Apparently factory farms (i.e. chickens, pigs, turkey, cows, etc.) are the largest abusers of antibiotics.
> Of all antibiotics sold in the United States, approximately 80% are sold for use in animal agriculture; about 70% of these are “medically important” (i.e., from classes important to human medicine). Antibiotics are administered to animals in feed to marginally improve growth rates and to prevent infections ...
Antibiotics Overuse in Animal Agriculture: A Call to Action for Health Care Providers
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4638249/