Why Does My Dog Eat Grass? What the Research Actually Shows | South Africa Guide

Most dogs eat grass because it is normal dog behaviour — not because they are sick and not because they are trying to make themselves vomit. That is the conclusion of the largest survey ever done on the subject, and of the only controlled feeding experiment on grass eating in dogs. The popular "upset stomach" explanation is the one thing the evidence does not support well.

Here is what researchers have actually measured, which explanations hold up, which don't, and the risks that matter far more than the behaviour itself: what's on the grass.

What the research found

In 2008, Karen Sueda, Benjamin Hart and Kelly Cliff of UC Davis published Characterisation of plant eating in dogs in Applied Animal Behaviour Science. They ran three surveys, and they designed them specifically to test the illness-and-vomiting hypothesis.

• 25 veterinary students with dogs: every one reported their dog ate grass. None had ever seen signs of illness beforehand. Only 8% said their dog regularly vomited afterward.
• 47 owners at a veterinary teaching hospital: 79% had seen their dog eat plants, grass most often. Among the 33 who answered the follow-up questions, signs of illness beforehand were reported for four dogs and occasional vomiting afterward for six.
• 1,571 usable web surveys from plant-eating dog owners: 68% ate plants daily or weekly. Only 8% frequently showed signs of illness before eating plants, and 22% regularly vomited afterward.

Hart's own summary of the work is blunt: grass eating is a common behaviour in normal dogs, unrelated to illness, and "vomiting seems to be incidental to, rather than caused by, plant eating."

The survey also found no link between grass eating and diet. Dogs fed table scraps or raw food were no more likely to eat grass than dogs on commercial diets, and dogs on lower-fibre diets were no more likely to eat grass than dogs on higher-fibre ones.

The one controlled experiment

The result that undercuts the emetic theory: across 709 separate grass-eating events, there were five vomiting events, involving three dogs. If grass were a self-administered emetic, that number should be far higher.

What the experiment did find was a hunger pattern. Dogs spent significantly more time eating grass before their daily kibble meal than after it (2.67 vs 0.53 minutes), and grass eating declined through the day. The longer since the last meal, the more grass. The authors' reading: dogs may simply treat grass as a minor food item.

The explanations, ranked by evidence strength

Best supported: normal inherited foraging behaviour

Grass and plant material turns up in a large share of wolf and cougar scat and stomach samples. Combined with how universal the behaviour is in healthy pet dogs, the leading interpretation is that it is an innate predisposition carried over from wild ancestors, not a symptom.

Moderately supported: hunger and food-seeking

The pre-meal spike in the controlled trial is a real, measured effect in a small sample of 12 dogs. It suggests appetite plays a role, but it comes from one study and should not be over-read.

Plausible but thinly evidenced: boredom, habit, attention

Veterinary behaviour sources commonly list under-stimulation as a contributor, and owners often report the behaviour clusters in dull, unsupervised yard time. There is no controlled study quantifying this in dogs. Treat it as a reasonable working theory, not a finding.

Popular but weakly supported: gastrointestinal discomfort

Real in a minority of dogs, wrong as a general explanation. Hart notes that subclinical gastric upset may occasionally trigger it, and dogs showing illness signs beforehand were more likely to vomit after — but that is a small subset, not the norm.

Speculative: parasite purging

The chimpanzee leaf-swallowing parallel is documented in primates. Extending it to dogs is a hypothesis, and the researchers who proposed it say so.

Not supported: dietary deficiency

The survey data found no diet or fibre relationship. No food or supplement has been shown to stop grass eating, and any product claiming to is making a claim the research does not back.

The risks that actually matter

The behaviour is usually harmless. What the grass is carrying often isn't.

