The Supreme Court ruled unanimously Monday that regulation of the nation’s slaughterhouses is exclusively up to the federal government and struck down a California law that required immediate euthanization of livestock that are too sick or weak to walk.

The justices agreed with the National Meat Association that California’s 2009 law violates a federal statute that sets national standards for meat safety and gives federal inspectors the final word on what to do about “nonambulatory” livestock.

The case involved swine, and Justice Elena Kagan said the Federal Meat Inspection Act is clear.

FMIA “regulates slaughterhouses’ handling and treatment of nonambulatory pigs from the moment of their delivery through the end of the meat production process,” Kagan wrote. She added that California’s law “endeavors to regulate the same thing, at the same time, in the same place — except by imposing different requirements. The FMIA expressly preempts such a state law.”

California passed its law after the Humane Society of the United States released a video of “downer” livestock being shocked and dragged at a slaughterhouse.

In this April 22, 2010 image provided by the Humane Society of the United States, a downer dairy cow too sick or injured to walk lies on the ground at the Hallmark Meat Packing Company in Chino, Calif. In 2009, California barred the purchase, sale and butchering of animals that can't walk and required slaughterhouses to immediately kill non-ambulatory animals. (Anonymous/AP)

The law bans slaughterhouses from buying, butchering or selling downer livestock for human consumption and calls for immediate euthanization of the animals.

FMIA, however, allows federal inspectors to determine whether a nonambulatory animal is fit for human consumption.

Nonambulatory pigs, the meat association said in its brief, may not necessarily be diseased but rather may be reacting to the trip to the slaughterhouse.

“The vast majority of nonambulatory pigs are merely overheated, stressed, fatigued, or stubborn and, if allowed to rest, will stand and walk unassisted,” it said.

An inspector may then decide whether it can become part of the nation’s food supply.

Kagan’s opinion said the number of pigs becoming nonambulatory after delivery to the slaughterhouse is estimated at 100,000 to 1 million of the 100 million swine slaughtered each year.

California had contended that its law did not run afoul of the federal regulations because it was not regulating animals that were going to become meat. In effect, the state argued, it was removing livestock before the federal requirements kicked in.

The U.S. Court of Appeals for the 9th Circuit, in San Francisco, had upheld the law.

Wrote Kagan: “The FMIA’s scope includes not only ‘animals that are going to be turned into meat,’ but animals on a slaughterhouse’s premises that will never suffer that fate,” such as diseased hogs.

The case is National Meat Association v. Harris.