Using milk for babies, in particular cows milk, has to be done with a certain caution. While there are some that can handle the dairy with no issues, there are many who will experience a little discomfort. When visiting a pediatrician, the recommended age for cow's milk to be introduced to an infant's diet is not until they have reached between 12-14 months, as this allows for their digestive system to fully develop, and handle the many different nutrients that are in the milk.
However, when given the green-light to introduce this, it can help to build a strong system, as they continue to grow. Before this age, there are other products that can be used. Many different formulas have proteins, if you are using these. Otherwise, the milk supplied by the mother is sufficient to produce all the nutrients needed, until the infant is old enough to handle a more solid and substantial diet.