A case study of a dog that was a chronic grass eater was published recently. The authors told the story of a dog that ate grass and subsequently vomited for 7 years. They ran all the usual tests and even conducted a food trial and endoscopy to help find this dogs problem. In the end they increased the dogs fiber intake. The vomiting and grass eating stopped and had not returned at 13 months. While not a scientific study, it raises the question that maybe some dogs eat grass because they are not getting enough fiber in their diet.
This lead me to two conclusions:
First, each patient is an individual. While some dogs may do very well on one type of dog food, others pay actually be harmed by it. This is why when you go to the vet's office they want to run tests. If we start treating every dog the same, we will injure a lot of dogs.
Second, there is still so much we don't know. Chasing down these cases is what makes medicine so much fun.
A high fiber diet responsive case in a poodle dog with long-term plant eating behavior. Kang B-T, Jung D-I, Yoo, J-H, et al. J VET MED SCI 69:779-782, 2007.