Scavengers 'R Us
Dogs, unlike their catty counterparts, are not carnivores. But they're not like your garden-variety omnivores, either. For tens of thousands of years, these opportunistic scavengers have devoured anything and everything, as long as it fulfilled their basic dietary requirements.
The modern dog, partly because of evolution and domestication, is no longer like its ancestors, which frequently ate their prey entirely, including the stomach contents of plant-eating animals. Instead, dogs today seek out plants as an alternative food source. Most commonly the plant is grass -- since that is what is closest at hand -- but wild canines are known to eat fruits, berries, and other vegetable matter, too.
Clearly, dogs can find their nutrients in a wide range of plant foods, but that doesn't explain why Fido usually throws up after eating grass.
When the Tummy's Grumblin'
A dog will seek out a natural remedy for a gassy or upset stomach, and grass, it seems, may do the trick. When ingested, the grass blade tickles the throat and stomach lining; this sensation, in turn, may cause the dog to vomit, especially if the grass is gulped down rather than chewed.
Although dogs don't typically graze on large amounts of grass like a cow, they may nibble on grass, chew on it for a while, and not throw up (an unwell dog will tend to gulp the grass down in big bites and then throw up). This may be because they find the texture of the grass palatable, or just because they need to add a little roughage to their diet.
Whatever the reason may be, most experts see no danger in letting your dog eat grass. In fact, grass contains essential nutrients that a dog might crave, especially if they're on a commercial diet. If you notice that your dog has been munching away on grass or houseplants, then you may want to introduce natural herbs or cooked vegetables into their diet. Dogs aren't finicky like cats, but they're not too fond of raw veggies either. They're kind of like big furry kids that way.
So, when you think about it, grass munching isn’t that bad at all. However, watch out for a sudden increase in grass eating; it could be a sign of a more serious underlying illness that your dog is trying to self treat, and that requires immediate veterinary assistance.
You may also want to buy a small tray of grass just for the dog, or start an herbal home garden. This will give your poor pooch an alternative to the outdoor grass and landscaping, the eating of which could lead to accidental ingestion of pesticides, herbicides, or chemicals that have been used to treat your (or your neighbor's) yard.