Marion Nestle, in her book Pet Food Politics, says:
"Pet foods have always been made from the leftover parts of slaughtered farm animals that are not going to be used for human food—the bones, organs, ears, and other nutritious by-products. The need for an outlet for the leftovers of animal slaughter is one of the reasons commercial pet food exists."Where do those animals that get made into dog and cat food come from? Foer says:
"Upwards of 99 percent of the animals that are raised for meat in this country come from factory farms."