Every dog owner has seen it. You let your dog out in the garden, and instead of doing their business, they start munching on grass like a tiny cow. Then, twenty minutes later, they throw it all up on the kitchen floor. Fun.
If this sounds familiar, you’re not alone. Roughly 80% of dog owners report their dogs eating grass at least occasionally, according to a study published in Applied Animal Behaviour Science. It’s one of the most common behaviours vets get asked about — and one of the least understood.
The “upset stomach” theory — mostly wrong
The most popular explanation you’ll hear is that dogs eat grass to make themselves vomit when their stomach is off. It’s intuitive. It feels right. But the data doesn’t really support it.
That same study found that only about 22% of dogs regularly vomit after eating grass. And only 9% of dogs showed signs of illness before eating it. Most grass-eating dogs aren’t sick beforehand and don’t throw up afterward. They just… eat grass.
The dogs who do vomit tend to gulp grass quickly, barely chewing. Dogs who eat grass leisurely, actually grazing and selecting specific blades, almost never vomit. This suggests two different behaviours that get lumped together.
So why do they actually do it?
Honestly, nobody knows for certain. But there are a few theories with decent evidence behind them.
The instinct angle is probably the strongest. Wild canids eat plant matter regularly. Wolves, coyotes, and foxes all consume grass, berries, and other vegetation as part of their natural diet. It’s been documented in wild canids for as long as people have been studying them. Your dog might just be following a deeply wired behaviour that predates kibble by a few thousand years.
Some dogs may be fibre seeking. There’s anecdotal evidence from vets that switching a chronic grass-eater to a higher-fibre food sometimes reduces or stops the behaviour. Not always, but often enough to be worth noting. If your dog eats a lot of grass and their food has less than 4% crude fibre, a diet change might help.
Then there’s the boring answer nobody wants to hear: some dogs just enjoy eating grass. The same way some dogs enjoy eating sticks, socks, or cat poo. Not everything a dog does has a medical explanation. Fresh spring grass in particular seems popular — possibly because it’s sweeter and more tender.
Boredom and anxiety can play a role too. Dogs left alone in gardens for long periods eat more grass than dogs who get regular walks and interaction. If the grass-eating is compulsive — same spot, same time, frantic nibbling — it could be a displacement behaviour rather than a dietary one.
When it’s actually a problem
Most of the time, grass eating is completely harmless. But there are situations where you should pay attention.
Treated lawns are the biggest risk. Herbicides, pesticides, and fertilisers can make your dog genuinely sick. If you treat your lawn or your neighbours do, keep your dog off freshly sprayed grass for at least 48 hours (longer for granular fertilisers). The grass itself isn’t the danger — the chemicals on it are.
Parasites are another concern. Grass can carry intestinal parasite eggs, particularly in areas where foxes or other dogs have been. This is more relevant for dogs who eat grass in parks and public spaces. Keep your dog’s worming schedule current.
Frequent vomiting after grass eating — more than once or twice a week — warrants a vet visit. Not because the grass is causing the vomiting necessarily, but because the underlying stomach issues driving the grass eating might need attention. Conditions like inflammatory bowel disease, acid reflux, or food sensitivities can all trigger increased grass consumption.
The “purge” question
One theory that gets a lot of attention: dogs eat grass to purge intestinal parasites. The idea is that the grass wraps around worms in the gut and helps expel them. This has actually been observed in wild chimpanzees who swallow rough leaves whole — the leaves come out the other end wrapped around parasites.
Whether domestic dogs are doing the same thing is unclear. They’ve been dewormed by humans for generations, so the evolutionary pressure to self-medicate might be vestigial at this point. But it’s a plausible theory for why the behaviour persists in the species, even if your individual dog isn’t actually doing it for that reason.
Should you stop it?
If your dog occasionally nibbles grass on walks and seems fine otherwise, there’s no reason to stop them. It’s normal behaviour for the species.
If you want to redirect the behaviour, try offering alternatives. Some dogs enjoy raw apple slices, carrots, or cucumber as a crunchy, high-fibre snack. If your dog is a texture eater — they like the crunch of grass — these might satisfy the same urge.
Increasing fibre in their diet can help too. A spoonful of plain pumpkin puree (not pie filling) or cooked sweet potato mixed into their meals is a simple adjustment. Or switch to a food with higher fibre content.
And sometimes the answer has nothing to do with food at all. More exercise and mental stimulation can reduce compulsive grass eating, especially if it happens mostly when your dog is bored in the garden.
FAQ
Is it safe for puppies to eat grass?
Generally yes, with the same caveats as adult dogs — avoid treated lawns and keep parasite prevention up to date. Puppies tend to put everything in their mouths anyway, so some grass nibbling is expected. If they’re eating large amounts or vomiting frequently, check with your vet.
My dog eats grass and then has diarrhoea. What’s going on?
The grass might be irritating their digestive system, or the underlying cause of the grass eating (stomach issues, food sensitivity) is also causing the diarrhoea. Either way, if it happens regularly, a vet check is a good idea. Bring a stool sample if you can.
Does eating grass mean my dog’s food is bad?
Not necessarily. Dogs on premium diets eat grass too. But if your dog eats grass compulsively and their food is low in fibre (under 3-4% crude fibre), it might be worth switching. Talk to your vet about a fibre supplement or food change before assuming the worst.
Should I grow “cat grass” or wheatgrass for my dog?
You can. Some owners grow a pot of wheatgrass indoors specifically for their grass-eating dogs. It’s pesticide-free and gives your dog a safe option. Just don’t expect it to stop them eating outdoor grass too — they seem to treat indoor and outdoor grass as different things entirely.
