From ANIMAL PEOPLE, November/December 2013:
WASHINGTON D.C.––The U.S. Food & Drug Administration on
December 11, 2013 announced that the drug makers Zoetis and Elanco,
which produce the majority of antibiotics used to promote livestock
growth, have agreed to participate in a voluntary phase-out of
non-therapeutic use of antibiotics in animal husbandry.
Routine antibiotic doses promote faster growth by suppressing
infections that often result from housing large numbers of animals in
close proximity under unsanitary conditions.