This link from kellymom has really great info on introducing cow's milk to a baby (click on link for more wonderful tips regarding milk) ...
"Many nursing moms are told that they must introduce cow's milk at a year. Your nursing toddler is already getting the best milk he can get - mother's milk! Breastmilk has a higher fat content than whole cow's milk (needed for baby's brain growth), and all the nutrients of human milk are significantly more bioavailable than those of cow's milk because it is species specific (not to mention all the components of mother's milk that are not present in cow's milk).
There is no need for additional milk or (or the equivalent nutrients from other foods) as long as your baby is nursing 3-4 times per day. Cow's milk is really just a convenient source of calcium, protein, fats, vitamin D, etc. - it's not required. There are many people in many parts of the world who do not drink milk and still manage to get all the calcium, protein, fats, vitamin D, etc. that they need."
If you still want to introduce cow's milk, it is generally said to wait until at least one year old. Dr. Sears' advice is "no cow's milk before age one, whole milk until age two." And I would recommend going organic when you do introduce it. Hope this helps!