Adding a constant stream of antibiotics to animal feed to promote faster growth has been common practice since the 1940's.
I've seen some theories tossed around to explain the mechanism, but there is no consensus, aside from the fact that it does work.
Unfortunately, it's also led to us having human disease that no longer responds well to antibiotics that are safe to use in humans.
For instance, with Extensively Drug Resistant Tuberculosis:
> Second-line drugs are more toxic than the standard anti-TB regimen and can cause a range of serious side-effects including hepatitis, depression, hallucinations, and deafness. Patients are often hospitalized for long periods, in isolation. In addition, second-line drugs are extremely expensive compared with the cost of drugs for standard TB treatment.
Has there been any research to determine why growth improves with diminished gut bacteria?