Why Do Dogs Eat Grass?

Sometimes the simplest questions are the hardest to answer, for example “Why do dogs eat grass?” While there are a whole pack of theories out there, it turns out most of these are flawed or false.

The most popular suggestions are:

  • As a form of herbal cleansing because the dog feels unwell
  • The dog eats a refined diet and craves roughage
  • To correct a dietary deficiency
  • To get rid of worms
  • No-one knows for sure

What is your pet theory?

So would it intrigue you to know that experts now think they have the answer?

Dog Eating grass

The Wrong Reasons

For a long time there’s been no specific research into the subject of why dogs eat grass and one guess was as good as another. But this bugged Benjamin Hart, DVM, PhD, DACVB, working at the School of Veterinary Medicine, University of California, Davis, so he decided to investigate

Prof Hart likes getting answers to tricky questions (such as ‘Why do elephants yawn?’) and set his expertise to the chewy topic of dogs eating grass. To do this he sent questionnaires out to dog-owning vet students, and then widened the survey to include the dog owning public. The results threw out of the park some of the most popular theories about why dogs eat grass.

Let’s look at Prof Hart’s findings in more detail:

Is Eating Grass Common?


OK, so we all know dogs love to eat grass, but Prof Hart put a figure on it. Between 68-70% of dogs eat grass regularly once a week or more. That’s a fact.

Indeed, their wild ancestors, the wolf, also eat grass. We know this because someone took the time to analyse wolf scats (Andersone and Ozolins, 2004), and found it was composed of 2 – 74% herbage. So deep down our pet dogs have an in-built desire to follow in their ancestors paw prints: But how does this benefit them?

Grass as a Medicine?

Any owner who’s dog has thrown up after chowing down on the green stuff, will probable vote for the dog feeling ill and trying to make themselves feel better. But those owners would be wrong to do so.

Prof Hart’s statistics diss this theory. He found that no vet students noticed signs of illness prior to eating grass, whilst amongst 3,000 dog owners only 8% noted their dog being off colour ahead of dining on grass. So with the majority of chow-hounds being full of bounce before their outdoor buffet, using grass as a medicine seems unlikely.

Indeed, the idea of eating grass to induce vomiting as a sort of herbal detox also falls flat as theory. Only 8% (vet student dogs) and 22% (regular owners’ dogs) reported their dogs were sick afterwards so we can kick this theory into the long grass.

However, an interesting trend did show up, which was the dogs that felt unwell before eating grass, were more likely to be sick afterwards. So perhaps there’s a tiny bit of truth in this theory.

dog playing on grass

Eating Grass as a Source of Fibre?

When a wolf in the wild makes a kill, they don’t just eat their prey’s muscles but also whatever is in their stomach at the time (including grass – a natural source of roughage). So it’s tempting to think a dog fed a highly refined diet, might lack for fibre. Might modern pets be leaning towards their wild ancestors’ habits, to increase roughage intake?

Actually, no.

Prof Hart’s survey included questions about dog foods. These ranged from commercial highly processed wet and dry foods, to raw feeding, and home prepared diets. Guess what? The dogs diet didn’t influence whether they ate grass or not, making the roughage theory hard to swallow.

Indeed, with the vast majority of dogs regularly eating a well-balanced food, it also seems unlikely that dietary deficiency is a driver behind a grass-habit.

What are We Left With?

Which pretty much leaves getting rid of parasites as a final option (along with ‘No one knows.’)

But maybe, just maybe, this one makes sense.

Eating Grass to Get Rid of Intestinal Worms

Prof Hart concluded that whilst we can’t be certain why dogs eat grass, a reasonable theory is because it acts an anti-worming treatment.

Let’s take chimpanzees as an example. It’s known that wild chimpanzee eat certain types of leaves without chewing, which then pass undigested through the intestinal tract. These leaves wrap around some of the unpleasant wriggles in the gut, dislodge them, and drag them out in poop. When those worms rob the host of vital nutrition, then decreasing the number of parasites has a real and beneficial effect.

Transfer this knowledge to dogs and it’s a plausible explanation. Not only might grass tangle with worms and make them pass along, but there’s another effect. Grass irritates the lining of the intestine causing muscular spasms. These spasms may shake worms free and squeeze them along and out of the bowel. Hence, acting as a natural de-wormer.

This also fits with the fact that young dogs are more likely to eat grass than older ones. Pups have a higher worm burden, because they are born with worms eggs that passed across the placenta and from their mother’s milk. Young dogs also have a lot of growing to do, so their drive to get the most out of their food is higher.

Does this mean you can stop worming your dog because they eat grass? No!

Even if this theory is correct, eating grass can only get rid of a small percentage of their worm burden. Don’t forget, wild dogs and wolves don’t have the life expectancy our pets do, and death from parasitic disease is more common.

Dog eating grass – Solved?

The most likely explanation is this is a natural and normal behaviour inherited from their canine ancestors. Eating grass may have beneficial actions that help lower their worm burden. But of course eating grass is no substitute for regularly de-worming treatments from your vet. Nice as it to avoid the overuse of medications, sadly eating grass just doesn’t cut it where your dog’s health is concerned.