Every cow killed and eaten means not only so much less meat available but so much less of an adequate substitute.
Lean meat contributes to the diet chiefly protein and iron.

We eat it primarily for the protein.

Hence in comparing meat and milk we think first of their protein content.

One and one-fourth cups of milk will supply as much protein as two ounces of lean beef.

The protein of milk is largely the part which makes cottage cheese.