Dogs Have Been Eating Grass Forever, and Science Says That Is Completely Fine
If you have a dog, you have almost certainly watched it ditch its perfectly good food bowl to go chew on a patch of grass in the backyard or on a walk. It is one of those things that makes dog owners instantly anxious. The most common explanation people reach for is that the dog must have an upset stomach and is trying to make itself vomit to feel better. It sounds logical. But when researchers actually looked into it, the data told a very different story — and the real explanation turns out to be much more interesting than a sick tummy.
Grass-eating is not a quirk belonging to a handful of strange dogs. It is remarkably widespread. A 2008 study from the University of California, Davis surveyed dog owners and found that 79 percent of dogs with regular access to plants ate them. A follow-up internet survey covering more than 1,500 owners found that 68 percent of dogs grazed daily or weekly, and grass was by far the most popular plant. When a behavior shows up in roughly three out of four dogs, it stops being a warning sign and starts being a defining trait of the species.
The sick-stomach theory falls apart even further when you look at what actually happens before and after grazing. The 2008 study found that only about 9 percent of dogs appeared unwell before eating grass, and only around 22 percent vomited afterward. Dr. Melissa Bain, a professor of clinical animal behavior at the University of California, Davis, points out that the pattern goes in the opposite direction from what people assume: when a dog does get sick after eating grass, it usually seemed unwell before it started grazing, not after. The grass looks like a side effect of the sickness, not the cause of the vomiting. Diet did not change anything either — dogs eating raw food, dry kibble, or even vegetarian diets all grazed at similar rates, suggesting they were not compensating for a missing nutrient.
Once illness and diet are ruled out, the answer that remains is much simpler: dogs appear to genuinely enjoy eating grass. Dr. Carlo Siracusa, a professor of clinical small animal behavior and welfare at the University of Pennsylvania, puts it plainly — most dogs eat grass because it is something they like. Bain has noticed that dogs tend to seek out moist, long-stemmed, tender grass, especially the fresh growth that appears in the early morning. They are not just mindlessly chewing whatever is underfoot; they are making a preference. That is the behavior of an animal choosing a snack, not self-medicating.
Where does this preference come from? Researchers think it may be an instinct — an inborn behavior — inherited from wild canid ancestors. Canids are the family of animals that includes wolves, foxes, and domestic dogs. Studies of wild canid droppings, the waste left behind by these animals, have found long strands of plant material inside, sometimes with intestinal parasites — organisms that live inside a host and feed off it — tangled up in the fibers. One theory is that fibrous grass helped wild ancestors clear those parasites from their digestive systems, wrapping around the worms and pulling them out. Whether that is the true origin of the behavior is still an open question, but the clues from wild relatives are compelling. A 2021 study of domestic cats found a similar pattern: very few cats looked sick before eating plants, and the behavior appeared to be instinctive rather than a response to feeling unwell.
For the overwhelming majority of dogs, grass-eating is harmless and normal. There are a few situations worth paying attention to, though. Compulsive grazing — eating grass obsessively and constantly rather than casually — can occasionally lead to an intestinal obstruction, which means a blockage inside the digestive tract. Siracusa notes this can happen in anxious dogs, though it is rare. Bain, who has nearly three decades of experience, can recall only one truly compulsive grass-eater in all that time. If a dog grazes nonstop, looks sick beforehand, or vomits frequently after, a visit to the veterinarian makes sense. It is also worth keeping dogs away from lawns treated with chemical pesticides or fertilizers, and away from plants that are known to be toxic to dogs. For the typical healthy dog, though, none of those concerns apply. Grass is just one of the small pleasures in a dog's day — and now you know why.
Source: Popular Science