Not implying Diamond or Proctor and Gamble do it themselves but it is sometimes purchased by the larger pet food companies as it is cheap protein. Rendering companies mix "unnamed" animal by-products and protein and sell to the food companies as it is cheap. This is why you should always buy pet foods with specific proteins such as chicken, beef, lamb, etc. and no by products. There are some by-products that are better than others but you don't know which is in the food you buy.
I just want to know where all the ingredients come from and if any preservatives are used (preferrably none as most are toxic) and it to be made in a clean, private plant. I am also assured a good quality product since it is rated human grade and the great majority of pet foods are not human grade. The fact it is sustainable is a nice bonus. You can't even get that at McDonalds!