B12 is basically never made by eukaryotes. We humans use bioreactors or traditional fermentation cats to have bacteria do the job. Ruminants like cows get B12 by fermentation in the gut. Animal with smaller guts like chicken need it in food like us, generally via fermented food. There is no significant difference in bioavailability that I know of for B12, but there’s an argument to be had that meat is more palatable than pills or weird fermented acquired-taste foods.

Iron is probably the only nutrient I can immediately think of where there’s a significant (~2x) bioavailability difference between plants and animals. Not big enough to make the animal source "more efficient" from a resource use perspective considering the feed conversion still.

> We humans use bioreactors or traditional fermentation cats to have bacteria do the job.

OK, same line of reasoning as above, I guess. Answer me this question please: how did we get the B12 we needed to stay helahty before we knew about B12, or any other vitamin, or were capable of creating in bioreactors?

> Why do they still need to be injected with supplements like vitamin B12?

I've had this conversation many times before: maybe animals need to be supplemented with B12 where you live, but not where I live and I'm pretty sure not in most of the world, either. If we couldn't get enough B12 from meat and animal products we'd have been in real trouble a very long time ago. Like a couple hundred thousand years ago, at least.

But I should not use the word "efficiency" in this kind of context because it's really so vague what "efficiency" means that it can be used to mean anything at all and justify any kind of conclusion at all, even completely contradictory conclusions. For example: are chickens "efficient"? Sure, because you can pack a hundred of them in a square meter! Aren't chickens really "inefficient"? Obviously, since you need one chicken to lay one egg a day!

Mea maxima culpa. Animals are not "efficient" producers of B12. Instead, eating the meat of animals is how we naturally acquire our B12. Regardless of who actually makes it. How does that sound? Better I hope.