Cows are not extracting protein from grass (well not much), they are converting the cellulose in the grass into microorganisms and then extracting the proteins from the microorganisms. The ruminant digestive system is an amazing system [1].
Cows actually can't digest cellulose at all - what they can do is harbour the microorganisms that can. These microorganisms are then eaten by the cow and provide most of the protein in a cows diet.
1. https://en.m.wikipedia.org/wiki/Ruminant