The FDA may already be too late. Several owners of clones have been selling semen to farm clubs and others vying to grow prize-winning cattle. Most of those animals end up being slaughtered, sold and eaten, experts said.

"That you can go online today to any number of different Web sites and purchase semen from cloned bulls tells you there are cloned sires out there fathering calves in the food supply," Walton said.

Like it or not, Walton and others said, the clones are out of the barn.