All a rabbit needs to thrive is a GOOD QUALITY pellet (not the cheapest bag at the feed store), fresh grassy hay (alfalfa is already in the quality pellet), a source of constant clean water, and adequate housing (dry and free from drafts). Any grain supplements should be fed carefully and in moderation, depending upon the duty of the rabbit, whether it be a kindling doe, meat production rabbit, or show rabbit. Today's modern milled pelleted feed has been engineered to provide complete nutrition.
As far as preventative antibiotics, it does more harm than good and is totally unneeded. Breeders have to understand that genetics is the most important factor in raising healthy, quality, rabbits. Any herd of rabbits that tends to get sick at the drop of a hat without constant treatment is genetically weak and should be culled from top to bottom.
I have raised rabbits on and off for the last 30 years. I showed rabbits seriously through the 90's, and would attend 30 to 35 regional shows a year, as well as the Nationals. I have had many rabbits place at the top of their classes at Nationals. My show rabbits were fed
MannaPro 16% pellets, quality timothy hay, and about a tablespoon of whole oats or BOSS each daily. That's it. Kindling does, or does raising a litter would recieve a tablespoon or 2 of Calf Manna daily to help keep them from drawing down.
Very rarely, a rabbit would get sick, and would always be an isolated incident. The rabbit would be removed from the others and treated with injectable Galimycin. I would have to treat possibly one or 2 animals per year, and that is while operating a 175 hole rabbitry.
It often seems like people tend to overcomplicate things, and overlook the basics.