In all the surveys, we asked specific questions about the dog's diet. There was no indication that dogs fed primarily table scraps or raw food were more prone to grass eating than those on a commercial, nutritionally balanced diet. Nor was there any indication that dogs receiving less fiber in their diets tended to eat plants more than those getting more fiber.

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.

WHAT ABOUT CATS?

AN ETHOLOGIC EXPLANATION: HERBAL PROPHYLAXIS

Our current hypothesis is that plant eating is a common behavior that usually occurs in normal dogs and cats. It is generally unassociated 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 (that were always exposed to intestinal parasites). As observed in wild chimpanzees, which eat whole leaves from a variety of plants, the plant material passes through the intestinal tract, increasing intestinal motility and wrapping around worms and thereby purging the tract of intestinal nematodes.^8,9 In our study, younger animals were observed to eat plants more frequently than did older animals.⁷ Perhaps young animals eat plants more often because they are less immune to intestinal parasites and are actively growing, thus nutritional stress could be more costly than in adults.

Whether intestinal parasites in wild ancestors of domestic cats were less prevalent than in wild ancestors of domestic dogs is an open question. Certainly cats are more fastidious about making their feces, a major source of intestinal infestations, less available for incidental ingestion.

CONCLUSION

When owners ask about their pets' tendency to consume plants, let them know that their pets are fairly typical—most dogs and cats consume some plant material. In addition, plant consumption is not usually associated with gastrointestinal illness but instead may be a trait inherited from their wild ancestors. Advise owners to keep their grass-eating dogs and cats away from chemically treated lawns and toxic plants.

Benjamin L. Hart, DVM, PhD, DACVB
Department of Anatomy, Physiology and Cell Biology
School of Veterinary Medicine
University of California
Davis, CA 95616
Coauthor of Canine and Feline Behavior Therapy, 2nd edition, Blackwell Press 2006.

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