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.
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?
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.
But we don't only need calories. In fact we already produce and eat many more calories than we need (but we distribute them unevenly so that a third of the world eats too many and 2/3s not enough). The reason we feed all this grain to animals is so that they turn it into nutrient-dense meat, dairy and eggs. Those are rich in micronutrients (vitamins and minerals) that we can't get as well from grains (or not at all like B12 vitamin).
So maybe farming looks "inefficient" if you consider only calories, but when you consider nutrients in general, meat (including seafood), dairy and eggs are actually the most efficient source of those we need to stay healthy and thrive.
But even if we only count calories, the amount of cropland that we would free for rewilding if we stopped farming meat completely and produced no meat from ruminants, no pork or poultry, no dairy, and no eggs, is miniscule:
https://ourworldindata.org/land-use-diets#more-plant-based-d...
In short, we'd go from 1.17 billion ha of cropland for farming animals and growing crops, to using 1 billion ha of cropland.
That's a tiny change and it would most likely quickly need to grow as the global population increases. And it's such a tiny change because most of the land used to farm animals, or grow grains for animal feed, is not cropland, but pasture, which is for the most part not suitable for growing crops.