I have heard both good things and bad things about Purina. When I had horses, we usually bought them the Omolene made by Purina, and Omolene was considered to be one of the best horse feeds on the market. We also fed our dogs Purina Dog chow, with no ill effects; but of course, we also supplemented that with meaty bones from the butcher shop for the dogs to chew on. They would cut up a lot of beef, and in hunting season, there was always venison, or elk bones in the barrel, and the butcher shop would give you the barrel of dog bones, as long as you brought the barrel right back for them to re-use.
We had Great Danes back then, and they really loved chewing on those big knuckle bones, and whatever else was in the barrel.