I lean more towards deere_9600's comment from the thread below:

I've fed cattle for 35 years and found the best pnuemonia preventer was to get em outa the barn (with windbreak) . quote from deere_9600 in thread about feeding under roof below

Here is a picture from a Baytril ad I received in an email today. Note the 28 day statement. Seems like purchasing pre-conditioned calves from a reliable source and a preventive medicine vaccination program would go a long way towards not needing to keep too large an arsenal of antibiotics on hand. jmho.

(Baytril 100 ad from Drovers.jpg)

Baytril 100 ad from Drovers.jpg (23KB - 636 downloads)