Why Do Dogs Eat Grass? What Science Says About This Very Common Habit

It happens on almost every walk. Your dog stops, sniffs a patch of lawn with great seriousness, then starts tearing at the grass and chewing it with obvious pleasure. Sometimes it is a few blades. Sometimes it is a determined grazing session that would impress a cow.

Ask around and you will hear the same explanation again and again: dogs eat grass when they have an upset stomach, because it makes them sick and helps them get rid of whatever is bothering them. It is a neat theory. It is also, according to the best evidence available, mostly wrong.

The quick answer

Eating grass is very common in healthy dogs. Most dogs that graze do not seem ill beforehand and do not vomit afterwards. Researchers think the habit is largely normal behaviour, possibly inherited from the wild ancestors of dogs, and that it is only occasionally connected to digestive problems. That said, there are situations where you should pay attention, and some plants and lawns are genuinely dangerous.

The study that tested the "sick dog" theory

The most detailed research on the question comes from the University of California, Davis. Veterinary behaviourist Benjamin Hart and his colleagues Karen Sueda and Kelly Cliff surveyed dog owners about plant-eating and published their findings in the journal Applied Animal Behaviour Science in 2008.

They started small. Among 25 veterinary students who owned dogs, every single one said their dog ate grass. None had noticed their dog looking ill before eating it, and only 8 percent said their dog regularly vomited afterwards.

The team then ran a much larger online survey of owners whose dogs ate plants. They received more than 3,000 responses and used 1,571 of them that described dogs the owners watched closely. The results were clear:

• 68 percent of owners said their dog ate plants daily or weekly.
• Only 8 percent said their dog frequently showed signs of illness before eating plants.
• Only 22 percent said their dog regularly vomited afterwards.

Grass was by far the most common plant eaten. Younger dogs ate plants more often than older dogs and were less likely to seem ill beforehand or to throw up afterwards.

In other words, the classic picture of a queasy dog eating grass to make itself sick does exist, but it describes a small minority of cases. For most dogs, grass-eating appears to be just something they do.

So why do they do it?

Nobody has a complete answer, but several explanations are taken seriously by vets and researchers.

1. An inherited instinct

Hart's team proposed an evolutionary explanation. Studies of wild wolves have found plant material in their stomachs and droppings. Many wild carnivores carry intestinal worms, and regularly eating indigestible plant fibre may help move parasites through the gut. A dog that inherited a tendency to graze from its ancestors might still do it today, even if it is regularly wormed and has never seen a wolf.

This would also explain why the dogs in the survey did not need to be ill to eat grass. The behaviour would be a routine, preventive habit rather than a response to feeling unwell.

2. They like it

Plenty of dogs simply seem to enjoy grass, especially fresh, young shoots in spring. Owners often notice that their dog is picky, selecting particular types or patches. The texture, the taste or the smell may all play a role. Dogs are omnivorous scavengers by nature and explore the world with their mouths.

3. Boredom or attention

Dogs that are under-stimulated may chew on whatever is available, and grass is always available. If your dog grazes mainly when it is left alone in the garden for long periods, more exercise, play and mental challenges such as sniffing games or food puzzles may reduce the habit. Some dogs also learn that eating grass gets a reaction from their owner, which is a reward in itself.

4. Diet and fibre

A common belief is that dogs eat grass because something is missing from their food, especially fibre. There are individual reports of dogs grazing less after a switch to a higher-fibre diet, but there is no strong evidence that grass-eating is generally caused by nutritional deficiency. Dogs on complete, balanced diets eat grass too. If you are worried, your vet can review your dog's diet.

5. Sometimes, an upset stomach

The old theory is not entirely wrong. Some dogs with digestive trouble do eat grass frantically and then vomit. Vets sometimes see this in dogs with gastrointestinal disease. The important difference is in the pattern: urgent, frantic grass-eating in a dog that seems unwell is a different thing from relaxed grazing on a walk.

Is eating grass bad for dogs?

Grass itself is generally not harmful to a healthy dog in small amounts. The risks come from what is on the grass, or from plants that are not grass at all.

• Pesticides, herbicides and fertilisers. Treated lawns, parks and verges can carry chemicals that are toxic to dogs. Avoid letting your dog graze where you do not know how the grass has been treated, and check product labels before using anything on your own lawn.
• Poisonous plants. Many common garden plants are toxic to dogs, including lilies of the valley, foxgloves, daffodil bulbs, azaleas and rhododendrons, sago palms and yew. Learn what grows in your garden.
• Parasites. Grass contaminated with the droppings of other animals can carry parasite eggs. Regular worming, as recommended by your vet, matters for dogs that graze.
• Grass seeds and awns. In summer, the sharp, arrow-shaped seed heads of some wild grasses can lodge in a dog's mouth, nose, ears, eyes or paws and work their way under the skin. Check your dog after walks through long, dry grass.
• Swallowing long blades. Occasionally a long strand of grass gets caught in the throat and causes gagging. Usually it comes back up on its own.

When to call the vet

Occasional grass-eating in a dog that is otherwise bright, eating normally and full of energy is not a cause for concern. Contact your vet if you notice any of the following:

• sudden, frantic grass-eating that is new for your dog;
• repeated vomiting, or vomiting with blood;
• diarrhoea, black or tarry stools, or straining;
• loss of appetite, weight loss or lethargy;
• signs of pain, such as a hunched posture or reluctance to be touched around the belly;
• eating non-food objects such as stones, fabric or soil, a behaviour called pica.

If you think your dog has eaten a poisonous plant or a treated lawn, call your vet or an animal poison line straight away, even if your dog seems fine.

Should you stop your dog eating grass?

For most dogs there is no need to stop the habit altogether. A sensible approach is to let your dog graze on clean, untreated grass that you trust, steer it away from roadsides, sprayed parks and unknown plants, and keep up with worming and flea treatment.

If you want to reduce the habit, train a reliable "leave it" cue and reward your dog generously when it walks away from the lawn. Make sure your dog gets enough exercise and mental stimulation. Some owners grow a small tray of dog-safe grass at home, such as wheatgrass or oat grass, so that the dog has an approved place to graze.

What about cats?

Cats eat grass too, and the same UC Davis team looked at them. Their preliminary findings suggested that plant-eating is somewhat less common in cats than in dogs, that cats rarely seem ill beforehand, and that most do not regularly vomit afterwards. Cats also tend to nibble a wider range of plants, which matters because some house plants, especially lilies, are extremely toxic to cats.

The bottom line

The next time your dog stops for a mouthful of lawn, you can relax. In most cases it is not a cry for help and it is not a sign of a missing vitamin. It is a normal, probably very old canine habit. Keep the grass clean, know your plants and watch for the warning signs above. Otherwise, let your dog enjoy its salad.