I have an interesting article titled Why do dogs and cats eat grass? by Benjamin L. Hart which was published in the journal Veterinary Medicine (December 2008, 103, 12 pp648-649).
The author did several small surveys of students and customers who were dog owners based on the assumption that plant eating is associated with illness or dietary deficiency. The results indicated that illness was unlikely to be the reason for eating grass and that vomiting afterwards is not as common as thought.
To dig a bit deeper they used an online survey to ask dog owners about plant eating behaviour. With 1571 useful responses:
67% said their dogs eat plants daily or weekly.
8% reported their dogs showing signs of illness before eating plants.
22% said their dog regularly vomits eating plants
Younger dogs are seen eating plants more often than older dogs
There was no indication that dogs with lower fibre in their diet ate plants more than other dogs.
Contrary to popular opinion, most dogs that eat grass or other plants are not unwell and most do not vomit afterwards. So why do they eat grass?
Both domestic and wild dogs eat plant material. It does not seem to be associated with illness or dietary deficiency but is a common behaviour so presumably serves some purpose.
The theory put forth by the author of this article is that eating plants helps purge parasites such as intestinal worms from the gut by increasing how quickly material moves through the gut and also by wrapping around the worms and carrying them out of the body
My question then is; why does my disgusting dog eat her own poo? She does eat grass but the theory of removing parasites is defeated by her coprophagy!