If your dog stops mid-walk to chew on a patch of lawn, you’ve probably wondered whether something is wrong. Of the delicious food options available to them, why would they choose leafy, bitter grass? Many owners assume the worst: that the dog has an upset stomach and is eating grass to make itself throw up.
Dr. Melissa Bain doesn’t see it that way. “My dog enjoys it every day,” says Bain, a professor of clinical animal behavior at the University of California, Davis. “If we ever mow the grass, [he’ll] go out there and just start chomping on it.” To her, it reads as a snack, not a symptom.
The idea that dogs graze to purge a sick stomach is one of the explanations owners reach for most. But it’s not what the research shows.
Eating grass is normal dog behavior
Grass eating is extremely common. In a 2008 UC Davis study, 79 percent of owners whose dogs had daily access to plants said their dog ate them. A follow-up internet survey of more than 1,500 owners found that 68 percent of dogs grazed daily or weekly, and grass was by far the plant they ate most.
If a behavior turns up in roughly three out of four dogs, it’s hard to call it a sign of illness.
Most dogs don’t get sick from grass
If dogs really ate grass to purge, you’d expect them to look ill first and vomit afterward. Most don’t.
The same 2008 study found that only about 9 percent of dogs seemed sick before grazing, and only around 22 percent regularly vomited after.
Diet made no difference either. Whether dogs were fed raw food, kibble, or a vegetarian diet had no bearing on whether they ate grass.
“There is no nutritional basis for that that we know of,” Bain says of the theory that grazing makes up for something missing in a dog’s food. It’s a normal behavior, she adds, and one she sees mostly in healthy animals.
Her interviews with owners point in the same direction. When Bain asked what a dog was doing right before it ate grass, the dogs that already seemed unwell were the ones more likely to throw up afterward. The dogs that seemed fine usually didn’t. So, when sickness does show up, it tends to come before the grass, not because of it. The vomiting looks like a side effect, not the goal.
A popular version of that idea is that dogs graze to flush intestinal worms out of their gut. But many of the dogs in the survey were on monthly heartworm medication, which also clears intestinal worms—so those dogs had nothing to flush out. They grazed anyway.
They probably just like it
Once you set illness and diet aside, the explanation that’s left is appetite. “Most dogs eat grass because it is a food they enjoy,” says Carlo Siracusa, professor of clinical small animal behavior and welfare at the University of Pennsylvania.
Bain has noticed the same thing. Dogs tend to go for moist, long-stemmed grass, the tender kind that comes up early in the morning. They’re choosing what tastes good to them.
The behavior may be inherited from wild ancestors
Why dogs like grass in the first place is harder to answer. The 2008 study proposed that grazing is a normal behavior, possibly an instinct carried over from wild canid ancestors.
Bain finds that idea convincing. One ecological version of that idea holds that grass once helped wild canids clear intestinal worms—the fibrous strands wrap around the worms and carry them out in the droppings. Bain points to wild-canid droppings to support this idea: They often hold long strands of plant material, sometimes with parasites tangled in it. But it isn’t proof, she says.
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A 2021 study of domestic cats had similar results: Very few cats looked ill before eating plants, and the behavior appeared normal and likely innate rather than a reaction to feeling sick. (Cats did vomit more often than dogs—up to a third of the time—which the authors say may reflect some gastric upset.) Why the instinct exists at all is still an open question.
When it’s worth a second look
Only rarely does grass-eating become a problem, Bain says—when it becomes compulsive. Siracusa says it can turn excessive enough to cause an intestinal obstruction.
“I have seen this in anxious dogs, but it does not represent the norm,” he says. In nearly three decades of practice, Bain can remember only one dog whose grazing was truly compulsive, and that dog obsessively ate everything, not just plants.
What matters is the pattern. A dog that grazes constantly, looks sick before eating, or vomits regularly afterward is worth a trip to the veterinarian, since the underlying cause may be nausea or another gut problem. It’s also a good idea to keep grass-eating dogs off chemically treated lawns and away from plants that are toxic to dogs.
For most dogs, though, none of that applies. “Most owners should not be concerned if their dog eats grass,” Siracusa says. For a lot of dogs, grass is just the first snack of the day.
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