Why do dogs and EAT GRASS?
A) They are sick and need to vomit.
B) They have a dietary deficiency.
C) Studies point to a third option that may well be the correct answer to this often-asked client question.
BY BENJAMIN L. HART, DVM, PhD, DACVB
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Dogs and cats are frequently seen eating grass and other plants
of no apparent nutritional value. Unfortunately, little on this topic appears
in the veterinary literature. Although the prevalence of plant eating in domestic
dogs and cats has not been documented, wild canids and felids are known to eat
grass and plantsplant material has been found in 2 percent to 74 percent
of scats and stomach content samples of wolves and cougars.1'6
Clients commonly ask questions about plant eating in their pets: Do animals eat grass to induce vomiting because they're sick? Do they eat plants because they have a dietary deficiency? In a recent study by Dr. Karen Sueda, Dr. Kelly Cliff, and myself, three surveys of pet owners were conducted to find answers to these and other questions about plant eating in dogs.7
Finding the answers
Our hypothesis was 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. First, we surveyed veterinary students who owned a total of 25 pet dogs about the frequency of grass eating in their own dogs and whether the students observed signs of sickness before
cats grass consumption or vomiting afterward. All of the students reported that their dogs ate grass. None reported observing signs of illness before their dogs ate grass, and only 8 percent said that their dogs regularly vomited afterward.
Next, we surveyed 47 dog owners who had brought their dogs to our teaching hospital for outpatient care. We asked these owners for their observations on their pets' consumption of plants and the animals' behavior before and afterward. Of this group, 79 percent had observed their dogs eating plants (grass was the plant most frequently consumed). Thirty-three owners answered questions about their dogs' behavior before and after eating plants. Of these owners' dogs, signs of illness were infrequent (four dogs), and vomiting afterward was only occasional (six dogs).7
Faced with the prospect of a null hypothesisthat plant eating is not particularly related to illness or vomitingwe conducted a large Web-based survey targeting owners of plant-eating dogs. More than 3,000 owners responded to our survey. We asked these owners about their dogs' plant-eating habits and diet, and we gathered data on the dogs' sex, gonadal status, breed, and age. After excluding responses from owners who spent less than six hours a day with their dogs as well as dogs that appeared to only chew but not ingest plants, we ended up with 1,571 usable surveys.
Our findings included the following:
Sixty-eight percent of the respondents said their dogs
ingest plants on a daily or weekly basis.
Eight percent reported that their dogs frequently show
signs of illness before plant eating.
Twenty-two percent said their dogs regularly vomit afterward.
Of the plant-eating dog population, younger dogs ate
plants more frequently than did older dogs and were less likely to appear ill
beforehand or to vomit afterward.
Our study revealed that a few dogs do appear to be ill before
eating plants and that vomiting does sometimes follow plant eating. 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 that if dogs showed signs of illness before eating
plants, they were more likely to vomit afterward than were dogs with no signs
of illness beforehand.
In all the surveys, we asked specific questions about the dogs' diet. There was no indication that dogs fed primarily table scraps or raw food were more prone to grass eating than those receiving 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?
In an ongoing study with my colleagues Drs. Sueda, Melissa Bain, and Gretel de la Riva, preliminary findings suggest that plant eating is less common in cats than in dogs. As in dogs, cats typically do not appear to be ill before eating plants nor do they regularly vomit afterward. Our preliminary data suggest that cats eat more nongrass plants than do dogs.
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 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. As observed in wild chimpanzeeswhich eat whole leaves from a variety of plantsthe plant material passes through the intestinal tract, increasing intestinal motil-ity 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 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
We found that grass eating is a common behavior in normal dogs unrelated to illness.
open question. Feces is a major source of intestinal infestations, and cats are certainly more fastidious about making their feces less available for incidental ingestion.
What to say to clients
When owners ask about their pets' tendency to consume plants, let them know that their pets are fairly typicalmost 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. However, if the dog or cat appears ill before eating plants or if vomiting persists, a medical checkup is in order. Also, advise owners to keep their grass-eating dogs and cats away from chemically treated lawns and toxic plants.
1. Andeisone Z, Ozolins J. Food habits of wolves Cmistupusin Latvia. Ada Theriobgic 2004,49357-367
2. Andersone Z. Summer nutrition of wolf (Cants /upus) in the Slitere Nature Reserve, Latvia. Proc LatvimAcadSci 1998^2:79-80.
3. Papageorgiou N, Vlachos C, Sfougaris A, et al. Status and diet of wolves in Greece. Acta Theriologica 199429:411-416.
4. Mech LD. Resultsthe Timber wolf and its ecology. Fauna of the National Park of the United States: The Wolves of Isle Royale. National Park Service. 3 September 2004. http://www.anps. gov/history/online_books/fauna7/fauna5g.htm.
5. Stahler DR, Smith DW, Guernsey DS. Foraging and feeding ecology of the grey wolf (Cams lupus): lessons from Yellowstone National Park, Wyoming, USA. JNufr 2006;136:1923S-1926S.
6. Robinette WL, Gashwiler JS, Morris OW. Food habits of the cougar in Utah and Nevada. } ViM Manege 1959;23:261-273.
7. Sueda KLC, Hart BL, Cliff KD. Characterisation of plant eating in dogs. Appl Anim Behav Sti 2008;111:120-132.
8. Huffman MA, Canton J. Self-induced increase of gut motility and the control of parasitic infections in wild chimpanzees. M J Primtol 2001^2:329-346. ,
9. Huffman MA, Page JE, Sukhdeo MVK, et al. Leaf-swallowing by chimpanzees: a behavioural adaptation for the control of strongyle nematode infections, hit} Primatol 1996;17:475-503.
Benjamin L. Hart, DVM, PhD, DACVB, is a distinguished professor
emeritus in the Department of Anatomy, Physiology and Cell Biology at the University
of California-Davis' School of Veterinary Medicine. Dr. Hart's research is supported
by UC-Davis' Center for Companion Animal Health.
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