80% of U.S. antibiotics are used in animals

From a report at Wired, citing the Center for a Livable Future, which used data from the FDA:

Most important to note: Most of the drugs used in animal agriculture and in human medicine are functionally identical. That’s one reason why the overuse of antibiotics in animals is such a concern: When organisms become resistant on the farm to drugs used on livestock, they are becoming resistant to the exact same drugs used in humans. (One major drug category used in animals, ionophores, do not have a direct human analog...)

The next battle, which industry has already begun, is defining what non-therapeutic use will constitute. Producers are already claiming that the use of antibiotics for growth promotion has decreased, maintaining current low-dose usage is aimed at disease prevention. Regardless, all low-dose usage of antibiotics can lead to a significant increase in antibiotic resistance.

Additional data and discussion at the links.