See subject. When they have a tummy ache.
Srsly ?
That’s what I was always told for dogs and I’ve seen it a lot. We have cats now and I’ve sometimes seen grass when they throw up…but then cats make a habit of throwing up… 
Another dog behavior I’ve seen is chewing with their lips open and away from their teeth when they get something they don’t like - to minimize the taste?
For what it’s worth, PetMD has an article about it. I can’t speak to it’s accuracy.
My dogs eat grass like they’re goddamn goats. It never makes them throw up.
Yep. Dogs are omnivores. Especially during a wet spring, grass can be especially tempting. They cannot really digest it, but they seem to enjoy it.
I have two cats, and both, although obligate carnivores. nibble on grass. Pet stores, in addition to selling catnip, sell wheat grass. My cats like the wheat grass but also nibble on other grasses. I think they do it, not as an emetic (because they don’t vomit afterwards) but because it settles their stomachs.
I had an 80 lb dog who liked to eat grass, and in the spring and summer I had to wait every friggin’ morning while she did the ol’ heave ho. In the winter, she didn’t do this. She’d go outside, poop/pee come to the door.
After years of this I just decided either
- She liked nature’s natural organic weed filled salad
or - She was into binge and purge as part of nature’s plan for weird dog behavior.
It didn’t do any harm so I just planned my mornings around it.
Yeah, my black lab grazes like a black Angus.
My dog eats some specific thing in our grass that she hunts down and finds. I have no idea what it is, but our yard is all “free range and organic” so there’s tons of little non-grass plants.
One of our cats loves the grass I occasionally pick for him, plus lettuce, spinach, raw butter beans, and most other green things. He begs for them like they’re tuna squeezins.
In case you’re wondering about Claverhouse’s bemusement, dogs eating grass to puke has been a cliche for like, ever! If from no other place it’s been a staple of cartoons almost as much as dogs chasing cats has (cartoons is where I first saw it as a kid). Like any cliche it isn’t always 100% true, but most everyone’s heard of it…