There seemed to be broad agreement among the justices that the Federal Meat Inspection Act, which regulates slaughterhouses, barred most or all of the California law, which requires the immediate euthanasia of “downer” animals and bars their slaughter or sale.
The state law was enacted in 2009 in response to videos showing the inhumane treatment of cows that could not walk.
The federal law allows federal meat inspectors to decide what is to be done with such animals, and it says that states may not impose additional or different requirements.
Federal inspectors sometimes determine that the animals may be revived and slaughtered, and they sometimes order animals to be kept alive long enough to inspect them for contagious diseases.
The United States Court of Appeals for the Ninth Circuit, in San Francisco, ruled last year that there is no conflict between the California law and the federal one because the state had merely disqualified certain kinds of animals from being used for food. That is different, the appeals court said, from issuing additional or different regulations concerning the operations of slaughterhouses.
“States aren’t limited to excluding animals from slaughter on a species-wide basis,” Chief Judge Alex Kozinski wrote for a unanimous three-judge panel of the court.
Similarly, he wrote, the California law “withdraws from slaughter animals that are unable to walk to their death.”
At Wednesday’s argument, Steven J. Wells, a lawyer for the National Meat Association, which is challenging the law, said that so fine a distinction was unworkable.
“It’s easy to identify what’s a horse and keep it off the premises,” Mr. Wells said. “That’s not true with nonambulatory animals.”
Susan K. Smith, a deputy California attorney general, disagreed. “When an animal becomes nonambulatory, it is readily apparent,” she said.
More broadly, Ms. Smith said the state was entitled to bar the slaughter of both certain species of animals and certain kinds of animals. In both cases, she said, “California is not regulating animals that are going to be turned into meat.”
Justice Elena Kagan seemed uncomfortable with where that argument was leading.
“But then you have to, you know, ask yourself the question: Are nonambulatory swine so easily excludable as horses?” she said. “Why couldn’t the state then exclude swine with various kinds of diseases? And then it would be clear that the state was doing something that the federal statute is supposed to be doing.”
The federal government supported the trade association in the case, National Meat Association v. Harris, No. 10-224 .
Though much of the argument was a semantic tangle, there was at least one moment of clarity, when Justice Antonin Scalia reviewed the broader rules about which species may be slaughtered for food. “Lassie and Kitty are no good,” he said, “but Dobbin is all right.”