The dairy cow is the most efficient means of turning grass on a rocky hillside into human-usable protein [0].
However, we(humans) as we always do, took things too far, and have been turning lots of wild land to make grass as food for these cows that eventually turn them into proteins. That only leads to more and more grass production and burping cows :-(
This is not the reference, but something I just found on google books about the efficiency of dairy cows: https://www.google.com/books/edition/Farming_the_Business_Ma...
(responding to request for the reference for the dairy cow as an efficient converter of grass on rocky hillsides).