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I think that banning of antibiotic use in livestock generally focuses on the preemptive addition of antibiotics in feed. Antibiotic additives are associated with increased weight gain for the animals, as well as letting feed lots put more animals in denser quarters while keeping disease rates down. These direct economic drivers are why industry has fought the regulations for so long - despite the long-term detrimental societal health costs. It's pretty much a classic case of an economic externality needing regulation.

Edit: here's a reference link http://www.ucsusa.org/food_and_agriculture/our-failing-food-...