One minute your dog is walking normally, the next, he lowers his head and starts happily chewing on grass.

Naturally, you wonder: is he feeling sick? Does he have an upset stomach? Is he trying to make himself vomit? Or is he slowly turning into a cow?

This is one of the most widespread beliefs about dog behaviour. But according to the available scientific data, it is very likely false.

The common misconception: “he eats grass to make himself vomit”

This idea became so widespread that even veterinary professionals repeated it for years, despite limited scientific evidence.

To test it, Dr Benjamin Hart and his team at the UC Davis School of Veterinary Medicine surveyed 1,571 dog owners.

The results were clear:

• Only 9% of dogs showed signs of illness before eating grass.

• Only 22% vomited afterwards.

• Most dogs were perfectly healthy before eating grass and never vomited afterwards.

• The theory that dogs eat grass to make themselves vomit is therefore not supported by current evidence.

Important nuance

If your dog eats grass while showing signs of distress, such as excessive salivation, agitation, visible abdominal pain or repeated attempts to vomit, he should be examined by a veterinarian.

So why do dogs eat grass?

There probably isn't a single explanation. Researchers believe several mechanisms may be involved, and they are not mutually exclusive.

1. An instinct inherited from their ancestors

Plant material is commonly found in wolf droppings, with studies reporting it in 11% to 47% of samples.

Wild canids also carried intestinal parasites far more frequently than most pet dogs today.

One hypothesis is that eating grass helped stimulate intestinal motility, making it easier to eliminate parasites naturally through the digestive tract. Even though most modern dogs are regularly dewormed, this instinctive behaviour may still persist.

2. A source of dietary fibre

Dogs do not need to eat grass specifically, but dietary fibre plays an important role in digestive health.

In one published case report, a Miniature Poodle had eaten grass almost every day for seven years. After switching to a diet containing more fibre, the behaviour disappeared completely (Sato et al., 2008).

Of course, a single case cannot prove that fibre deficiency explains every instance of grass eating. It does, however, suggest that nutrition may sometimes play a role.

3. Because they simply enjoy it

Not every behaviour has a medical explanation.

Some dogs seem to enjoy the taste or texture of fresh grass, especially young shoots in spring. For them, eating grass may simply be another way of exploring their environment, much like sniffing or chewing sticks.

Others may chew grass more often when they're bored, frustrated or experiencing mild stress, particularly if they spend long periods alone in the garden.

When should you be concerned?

For most healthy dogs, eating grass is completely normal.

However, a few situations deserve closer attention:

• Chemically treated lawns. Grass exposed to pesticides, herbicides or lawn treatments can be toxic.

• Parasite contamination. Grass contaminated with the faeces of other animals may expose your dog to intestinal parasites.

• A sudden change in behaviour. If your dog suddenly starts eating much more grass than usual, it's worth paying attention.

• Other clinical signs. Vomiting, loss of appetite, abdominal pain, lethargy or repeated attempts to vomit should always prompt a veterinary consultation.

What can you do?

If your dog is otherwise healthy, occasional grass eating is usually nothing to worry about.

You can simply:

• Make sure your dog's diet provides enough dietary fibre.

• Prevent access to chemically treated lawns whenever possible.

• Keep an eye on any sudden changes in frequency or associated symptoms.

Most importantly, look at the whole dog, not just the behaviour. Grass eating on its own is rarely a sign of disease.

Sources

1. Sueda KL, Hart BL, Cliff KD. Characterization of plant eating in dogs. Applied Animal Behaviour Science. 2008;111:120–132.
2. Hart BL. Why do dogs eat grass? Veterinary Medicine. 2008;103:648–649.
3. Sato J, et al. Case report on chronic grass eating resolved after increased dietary fibre intake. PubMed PMID: 17675815.