If your dog is like the majority of dogs out there, they eat grass. And if you're like the majority of owners out there, you have no idea why.
As it turns out, there is a good reason for that confusion. Grass eating behavior in dogs is far more complicated than most of us would guess. So, to finally answer this age-old question, we looked into the various theories for why dogs eat grass and the scientific studies constructed to test those theories.
So, why do dogs eat grass? The answer is likely to surprise you.
Common Theories for Why Dogs Eat Grass
There are a handful of theories out there as to why dogs eat grass. If you've ever talked to other dog owners, your veterinarian, or even Googled the issue, then you've probably heard a few of these theories before.
But just because something is well known does not necessarily mean it's true. The more we dug into the science behind canine grass eating, the more we realized how few answers we have about this common behavior.
As a dog owner, you've more than likely come across grass-filled dog vomit on your carpet. Probably more than once. But does the presence of grass in a puddle of dog puke mean that dogs eat grass to make themselves vomit?
Various studies have looked at grass eating in dogs to see if there truly is a correlation between grass eating and vomiting. From those studies, we know that grass eating on a daily to weekly basis is something that about 68% of dogs partake in. Of dogs who eat grass, only about 22% of them frequently vomit afterward.
So, grass-eating certainly has the potential to cause vomiting. But does that mean dogs are intentionally eating grass to induce vomiting?
In this case, the data is much less clear. Only about 8% of owners report that their dog frequently appears sick before engaging in grass eating. When dogs appear sick before they eat grass, they are more likely to vomit afterward than if they were not sick, to begin with. This would seem to indicate that grass does not induce vomiting, but rather is sometimes thrown up when a dog is already feeling ill or as an unintentional side effect.
Help Settle the Stomach
Many people think that dogs eat grass not necessarily to make themselves puke, but rather to settle their stomachs when they are upset. This particular theory has been tested using a controlled research study.
In this study, two groups of dogs were fed a standard diet, with one group receiving a fructo-oligosaccharide (FOS) supplement meant to upset the stomach. In addition, both groups of dogs were given free access to grass throughout the study.
The FOS dogs developed runny, watery stools, while the control group had normal stools. Despite this obvious gastrointestinal upset, the FOS dogs engaged in less grass-eating behavior than the control group. The researchers concluded that this finding was either because the FOS dogs felt too ill to engage in normal grass-eating behaviors or because the supplement had them feeling more satiated than the control group.
While this study doesn't prove that dogs never use grass as a means to settle their stomach, it certainly calls the theory into question.
Increase Fiber Intake
Another common theory says that dogs engage in grass-eating as a way to increase the amount of fiber in their diet. While this seems like a valid idea, given how much fiber grass contains, the science doesn't necessarily back the theory up.
When comparing a group of dogs fed a 0% fiber diet with a group of dogs fed the same diet with 2% added fiber, scientists found that both groups of dogs engaged in grass-eating at the same rate.
The idea that the fiber content of a dog's diet influences their grass-eating behavior is further disproved by statistical data. A survey of over 1,500 dog owners showed that dogs fed low-fiber raw or fresh diets were no more likely to engage in grass-eating than dogs on commercial diets. And, within the group of dogs fed commercial food, the researchers found no correlation between fiber content and grass-eating frequency.
The Real Reason Dogs Eat Grass
If dogs don't eat grass to induce vomiting, settle an upset stomach, or balance a nutritional deficit, then why do they practice this strange behavior?
We know from several research studies that dogs are most likely to engage in grass-eating on an empty stomach. This behavior is most common in the morning before the dog's first meal and least common immediately after eating.
Surveys have shown us that younger dogs are more likely to engage in grass-eating behavior than older dogs. And that younger dogs are less likely to vomit after eating grass and rarely eat grass when ill.
The only real conclusion we can draw from this evidence is that dogs eating grass is not at all uncommon. It seems grass eating is a normal behavior that the majority of healthy dogs engage in. We see a similar pattern of behavior in wild canines.
Wolves, coyotes, and foxes all eat plant matter to some degree, despite lacking the digestive enzymes to break down and utilize the nutrients present in grass and other plants. One strong theory for grass-eating behavior in wild canids is that they do it to help clear their intestines of parasites.
Large amounts of fibrous material, such as leaves and grass, have proven an effective cleanser against worms and other intestinal parasites in many animal models. Since wild dogs, especially younger individuals, are prone to these types of infections, it makes sense that these animals would have developed behaviors to combat these pathogens.
So, why do dogs eat grass?
The most likely reason dogs eat grass is because their ancestors did. Grass-eating behavior evolved in wolves as a means to control digestive parasites. Many dogs continue this behavior today because their hardwired instincts drive them to and because eating grass increases feelings of satiation.
If your dog eats grass, know that this behavior is completely normal and rarely indicates a problem. As long as the grass they are eating hasn’t been treated with chemical fertilizers, herbicides, or pesticides, this odd habit shouldn’t cause any major problems. If you do use these types of products on your lawn, keep your dog away from the area and consider providing them with a safe place of untreated grass so they can indulge in this instinctual behavior.
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McKenzie, S.J., et al. Reduction in grass eating behaviours in the domestic dog, Canis familiaris, in response to a mild gastrointestinal disturbance. ScienceDirect. Feb 1, 2010. Vol 123, 1–2; 51-55.
Sara Seitz worked in the pet industry for over a decade. In addition to being a certified dog trainer, Sara gained experience working as the general manager of a dog daycare and boarding facility, as the creator and manager of a pet sitting company, as a groomer, and as a dog behavior evaluator. She also has a bachelors in animal behavior from CSU. Currently, Sara works as a freelance writer specializing in blog, article and content writing.