Interesting; see the article.
http://truthaboutpetfood.com/duplicate-minerals-in-pet-food/
Guess what form is so much cheaper?

Also, see this article on a veterinarian, like Cornell's vet, cautioning against the liver problems linked to changes in copper supplements in today's pet food.
http://truthaboutpetfood.com/a-veterinarian-takes-a-stand-against-a-common-pet-food-supplement/
The website link below is clickable to this article.