This slaughterhouse waste includes bones, blood, pus, and possibly diseased & cancerous meat, and is often contaminated with Salmonella (found in 25-50% of meat samples.) In 1990, the San Francisco Chronicle revealed that euthanized companion animals were being used in pet food. Although the pet food industry vehemently denied it, the American Veterinary Association confirmed the report.
As much as 97% of pet food is comprised of some form of grain product, usually corn. To hide the true proportions, the industry practices "splitting." In one of the most popular dog food brands, for example, Ground Yellow Corn & Corn Gluten Meal are listed after chicken, making it appear there is less grain than meat, even though the combined weight of the corn ingredients outweighs the chicken.
The final processed mixture is sprayed with a blend of preservatives, refined animal fat, kitchen grease and other substances you wouldn't feed a dog.