I found this on kellymom.com (http://www.kellymom.com/nutrition/milk/milk-supplements.html)
In general, only breastmilk or formula should be used if your baby is less than six months old.
Between six and twelve months, supplementing with solids (instead of formula) or very small amounts of cow, goat, soy or rice milk is less of a problem, as long as baby is still nursing for the majority of milk intake and baby is not allergic. However, babies under a year are more at risk for allergic reactions (see below) so it can be a good idea to wait.
After a year, other milks may be used, but are not needed (other sources provide the same nutrients). It's recommended that you limit the amount of cow's milk that your child receives (possibly other milks too, except breastmilk) to 2-3 cups (16-24 ounces) per day. Too much cow's milk in a child's diet can put him at risk for iron-deficiency anemia (because milk can interfere with the absorption of iron) and decrease the child's desire for other foods. More here on cow's milk after a year.
Cow's Milk
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.
Yogurt and cheese
Some experts consider yogurt and cheese to be okay for most babies after 6 months. Others prefer waiting until 9-12 months. Of course, if baby has a cow's milk allergy or there is a strong history of allergy to cow's milk in the family, yogurt, cheese and all other foods made with cow's milk should be avoided until 12 months or later. The main difference between yogurt and milk is that the lactose in yogurt has been converted into lactic acid. Cheese differs from milk in two ways: the whey proteins are drained off in the cheese-making process (so only casein proteins remain), and the curdling enzymes break down the casein proteins into smaller protein molecules. For babies and toddlers, try to find the whole milk yogurt (not low or no fat), and avoid the yogurt with artificial sweeteners lots of sugary fruits.
