Pentobarbital and phenobarbital are both used to put animals to sleep. The thing is, when you find it in dog food, you know it came from a euthanised dog or cat. They don't use either of these on livestock. It's too expensive.

The study is old though -- since Nutro was alerted of the problem, they got everything under control and there is no more pento in their food. Pedigree on the other hand does have it. They are now famous for putting euthanised pets in their foods.