Subtherapeutic use of antibiotics is clearly the best way to develop resistance becuase you're giving the bugs just enough antibiotic to promote their evolution, but not to kill them. The more basic question is whether it is appropriate to be using subtherapeutic levels of any antibiotics in agriculture. Europe has cut out the subtherapeutic use of antibiotics. But U.S. agricultural producers argue that the U.S. is not Europe, because Europe generally has smaller operations. So it's an extremely complex issue.
Therapeutic uses are also complex.
If chickens are sick, we should treat them. But the FDA approved quinolone use on that logic, and now we have a major problem with resistance in humans. That makes people more susceptible to infection, so we use antibiotics more frequently. Vancomycin-resistant enterococci were first seen in the late 1980s. They spread rapidly from the mid-Atlantic region and are now a major problem in hospitals across the country.