I recently learned that pet food uses "4-D meat". This is flesh of animals who arrive at the slaughterhouse already dead, dying, diseased, or disabled. It includes cancerous tumours, worm-infested organs and anything else deemed unfit for humans to eat.

Many rendering plants making pet food happily accept roadkill, dead zoo animals, and euthanised pets from animal shelters and veterinary clinics.

Included with the animal carcasses in the rendering step are cattle ear tags with fly repellants, pet collars and ID tags, and packaging (plastic, foam, boxes) from expired grocery store meat.

This podcast episode has a wider discussion of pet food and living with companion animals as vegans. https://pca.st/episode/e9f8eb3e-89e5-4a2a-acd3-55efd9ee4533