In a study conducted by Dr. Benjamin Hart, a distinguished professor emeritus at the University of California –Davis, Dr. Karen Sueda, an animal behavioral specialist, and Dr. Kelly Cliff, a contributing author in the "Textbook of Veterinary Internal Medicine," more than 3,000 dogs were observed for their grass-eating habits.

Initially, the team surveyed out-patient dogs and students’ dogs at the School of Veterinary Medicine to determine a base pattern of behavior. Each student reported their dog ate grass, while 79% of the out-patient dogs ate grass. Very few reported dogs vomiting after snacking in the yard.

With this knowledge in hand, the team launched a Web-based study with more than 3,000 dog owners.

According to Hart, the surveys “were designed to test the hypothesis that most plant eating in dogs is associated with illness or a dietary deficiency and that ingestion of plant material is usually followed within a few minutes by vomiting."

The team studied each dog’s grass-eating habits and diet, and gathered data on gender, breed, age and whether the dogs were spayed or neutered. Their findings revealed that:

•    Sixty-eight percent of the participants said their dogs ingest grass on a daily or weekly basis.

•    Eight percent of reported their dogs frequently showed signs of illness prior to eating grass.

•    Twenty-two percent reported their dogs routinely vomited after eating grass.

•    Younger dogs ate plants more frequently than did older dogs and were less likely to appear ill beforehand or to vomit afterward.

Is Illness a Reason Dogs Eat Grass?

Unlike most dogs who eats grass and vomits most of the time, the majority of grass-eating dogs don’t vomit after grazing. And most dogs do not show signs of illness prior to eating grass.

So, what do the veterinarians deduce from this data?

Writes Hart, “While we attempted to exclude dogs with known medical problems, it is possible that subclinical gastric or intestinal distress occasionally evokes grass eating, which may facilitate vomiting.

"In fact, we found from our large study that if dogs showed signs of illness before eating plants, they were more likely to vomit afterward than were dogs that did not show signs of illness beforehand."

So when it comes to dogs eating grass due to an upset stomach, what did the veterinarians conclude?

"Contrary to the common perception that grass eating is associated with observable signs of illness and vomiting,” writes Hart, “we found that grass eating is a common behavior in normal dogs unrelated to illness and that dogs do not regularly vomit afterward. Vomiting seems to be incidental to, rather than caused by, plant eating.

"In layman’s terms, it’s in Hart’s opinion that most dogs are not eating grass because they feel unwell.

Do Dogs Eat Grass Due to Dietary Deficiency?

Could your dog be eating grass because he’s lacking fiber in his diet? Some pet owners have reported that after changing their kibble, or switching them to a raw diet, they stopped eating grass. Coincidence?

In the grass-eating study, specific questions were asked about each dog’s diet: did the dogs eat table scraps, a raw diet or a commercially-produced kibble?

The results showed there wasn’t any indication that dogs receiving less fiber in their diets tended to eat grass more often than those getting more fiber.

Hart does point out that younger dogs were observed to eat grass more frequently than older ones; could this be because they are actively growing? “Nutritional stress could be more costly than in adults,” writes Hart.

Do Dogs Control Intestinal Parasites by Eating Grass?

Another ongoing theory is that hundreds if not thousands of years ago, our dogs’ wild ancestors — frequently exposed to intestinal parasites — would purge the pesky bloodsuckers by eating a variety of plants, including grass. The plants then pass through the intestinal tract, wrapping themselves around worms and pushing them out with feces.

Could our dogs still inhibit these ancestral habits? This remains an open question. Hart’s study does not mention whether or not they conducted fecal tests for parasites with any of the dogs being observed. If you're worried your dog is eating grass because he may have intestinal parasites, check your dog’s feces daily for signs of parasites. To learn more about pets and parasites, check out this helpful article on signs of parasites and treatment.

The Leading Reason Dogs Eat Grass

The good news: According to Hart, eating grass is not abnormal behavior for dogs. After studying their results, Hart, Sueda and Kelly believe grass eating is not associated with illness or a dietary deficiency, its an “innate predisposition inherited from wild canid ancestors.

Says Hart, “More studies are needed, but plant eating likely serves a biological purpose."

So, most of our dogs just like eating grass.