It rips grass in large clumps, and swallows it whole. This mash is then re-swallowed into the first big chamber, called the rumen. This is a fermentation sack where bacteria and enzymes live and grow by eating the cellulose in the grass. There is a fine mesh at the back of the rumen, which only allows liquid through. The liquid that passes through this mesh contains huge quantities of bacteria and enzymes, and these are digested and are absorbed by the stomach.
Cows live on a very rich animal protein diet.
In fact, when we have to buy winter supplements for their food, the manufacturer has to guarantee that any animal protein is not 'rumen degradable'. If it is, we are giving expensive food to bacteria not to the cow. Sorry, but these are proven experimental facts. Nobody would spend money on expensive animal protein supplements if they were not necessary, as vegetable protein alternatives are cheap. Your comments on land clearance were interesting, but unfortunately not correct.