It's absolutely incredible how much more efficient cow milk is per kg compared to beef. Some of that must be because milk is even more watery than meat is, but the rest must come from being able to get milk from cows throughout their (kind of miserable, sadly) lifespan.

A huge amount of muscle is grown and destroyed cyclically as part of a cow's natural life. It's horrific, but imagine if meat could be "harvested" from living animals - how much would that improve beef's efficiency? Is there a less-abusive to sentient critters equivalent of this idea? (mushrooms?)

I think the equuvalent that still produces meat is lab grown meat. Then you can just grow the parts you need to eat it, which should be about as efficient as it gets once it's all figured out.