Recently, I was giving a one-day course in dog behavior. During one of the breaks, a man came up to me and introduced himself as a veterinarian who was attending in order to catch up on the latest information about the way dogs think. He then went on to say:
"While I'm here, I thought I'd ask you something which I am always questioned about. Almost every week or two one of my clients asks me 'Why do dogs eat grass?' I normally gave them the answer that I seem to remember from when I was in veterinary school, namely that dogs eat grass when their stomach is irritated or upset. Supposedly it causes them to vomit and this may provide them with some relief. Unfortunately, when I looked for some scientific data to support that position, I could never find any, so now I usually add that an alternative reason is that dogs might simply like the taste of grass. Do you have a better answer to this question?"
If you Google the question, "Why do dogs eat grass?" you will find 2,520,000 websites that purportedly deal with the issue. I glanced at about two dozen of these and every one of them mentioned eating grass to deal with an upset stomach and to cause vomiting.
I have personally never found this explanation to be particularly satisfying because the cause and effect issues are all tangled up in it — "Do dogs eat grass to make themselves vomit, or do they vomit because they ate grass?" The other issue is that many dogs eat grass and don't end up vomiting (like my Beagle, Darby, who loved to munch grass as if it were a free salad and never once regurgitated from it afterward).
The second most popular answer is that dogs evolved from wild canines, such as wolves, who hunt and eat mostly grass-eating animals like mice, rabbits, or deer. When these canines consume their prey, they will usually eat pretty much all of it, including the grass-filled stomach of whichever creature they just killed. It is usually suggested that there is something in canine physiology that is satisfied by the nutrients that might be in the grass, or perhaps the actual roughage or fiber that comes from the plants is somehow useful. Regardless of which explanation is given, none of the websites I consulted cited any scientific research to support their claims.
When I searched the scientific literature, I was astonished to find that there was only one study dealing with the issue of grass eating and it was relatively recent (2008). The research was conducted at the University of California, Davis, by Karen Sueda, Benjamin Hart, and Kelly Cliff and published in the journal Applied Animal Behavior Science.* This report involved a series of surveys that were designed to test the hypotheses that most plant-eating in dogs is associated with illness and results in vomiting, or that grass eating is associated with some kind of dietary deficiency.
The initial study was a survey of 25 veterinary students who had pet dogs. All of the students reported that their dogs ate grass. None reported seeing any signs of illness before their dogs ate the grass and only 8% said that their dogs regularly vomited after eating it.
The researchers then conducted a second survey which involved 47 dog owners who had brought their pets to the UC Davis teaching hospital for outpatient care. This group reinforced the fact that grass-eating was quite common since 79% had observed their dogs eating plants (mostly grass). When questioned about their dog's behavior before and after eating grass it was found that signs of illness were infrequent (only four dogs) and vomiting afterward was also not common (only six dogs).
After these two preliminary studies, the researchers expanded their data sample by conducting a large web-based survey that netted 1,571 usable data sets from dog owners. Once again it was found that grass eating was common since 68% of the respondents said their dogs ate grass on a daily or weekly basis. Only 8% reported that their dogs frequently showed signs of illness before eating grass and 22% reported that their dogs regularly vomited afterward. In the grass-eating dog population they found that it was the younger dogs who ate grass most frequently and they were less likely to appear sick before or to vomit afterward. The research did find, however, that if a dog showed signs of illness before eating grass it was more likely to vomit afterward.
As for the notion that dogs are eating grass in order to make up for some kind of dietary deficiency, no support was found. Dogs that had their diet regularly supplemented by plant matter (vegetables or fruit) were no less likely to eat grass which seems to kill the idea that dogs are eating grass to make up for the absence of vegetable matter in their normal food intake.
The researchers conclude that grass eating is a common behavior that usually occurs in normal dogs and is generally not associated with illness or dietary needs. They go on to suggest that grass eating may reflect an innate predisposition inherited from dogs' wild ancestors.
This is supported by research on droppings left by wolves. Such research finds evidence of grass in 11 to 47% of the stool samples studied. The usefulness of grass eating in these wild canines is that it can help to purge intestinal parasites. The plant material passes through the intestinal tract and the fibrous matter increases the intestinal contractions and wraps around the worms or nematodes which may be infecting the animal. In this way, the grass helps to purge the system of these potentially harmful parasites. Although most pet dogs are free of such worms they nonetheless may still have that predisposition to eating grass which was helpful to their ancestors living in the wild.
An alternate suggestion, not made by the researchers, but suggested by the veterinarian who raised the question of grass eating with me, is that dogs may eat grass simply because they like the taste of it.
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* Sueda, K.L.C., Hart, B.L. & Cliff, K.D. (2008). Characterization of plant-eating in dogs. Applied Animal Behavior Science, 111, 120-132.