all federal requirements. Now, the USDA is obliged to assign meat inspectors to the plant.
The USDA also said it may soon issue similar grants for plants in Missouri and Iowa.
Horse meat cannot be sold as food in the United States, but it can be exported. Attempts to reach Valley Meat Co via a number listed online were unsuccessful.
Valley Meat would be the first meat plant to be allowed to slaughter horses since Congress banned it in 2006.
It is not known when the plant will start production, but two bills in Congress want to ban horse slaughter and President Obama has asked Congress to ban it.
The Humane Society of the United States and Front Range Equine Rescue threatened on Friday to sue the USDA, saying horses are raised as pets and as working animals. Because they are not intended as food animals, horses are given medications banned from other livestock, the groups said, questioning if the meat would be safe.
The USDA says it can test for residues of 130 pesticide and veterinary drugs. It also has safeguards to keep horse meat out of the food supply.
Congress effectively banned horse slaughter in 2006 by saying the USDA could not spend any money to inspect horse plants. Without USDA inspection, meat plants cannot operate.
The ban was part of the annual USDA funding bill and was renewed a year at a time through 2011. The prohibition expired in October 2011.
Lawmakers could vote on reinstating the ban in coming weeks when the USDA appropriations bills are debated in the House and Senate. But no date has been set to consider the bills and it could be months before work is completed.
The USDA said it was required by law to issue the grant of inspection because Valley Meat met all federal requirements. At one point, the company sued the USDA for an overly long review of its application. Once it issues a grant of inspection, the USDA is obliged to assign meat inspectors to a meat plant.
"Until Congress acts, the department must comply with current law," said a USDA spokeswoman.
Valley Meat retrofitted its plant for horses after drought weakened its cattle slaughter business.
Horse meat is sold for human consumption in China, Russia, Mexico and other foreign nations and is sometimes used as feed for zoo animals.
But in the United States, horses enjoy a higher stature, more akin to house pets, than to hogs, cattle and chickens.
An estimated 130,000 U.S. horses are shipped annually to slaughter in Canada and Mexico. Groups have quarreled for a decade whether a ban on slaughter will save horses from a cruel death or lead to abandonment by owners of animals they cannot afford to feed or treat for illness.
Early this year, regulators discovered that horse meat was being sold as beef in Ireland. The mislabeled meat was found in meatballs sold by Swedish retailer IKEA in much of Europe and in other outlets.
USDA conducts tests on domestic and imported products to identify the species that yielded the meat. The tests can distinguish beef, sheep, swine, poultry, deer and horse.
As well, USDA stepped up its species testing in April because of the meat adulteration scandal in Europe. (Reporting By Charles Abbott; Editing by Bernard Orr)
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