Does the dog food have a name, like Purina, or Natural Choice? If it is a no name brand or just a store brand, then it is probably not very nutrional. The best diets are the veterinary kinds, but those can be expensive. I use Natural Choice.
My dog sometimes eat poop. We don't have cats, or any thing that she has access to, so it's not a very common problem.
However, she likes to roll in our horse's poop. Most dogs will roll in poo because it makes them smell like their prey. (I don't know where they get the idea that horse is a prey). It might also be just because they like the smell.