If you look at the chain of food - not only for dogs - but for humans as well - you can see corn is at the base of it all. Even though we try to buy dog food containing some kind of meat (beef, lamb, chicken, salmon, etc) all of those creatures were fed corn.
Considering what goes into corn production these days I'd say we're ALL (dogs, cats, people) are being poisoned resulting in many diseases such as cancer. I'm hearing about dogs and cats getting diabetes. Diabetes!!! Strictly from being fed (allegedly) normal pet food.
Also - I just saw an article about pet Treats killing pets. They mostly blamed China (of course) - but there's direct evidence of pet food killing pets.