Why Does My Pet Eat Grass?

“Why do they do that?” It is the age-old query that pet parents ask when they see their dogs and cats happily eating grass. There are old wives’ tales and myths and theories, but no one has established the exact reason why grass is so attractive to our pets. Scientists, researchers, and veterinarians are still working to determine the answer to this question, but in the meantime, here are a few plausible hypotheses that we can examine.

Dogs Eat Grass to Ease Upset Stomachs, Don’t They?

One of the long-standing myths associated with dogs eating grass is that they engage in this behavior to quell an upset stomach. After all, many a canine parent has observed her dog rapidly eating some grass, then coughing, spitting, or vomiting afterward. Surely, the two actions are connected. However, it turns out that is not the case at all.

Dogs’ attraction to grass may be traced back to their closest relative, the wolf. A 2008 study from the University of California, Davis, points out that, despite the lack of research on grass-eating domestic pets, both wild dogs and cats in nature eat plants and grass. The UC Davis study found that plant material in the waste and stomach contents of cougars and wolves ranged from 2 - 74%.

Thus, it may be that dogs are prone to eating grass because it is written into their evolutionary genes. When wolves and other wild canines consume their catch, they tend to eat everything: meat, bones, fur, and whatever the animal had in its stomach, often grass or plant-based material. It may also be a method by which these wild animals effectively remove intestinal parasites. As the grass pushes through the intestines and progresses through the upper and lower digestive systems, it eliminates these parasites.

Another blow to the upset stomach myth comes from the University of New England in Australia. In 2009, these scientists took two sets of dogs to test the upset stomach theory. One group of dogs was fed a standard kibble diet; the second group was fed the same diet with an added supplement, fructo-oligosaccharide (FOS). This additive causes mild digestive distress and loose stools at high doses. Both groups of dogs ate grass. To the scientists’ surprise, the group that did not receive FOS supplements ate more grass for longer periods of time. Therefore, it did not appear that consuming grass relieved an upset stomach.

The University of California decided to take this test a step farther and observed 25 veterinary students’ dogs and interviewed their parents. Though all of the dogs ate grass, only 2 of the 25 showed signs of vomiting. The next step expanded the study to a group of 47 dogs belonging to teaching clients. Again, the resulting numbers were similar: only a small percentage of the group had dogs who were sick after consuming grass. Finally, the last group was conducted via an online survey. Of almost 1,100 dogs, only 22% were ill after eating grass.

A final idea about eating grass is also the simplest one: some dogs like the taste of grass.  Though grass hardly seems appealing to humans, it does have different flavors and smells to dogs, some sweeter than others. Does this mean dogs are compensating for a dietary deficiency? According to research from the University of California: no; even dogs who have their protein-based diets supplemented with additional plant-based materials are no less likely to eat grass than dogs whose diets are untouched. Ultimately, dogs might just eat grass because they like it.

Does the Same Research Apply to Cats?

The dog-grass myth also extends to the feline portion of the pet world. Some pet parents see their cats eating grass, then almost immediately throwing it up. However, with cats, the reason for the vomiting varies slightly from the myths about dogs. Like dogs’ wild ancestors, cats’ forbearers also consumed grass when they ate their kill. Does this mean that all cats will eat grass, and that many of them will vomit? Not necessarily. For some, it may merely be instinctive.

It is important to note that it is far more common for a dog to eat grass than a cat. That said, some cats do eat grass, and some vomit after eating it; there are also many cats who do not. So, why would a cat eat grass at all, especially if it might make him ill?

Eating Grass Is Good for a Cat’s Gut

Similar to wild canine ancestors, wild cats—like lions, tigers, cougars, and bobcats—chew on grass. Instinctively, our domestic cats may do the same thing for the same reason: grass cleans out their digestive systems. Cats are incapable of digesting grass; thus, some vomiting occurring after consumption should not be surprising. In fact, eating grass and thereby inducing vomiting might be the way a cat clears his digestive tract of anything that might cause him harm, such as parasites, feathers, fur, or bones; this is a vital defense mechanism for outdoor cats. As such, grass could be a natural hairball remedy.

Grass may have the same impact on a cat’s lower digestive system. Any pieces that get through the digestive tract are pushed in whole form through the intestines. There, the grass could clean out undigested hair and parasites, as well as prevent constipation.

The Mystery Continues

Until more scientific research is conducted and reported, pet parents will have to work with the currently established theories about grass consumption. Perhaps it is in our pets’ genetic makeup to eat grass, and maybe—vomiting and all—it does a pet’s body good. Ultimately, dogs and cats might consume grass because they like it, and no amount of science will convince them otherwise.