Medicated chick feed. I read in my book Chickens In Your Backyard by Rick and Gail Luttmann that you should definitely not feed medicated chick feed to your chicks after 12 weeks of age, because after about 8 weeks of age they are building up a natural immunity and may interfere with the development of this natural immunity. Well, the chick feed that I have is Purina Medicated Start and Grow and the package says up to 18 weeks for small to medium breeds and fancy and exotic breeds. I have buff orps and australorps which are heavy birds. Do I switch them to flock raiser? Ive read here that some continue to feed medicated up to 18 weeks. Im very confused!!