Gizmodo, of all places, published a great article on the new FDA rules on animal livestock antibiotic use, which went into effect on Jan 1st 2017:
The impetus here is the looming threat of so-called superbugs that have grown resistant to antibiotics. Critics have long warned that the widespread use of antibiotics in raising food animals has contributed to a superbug rise. Estimates suggest that somewhere around 70% of antibiotics are used in agriculture. That high percentage is due in large part to how those antibiotics are used—not to treat illness but to stave it off in dismal, disease-ridden conditions or to promote growth.
The quote above is all the mention of animal welfare we get from the article. Keep in mind that the main reason livestock even need antibiotics on that scale is due to the horrendous conditions in which they are kept.