Great Grass Mystery: Why Do Dogs Eat Grass?

By Sophia Yin DVM, November 2008

Do you ever wonder why your dog eats grass? Is it because he’s sick and he’s trying to induce vomiting? Is he missing fiber or some other nutrient from his diet? Maybe you suspect that eating grass decreases his intestinal parasite load? Well, finally, researchers at the University of California, Davis, Veterinary Medical Teaching Hospital are seeking answers to these questions.

Led by Dr. Karen Sueda, a veterinary resident in the clinical behavior program, the researchers devised an Internet-based survey that asked owners how frequently their dogs consumed plants, the types of plants consumed, the behavior patterns before and after eating plants, and the dog’s regular diet, as well as details of their dog’s medical and behavioral history.

For each hypothesis, they made predictions and then considered which predictions the results supported. For instance, if dogs eat plants because they feel sick or to induce vomiting, then dogs who tend to eat plants should act sick either before or after eating the plant, or should vomit after eating the plant.

This isn’t the end of the study—Sueda and her colleagues continue to investigate this puzzle. Stay tuned for updates.


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Update: More than 3,000 people responded and about 68% of the usable respondants stated their dog's ate grass on a weekly basis. What's interesting is that only 8% of respondents saw signs of illness before their dog's ate grass. Of the 22% of dogs that vomited after, more showed signs of illness before ingesting the grass.

So contrary to the common perception that grass eating is associated with observable signs of illness and vomiting, 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. ...Our current hypothesis is that plant eating is a common behavior that usually occurs in normal dogs and cats. It is generally unas-sociated with illness or a dietary deficiency but reflects an innate predisposition inherited from wild canid and felid ancestors. More studies are needed, but plant eating likely serves a biological purpose. One explanation is that plant eating played a role in the ongoing purging of intestinal parasites (nematodes) in wild canid and felid ancestors.

So on a final note, they concluded that grass eating is a common behavior and doesn't neccessary indicate illness.

Article first appeared in The Bark, Issue 33: Winter 2005

Sophia Yin, DVM, (deceased) was an applied animal behaviorist. A long-time The Bark contributing editor, she was also the author of two behavior books.