As it turns out, the short answer to this question is “We don’t know.” However, there are lots of fun theories out there about why our carnivorous companion pets eat grass and it turns out that the circumstances of grass or plant eating can be very different between individual animals.
Are they sick?
Many people report that their pets eat grass and vomit afterward, which has led to the commonly held assumption that they eat grass because they have an upset stomach. The idea is that the poky tips of grasses irritate the stomach lining enough to induce a vomiting reflex. Others have theorized that they eat grass because of the effects of internal parasites, and the fiber helps them feel better by mechanically removing the parasites from the intestines. This idea has actually been studied in great ape research, where some chimpanzees have been shown to have decreased fecal parasite loads after eating certain kinds of fibrous plants.
In research surveys, however, grass eating and vomiting don’t necessarily go together. A recent study of clients and veterinary students who’s dogs ate grass showed that only 18% of clients’ dogs that ate grass or other plants vomited afterward, and only 9% of veterinary-students’ dogs showed any signs of illness prior to eating grass.
Another interesting study took normal dogs and treated a small subset of them with a mild, diarrhea inducing drug and then exposed them to different grasses. The dogs not given the drug ate much more grass than the dogs that were given the drug, suggesting that dogs do not eat grass to self-medicate at least that type of GI upset. However, anecdotes abound of people who were first alerted to anything from gastric ulcers to inflammatory bowel syndrome by frequent grass eating behavior. One difference owners seem to consistently notice is that dogs who are seen to suddenly start the habit of gulping down large amounts of grass (and usually vomiting afterward) are more likely to have an underlying illness.
Are they lacking in nutrients?
The ancestors of our modern day dogs and cats probably ate small amounts of plants and grasses as a normal part of the diet. They may also have obtained plant fiber and nutrients indirectly by eating the intestines of wild herbivores (like rabbits or deer). Since most of our dogs and cats are lacking fresh herbivore entrails in their daily diet, some people believe they crave grasses in an effort replace what they would get in the wild by eating plant material directly. Anecdotally, this theory is somewhat supported by the experience ofpeople who have seen that feeding their dogs greens such as small amounts of parsley, kale, or other vitamin C containing foods will curb their dogs’ grass eating habits. In addition, one published report in Japan documented a poodle that stopped eating grass after the owners instituted a high fiber diet.
While they are stricter carnivores than dogs, a similar thought process is used to try to explain grass-eating in cats. Many pet stores now carry barley grasses that are free of pesticides that you can grow at home for your kitties to eat. Nutritional benefit is questionable, but if it does not cause vomiting or other problems it is probably safe for them to eat small amounts of grass.
Are they crazy?
Neurologic or behavioral maladies such as obsessive compulsive disorders have been suspected in some cases of grass eating. Pica, a condition characterized by a consistent craving for non-food items (usually dirt, rocks, etc.), may apply to grass eating in some cases, although it is thought that these cases are rare.
Are they hungry?
Although their intestines are not adapted for breaking down plants for nutrition the way herbivores like cows and horses are, dogs at least (unlike cats) are able to tolerate a much higher fraction of non-meat protein and carbohydrate sources in their diet. Research on wild canid populations (wolves, foxes, dingoes, etc.) has shown that all of them do consume some plant material, including grasses, in the wild.
Owners of dogs that frequently snack on grass without side-effects often say that the dogs are selective about which types of grasses they will eat, including some who will only eat newly grown shoots of certain kinds of grass (which are lower in the bitter tannins and higher in carbohydrates and protein). There are probably many dogs that just like the taste of grass and eat it as a snack, whatever the larger implications may be.
What do I do about it?
Why, ask your veterinarian of course! If your pet snacks on grass and never or only very occasionally vomits, there less likely to be an underlying problem. However, if it is a new behavior, they are eating large amounts of grass or vomiting frequently (more than once a month or so), then we would recommend you have your pet seen for a physical examination and possibly some follow up blood work or other diagnostics. Also make sure that any grass eating they do is on grass that has not recently had fertilizers, herbicides, pesticides or other chemicals applied.
As a side note, supplementing pet diets with herbs and vegetables is fine but always make sure that a high quality AAFCO approved, age-appropriate, nutritionally balanced commercial food is the main component of any diet you feed your pets.
If in doubt, give us a call!