Why do dogs and cats eat grass?

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D
ogs and cats are frequently observed eating grass
and other plants of no apparent nutritional value.
Unfortunately, little on this topic appears in the vet-
erinary literature. Although the prevalence of plant eating in
domestic dogs and cats has not been documented, wild canids
and felids in nature are known to eat grass and plants—plant
material has been found in 2% to 74% 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 deciency?
In a recent study by Dr. Karen Sueda, Dr. Kelly Cliff, and myself,
three surveys of pet owners were conducted to nd answers to
these and other questions about plant eating in dogs.7
FINDING THE ANSWERS
Our surveys were designed to test the hypothesis that most
plant eating in dogs is associated with illness or a dietary
deciency and that ingestion of plant material is usually
followed within a few minutes by vomiting.
First, we surveyed veterinary students who had pet dogs
(n=25) about the frequency of grass eating in their own dogs
and whether the students observed signs of sickness before
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% said
that their dogs regularly vomit afterward.
Next, we surveyed a group of dog owners (n=47) who had
brought their dogs to our teaching hospital for outpatient care.
We asked these owners for their observations on their pets’
Why do dogs and cats 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 may well be the correct answer
to this often-asked client question.
Benjamin L. Hart, DVM, PhD, DACVB
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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648 De c emb er 2008 VETERINARY MEDICINE

consumption of plants and the animals’
behavior before and afterward. Of this
group, 79% 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 hy-
pothesis—that plant eating is not particu-
larly related to illness or vomiting—we
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
questions about their dogs’ plant-eating
habits and diet, and we gathered data on
the dogs’ sex, gonadal status, breed, and
age. After applying inclusion criteria (e.g.
excluding responses from owners who
spent less than six hours a day with their
dogs, and excluding dogs that appeared
to only chew but not ingest plants), we
narrowed the usable surveys to 1,571. Our
ndings included the following:
• Sixty-eight percent of the respondents
said their dogs ingest plants on a daily
or weekly basis.
• Eight percent of the respondents re-
ported that their dogs frequently show
signs of illness before plant eating.
• Twenty-two percent reported that their
dogs regularly vomit afterward.
• Of the plant-eating dog population,
younger dogs ate plants more fre-
quently 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 fol-
low 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 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.
In all the surveys, we asked specic
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
ber in their diets tended to eat plants
more than those getting more ber.
So contrary to the common perception
that grass eating is associated with observ-
able 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 ndings 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 eat-
ing is a common behavior that usually
occurs in normal dogs and cats. It is
generally unassociated with illness or a
dietary deciency but reects 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 an-
cestors (that were always exposed to
intestinal parasites). As observed in wild
chimpanzees, which eat whole leaves
from a variety of plants, the plant mate-
rial passes through the intestinal tract,
increasing intestinal motility and wrap-
ping 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 an-
cestors 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’ ten-
dency to consume plants, let them know
that their pets are fairly typical—most
dogs and cats consume some plant mate-
rial. In addition, plant consumption is not
usually associated with gastrointestinal
illness but instead may be a trait inher-
ited from their wild ancestors. Advise
owners to keep their grass-eating dogs
and cats away from chemically treated
lawns and toxic plants.
❖
REFERENCES
1. Andersone Z, Ozolins J. Food habits of wolves Canis lupus in
Latvia. Acta Theriologic 2004; 49:357-367.
2. Andersone Z. Summer nutrition of wolf (Canis lupus) in the
Slitere Nature Reserve, Latvia. Proc Latvian Acad Sci 1998; 52:79-80.
3. Papageorgiou N, Vlachos C, Sfougaris A, et al. Status and diet
of wolves in Greece. Acta Theriologica 1994;39:411–416.
4. Mech LD. Results—the 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.cr.nps.
gov/history/online_books/fauna7/fauna5g.htm.
5. Stahler DR, Smith DW, Guernsey DS. Foraging and feeding
ecology of the grey wolf (Canis lupus): lessons from Yellowstone
National Park, Wyoming, USA. J Nutr 2006;136:39:1923S–1926S.
6. Robinette WL, Gashwiler JS, Morris OW. Food habits of the
cougar in Utah and Nevada. J Wildl Manage 1959;23:261-273.
7. Sueda KLC, Hart BL, Cliff KD. Characterisation of plant eating
in dogs. Appl Anim Behav Sci 2008;111:120-132.
8. Huffman MA, Canton J. Self-induced increase of gut motility
and the control of parasitic infections in wild chimpanzees. Int J
Primatol 2001;22:329–346.
9. Huffman MA, Page JE, Sukhdeo MVK, et al. Leaf-swallowing
by chimpanzees: a behavioural adaptation for the control of
strongyle nematode infections. Int J Primatol 1996;17:475-503.
Gender-specific behaviors
in dogs and cats
To hear Dr. Hart talk about
some surprising differences
between these species, go to
dvm360.com and click
on “Medicine.”
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