At the end of March 2019 FDA issued revised guidelines in a Q&A format that are designed to help veterinarians, feed distributors, and animal producers comply with the Veterinary Feed Directive regulations that have been in effect since January 2017. The guidelines require that medically important antimicrobials only be used in food-producing animals for therapeutic purposes and must be under the direction of a veterinarian. Growth-promoting uses of these drugs are prohibited.
More details on the guidelines for Veterinary Feed Directive regulations are available from FDA.
More information on use of growth promotants and production enhancers in food-producing animals and ways to help prevent development of resistance to antibiotics is available in the Merck Veterinary Manual.