Fun fact: Rabbits eat their fecal pellets (they have soft and hard poops, pellets are the hard ones). Those are filled with half digested plant matter. Since they are exposed to bacteria during the first digestion, those fecal pellets are full of vitamins. B12 is one of them. They often eat those dried pellets directly from their fur as breakfast to get the energy to start the day.
And how do the bacteria get there? Do they get there if they're fed sterilized food or other cows? And are the bacteria killed off by the antibiotics they're given?
Pre-seeded during breast feeding, as happens across all mammals. How many billions of years do you want to go back on this question?
> And are the bacteria killed off by the antibiotics they're given?
Evidently not, as in developed country sick animals don't get turned into meat foods on principle and by law --- so they don't show up deficient. If they lived for 9 decades like we do, they might (or not). But they don't.