[jinx]

Oral antibiotics, especially those that act on Gram
positive organisms, became widely used at sub-therapeutic levels for their consistent ability to
improve the growth of livestock (Crawford, 1983; Droumev, 1983). While part of the reason for
this practice is to reduce the risk of disease, it is also accepted that regular intake of oral
antibiotics as feed additives has a direct nutrient sparing effect and reduces the production of
urea, methane, and ammonia in the intestine, among other effects (Visek, 1978; Walton, 1983).
The rationale for the use of antibiotics as growth promoters has been established (Luetzow,
1997). A modulating effect on either the metabolic activity of certain intestinal microorganisms,
or a shift of the balance of the microbial ecosystem, which constitutes an essential
part of mammalian digestion, is the proposed mechanism of action. These effects are observed at
use levels which are far lower than those achieved in therapeutic use. More efficient digestion
during the administration of low levels of anti-microbials decreases the amount of feed necessary
to raise and to fatten domestic animals. The beneficial effects of sub-therapeutic doses of
antibiotics have not decreased since these effects became known in the 1950’s (Frost, 1991).

This is from the USDA link above. I realize it's from 2007, I don't think that's long enough ago to discount it based on age alone. More recent information on the topic points to legislation to end the practice, so obviously that hasn't happened yet...