Use of cow's milk before a year is controversial among experts. You might want to get your baby's doctor's opinion on this.
The American Academy of Pediatrics recommends NO cow's milk until after the first birthday.
Cow's milk is more specific to a baby cow than a baby human. Cow's milk formula is based on cow's milk but has been engineered to be closer to human milk (still a ways off, but closer). Many infants still have problems with cow's milk formula (allergies, GI problems, etc.). Babies who are exposed to cow's milk before their first birthday are more likely to be anemic, have diarrhea or vomiting, and/or experience an allergic reaction (the proteins in milk are more numerous than those in other milk products, such as the yogurt). The excessive protein load in cow's milk can also overload a baby's kidneys. It is deficient in vitamins C, E, and copper. It is harder to digest as well, often causing intestinal blood loss. A number of studies have also indicated that early introduction of cow's milk may contribute to the development of Insulin-Dependent Diabetes Mellitus.
Others see no problem with starting cow's milk toward the end of the first year unless there is a family history of allergy to it. Dr. Jack Newman, a renowned expert in the lactation field, is one of these. See the Breastmilk, cow's milk, formula, outside work and bottles section of his article "Breastfeeding and Other Foods." Notice that he qualifies this advice with the importance of the baby being well-established on a variety of solids and continuing to be breastfed (both of which should help make up for what cow's milk can't offer at this age). He also suggests that supplemental milk of any kind is not all that necessary if the baby is allowed to nurse frequently when with mom. He suggests offering the baby solid foods with some water or small amounts of juice instead.