I can't answer your question, but I tried various searches on Google and came up with some key words that might help you. The resulting information is not the easiest to digest but there should be something there. Try: growth rate rabbits vitamin B12 (assuming you haven't already).

From what I know of rabbits, their digestive systems more closely resemble horses than goats. I believe, however, that Vitamin B12 is water-soluable, which should mean that excess is excreted... which in theory should make it fairly safe to supplement.

I am probably not the right person to advise you because I lean strongly towards natural feeding. I feel that properly fed, no healthy creature should need extra vitamins. But that's just me. Why not hop on over to the Homesteading Today Raising Rabbits for Profit forum and ask you question there. Lots of show people and commercial producers as well as backyard breeders, so someone should be able to help you.

Here is a link that might be useful: HT Rabbit Forum

Thanks for that
Im trying the best I can to do everything through natural feeding as well, but last year that did not turn out as well as Id hoped. I just want to make sure that Im doing all that I can for the little guys. Thank you for the link as well, its turning out to be very informative