The city of Los Angeles alone, for example, sends some two hundred tons of euthanized cats and dogs to a rendering plant every month. Added to the blend are the euthanized catch of animal control agencies, and roadkill. When the gruesome mix is ground and steam-cooked, the lighter, fatty material floating to the top gets refined for use in such products as cosmetics, lubricants, soaps, candles, and waxes. The heavier protein material is dried and pulverized into a brown powder - about a quarter of which consists of fecal material. The powder is used as an additive to almost all pet food as well as to livestock feed.
Farmers call it 'protein concentrates'.
In 1995, five million tons of processed slaughterhouse leftovers were sold for animal feed in the United States. I used to feed tons of the stuff to my own livestock. It never concerned me that I was feeding cattle to cattle. They are no longer eating solid parts of other cattle, or sheep, or goats. They still munch, however, on ground-up dead horses, dogs, cats, pigs, chickens, and turkeys, as well as blood and fecal matter of their own species and that of chickens.