Why does my dog eat grass? The usual answer is wrong

Ask anyone why dogs eat grass and you will get the same answer: they have an upset stomach and they are making themselves sick.

It is one of those things everybody knows, and the research does not support it.

What the study actually found

The most-cited work here is a survey out of the University of California, Davis, published in 2008. It set out specifically to test the sickness theory. The findings:

• 68% of dogs ate grass daily or weekly. It is not unusual behaviour, it is ordinary behaviour.
• Only 8% showed any sign of illness beforehand.
• Only 22% regularly vomited afterwards.

So for roughly nine dogs in ten, there was no illness before and, for most, no sickness after. The theory has the causation backwards: dogs are not eating grass because they feel ill. A small minority happen to be sick afterwards, which is the bit people notice and remember.

The same study looked at whether grass-eating signalled a dietary deficiency. It found no support for that either.

So why do they do it?

The honest answer is that nobody is certain, and the most likely explanation is the least dramatic one: dogs eat grass because they are dogs.

Plant-eating is normal in wild canids too, so it is probably just retained behaviour rather than a symptom of anything. Younger dogs did it most in the study, and were less likely to look unwell beforehand or be sick afterwards — which is the opposite of what you would expect if it were self-medication.

Boredom plays a part for some. A dog with nothing else to do in a garden will graze in the same way it will dig or chew a hose. If that sounds like your dog, the answer is occupation rather than restriction — our guide to signs your dog is bored covers what actually helps, and a puzzle feeder turns one ordinary meal into ten minutes of work.

When it is worth a vet

The behaviour itself is not the signal. The change is.

• A sudden increase in a dog that never used to bother
• Frantic, urgent eating rather than casual grazing — particularly with lip-licking, drooling or swallowing repeatedly
• Repeated vomiting, with or without grass
• Anything else alongside it — weight loss, changed appetite, low energy, altered toileting

Casual grazing in an otherwise well dog is not a problem. A dog that has suddenly started doing it constantly is worth a conversation.

The thing that is genuinely dangerous

Not the grass. What is on it.

• Weedkiller, lawn feed and pesticides — the most common real risk, and easy to forget after treating your own lawn. Keep dogs off treated grass for as long as the product says.
• Slug pellets, which are dangerous in very small amounts.
• Grass seeds in summer — the arrow-shaped ones work into paws, ears and noses and cause real problems.
• Verges and public paths, which may be sprayed by councils without any notice you would see.

The short version

Your dog is almost certainly fine. Grass-eating is common, mostly unrelated to illness, and something a lot of dogs simply do. Watch for change rather than for the behaviour, and pay more attention to what has been sprayed on the lawn than to the lawn itself.