"The US Deptartment of Agriculture uses the term '4-D' to indicate flesh that's considered unfit for human consumption. the 'D's' are: dead, dying, disabled, and diseased. In all but a few states, flesh from 4-D animals can be sold to pet food manufacturers ... (Jim) Mason and (Peter) Singer report an average of 15 million pounds of cancerous tissues a year (added to slaughterhouse reject piles - much of which is rendered into) 'by-products' on pet food labels."
~ Barbara Lynn Peden, Dogs and Cats Go Vegetarian
(note: I can't find my copy of Dogs and Cats Go Vegetarian so I don't have the year of these statistics, but my dogs have gone vegetarian)