I found this posted on a vet site I go to.

Why do dogs eat grass may be my least favorite, frequently asked, question. No dog has ever confided in me why he or she does this! Several interesting and plausible theories exist to explain this behavior.

Dogs, unlike cats, are not purely carnivorous. Certain members of the dog family are known to eat wild berries and other plant foods. Wild carnivores living a predatory lifestyle will often eat the contents of their victim's gastrointestinal tract before they consume the carcass. This probably fulfills a need to balance the diet with vegetable material and roughage, albeit "predigested". Thus one theory is that a grass-eating dog is seeking the roughage that some modern low residue pet foods may lack.

A theory popular with holistic health advocates is that the dog is merely supplementing his diet. The thinking here is that grass supplies some nutrient that modern foods have lost in processing. They also believe that, in addition to grass, the dog would probably benefit from a diet supplemented with herbs, perhaps thinking that this is closer to what dogs' wild ancestors ate. However, there is no experimental scientific evidence to substantiate this.

Another explanation, suggested by an online purveyor of canine supplements, is that fresh grass contains organic sulfur, a compound thought to be essential for proper digestion. Not surprisingly, the remedy offered to curb a pet from grass eating is a sulfur supplement sold through the same online site.

Some animal behavior experts think that grass makes it possible for the dog to rid his stomach of something that is either indigestible or poisonous. The offending material - often just a thick liquid or mixture of liquid with something undigested - is bound up by the grass and expelled when the grass becomes an irritant and causes the dog to vomit.

Dr. Kathleen Houpt, a veterinary behaviorist at Cornell University's College of Veterinary Medicine theorizes that sometimes when a dog appears to be eating grass, he may actually be running it through his mouth to use his senses to detect what animals have walked on or urinated upon the grass. In such cases, the dog will usually not attempt to swallow the grass.

Another theory advanced by Dr. Houpt is that dogs eat many things - garbage, for example - that humans do not consider palatable, so it may just be that our canine companion is eating grass because he likes the taste of wants the canine equivalent of a salad.

The theory that some dogs simply want variety in their diet has proponents, but, to date, no studies have shown this to be true.

The bottom-line is that grass eating, with and without consequent vomiting, concerns pet owners. If it is not continuous and/or accompanied by loss of appetite, it most likely does not indicate a problem. Vomiting usually occurs shortly after grass ingestion so when you see "Candy" grazing in your yard, it is wise to keep her outside for a short while.