Here is a paragragh from the pitbull forums:1) I do NOT feed puppy foods, the main reason being they donít need it. The main difference between a puppy and adult formula is the protein percentage and the supplementation. The protein comes from questionable sources, usually grain, and the supplementation doesnít really help them grow. In some cases it can actually be detrimental. For the large breed dogs giving them too many supplements can cause growth problems, cause them to grow too fast, and it just isnít necessary. Genetics will determine how big a dog will be, food will influence the over all health but not the end results of growth.
My pit was 2yrs old when I got my first large breed dog, a rotti\siberian husky, Buying 2 different dog foods was a pain for me and the dogs, so I only gave her puppy food for the first month, since then she has been on good quality adult dry food, She is happy and healthy at 6yrs now and has never had a health issue. You will not harm your dog giveing it a good quality adult food, It's up to you.