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. So, I am afraid that it is true.
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. You might be interested to know that pellets containing cellulose and urea actually feed the rumen bacteria colony. If we do not supplement their diet with animal protein, then growing foetuses emerge very small and the milk flow is very poor. Sorry, but these are proven experimental facts.