Science Diet and Iams I believe are one of the commerical producers that have been known to use dead dogs/cats euthanized from shelters in their food. They are among the worst. I would never buy commercially produced pet food containing meat! Commercial pet food contains the lowest quality meat "the 3D's" (dead, diseased, dying).
Did you ever consider a homemade diet? My dogs eat a homemade diet consisting of a stew made of brown rice, lentils, pureed sweet potatoes along with a variety of other veggies (kale, spinach, squash, green beans, peas, etc.) with small amounts of poultry that I purchase from a local, organic meat supplier. I also add in dry vegetarian food (natural balance or v-dog brand) and on occassion feed canned vegetarian food in between batches. They have many meatless meals and 1lb of meat lasts my two dogs 3-4 days. I wish they could be veg., but I am just not confident enough to do it full time with them :(