• Lawn herbicides. A 2013 Purdue study (Knapp et al., Science of the Total Environment) detected herbicides including 2,4-D in the urine of dogs from 19 of 25 households after lawn treatment — and in 14 of 25 before treatment, plus 4 of 8 untreated households, consistent with chemical drift from neighboring lawns. Herbicide residues were also commonly found on grass from untreated lawns. Hart's own client advice is to keep grass-eating pets off chemically treated lawns.
• Slug and snail bait. Metaldehyde is the active ingredient in the majority of slug baits worldwide and causes agitation, tremors, seizures and dangerous hyperthermia. There is no antidote. This is an emergency, not a wait-and-see.
• Foxtails and grass awns. These barbed seed heads travel one way only — deeper. They migrate into ears, noses, between toes and occasionally into body cavities, typically causing abscesses. Peak season is summer, year-round in warm climates.
• Intestinal parasites. Grass fouled by other animals' feces can carry roundworm and hookworm eggs.
• Lungworm. Angiostrongylus vasorum uses slugs and snails as intermediate hosts. Dogs can pick it up by eating a gastropod hidden in grass, or the slime trail left on it. It is regionally distributed but potentially fatal.

A decision framework

Normal — just monitor

Occasional or even daily grazing, no illness before, no vomiting after, normal appetite, normal stools, normal energy. Nothing to fix.

Manage with environment and enrichment

Grazing that looks bored or repetitive, or happens on ground you don't control. Stop lawn treatments or check what your service uses and its re-entry interval, keep dogs off freshly treated grass including neighbors', clear slug bait from the yard, check paws, ears and armpits after walks in dry seedy grass in summer, keep parasite prevention current, and add a sniff walk or food puzzle before the yard shift.

Book a vet appointment

Grass eating that starts suddenly or escalates sharply; repeated vomiting or retching; reduced appetite; weight loss; ongoing diarrhoea; lethargy; or eating non-food items such as dirt, rocks, fabric or plastic. These are reasons for a veterinary assessment, not a diagnosis — several unrelated conditions can produce them.

Urgent care now

Suspected slug bait ingestion; tremors, twitching or seizures; blood in vomit or stool; repeated unproductive retching with a bloated or tight abdomen; sudden collapse; violent sneezing, head shaking or squinting after being in tall dry grass, which can signal a lodged awn.

Frequently asked questions

Should I stop my dog from eating grass?

Not for its own sake. Healthy dogs eating clean, untreated grass are doing something normal. Redirect based on where the grass is, not the behaviour itself.

Does grass eating mean my dog has an upset stomach?

Usually not. Only 8% of owners of 1,571 plant-eating dogs frequently saw illness signs beforehand. When a dog does look unwell first, that is worth a vet's attention.

Why does my dog eat grass and then throw up?

About 22% of dogs in the large survey regularly vomited afterward, but the controlled trial recorded only five vomiting events across 709 grass-eating episodes. Frequent vomiting is a reason to see a vet regardless of the grass.

Will more fibre or a diet change stop it?

The survey found no link between fibre intake or diet type and grass eating. No diet or supplement has been shown to stop it.

Is eating grass every day a problem?

68% of dogs in the survey ate plants daily or weekly. Frequency alone is not the warning sign; a sudden change in frequency, or grass eating alongside vomiting, appetite loss or lethargy, is.

Sources

• Sueda KLC, Hart BL, Cliff KD. Characterisation of plant eating in dogs. Applied Animal Behaviour Science 2008;111:120–132 — ScienceDirect
• Hart BL. Why do dogs and cats eat grass? dvm360, Dec 2009 — dvm360.com
• Knapp DW et al. Detection of herbicides in the urine of pet dogs following home lawn chemical application. Science of the Total Environment 2013;456–457:34–41 — PubMed
• Merck Veterinary Manual — Metaldehyde Poisoning in Animals — merckvetmanual.com
• Merck Veterinary Manual — Lungworm Infection in Dogs — merckvetmanual.com
• Companion Animal Parasite Council — Angiostrongylus vasorum — capcvet.org
• Pet Poison Helpline — Foxtail Grass Foreign Body — petpoisonhelpline.com

This article is for general information and does not replace veterinary care. If your dog is unwell, or if the behaviour has changed suddenly, contact your veterinarian. In a suspected poisoning, contact your vet or an animal poison control service immediately.