All cattle spend the majority of their lives eating grass on pasture. Grain finished beef spend the majority of their lives eating grass. What makes them different from grass finished beef is that during the last 6-8 months of their lives, grain finished cattle are free to eat a balanced diet of local feed ingredients. What this means for the consumers is that your steaks will be larger and there will be more of that beauitful marbeling that gives beef that distinct sought after flavor.
Grain finished beef, grain fed beef and conventional beef are essentially all the same. They refer to cattle that have been raised with the beginning of their life on grass, and then given grain at the end of their lives.
Therefore, all beef is antibiotic free whether it is labeled that way or not.