Dogs, cats, farm animals, lab animals, road kill, you name it, goes into the making of most dog food. Buy the book by Ann N. Martin "Food Pets Die For". She had the industry in and out of court for over seven years. She had undercover people inside those rendering plants. If you are sure that there is a dog food company that doesn't use these practices, then call and ask if they give "tours". If they have nothing to hide, then they will show you around inside the rendering plant.
Closer Look At A Rendering Plant The Dark Side of Recycling