We tried raising pasture fed calves and we just didn't like the taste of the meat. It is nutritionally better from all the data that I have read, but the flavor is different. What we do is calve in early spring, wean in December and then feed some grain and hay through the winter. I like to have them go onto pasture at 600 to 700 pounds and then in October or when we bring them in off pasture grain them for 90 days. They normally weigh around 1100 pounds by the time we butcher, have some fat on them, but not too much. You end up with a lot of the pasture feed qualities, but have better flavor. It is quite a bit cheaper to run calves on pasture than to feed them hay and grain and a lot less work.
I raise longhorn cross calves and have not had to use any antibiotics on them, If you raise them right and don't bring in any animals from off the farm they will be fine.
Bob