No, Denagard is an antibiotic. There are still many antibiotics that you don't need a prescription to use in livestock. Just go to your local feed store and you'll find not only Denagard but also chlortetracycline, penicillin, oxytetracycline, sulfadimethoxine, sulfamethazine, and tylosin, to name a few. (In CA specifically, some of these listed require a prescription, but not in other states.) All antibiotics are antimicrobials, although not all antimicrobials are antibiotics.
Preventative treatment is not used 100% of the time for antibiotics. Using this 100% of the time would also select for antibiotic resistance, and would make the medication less effective or possibly even useless for treatment if symptoms became severe. Chronic use of antibiotics at low doses for prevention, or repeatedly short durations, are specifically the reasons that so many antibiotics that had been available OTC and now prescription-only meds. Their misuse has caused significant problems.