In a statement, the U.S. Agriculture Department said it was required by law to issue a "grant of inspection" to Responsible Transportation of Sigourney, Iowa, because it met all federal requirements. USDA will also be obliged to assign meat inspectors to the plant.
"The administration has requested Congress to reinstate the ban on horse slaughter," the USDA said in a statement. "Until Congress acts, the department must continue to comply with current law."
An application from a Missouri company was also expected to win approval this week.
Valley Meats in Roswell, New Mexico, on Friday became the first horse plant to clear the USDA review process since a ban on horse slaughter ended in 2011.
Five animal welfare groups filed suit on Tuesday in U.S. District Court in San Francisco to overturn the approvals, saying the Agriculture Department did not conduct environmental reviews before acting. The groups say horses are given medications not approved for livestock so the waste products of slaughter plants may include pollutants.
"America is the original home of the horse and has never been a horse-eating culture," said Neda DeMayo, president of Return to Freedom, one of the litigants. "Horses have been our companions, fought battles with us, worked sun-up to sundown by our sides ... we will not abandon them now."
Horse meat cannot be sold as food in the United States, but it can be exported. The meat is sold for human consumption in China, Russia, Mexico and other countries and is sometimes used as feed for zoo animals.
Nearly 159,000 horses were exported from the United States to Canada and Mexico during 2012, most likely for slaughter, officials said.
Congress effectively banned horse slaughter in 2006 by saying the USDA could not spend any money to inspect the plants. Without USDA inspectors, slaughterhouses cannot operate.
The ban had been extended a year at a time as part of USDA funding bills, but the language was omitted in 2011.
Lawmakers may vote in coming weeks on horse slaughter as part of its work on Agriculture Department funding. In addition, two freestanding bills would ban horse slaughter and the export of horses for slaughter.
Groups have argued for years whether a ban on slaughter would save horses from an inhumane death or cause owners to abandon animals they no longer want or cannot afford to feed and treat for illness.
It was not known how soon Valley Meats or Responsible Transportation would begin operation. A spokesman for Responsible Transportation was not immediately available for comment.
Responsible Transportation said on its website that there are 90,000 to 100,000 unwanted horses in the United States annually.
"We believe it is our responsibility to restore the value of the horse industry," it said.
