Who hasn’t wondered, “Why does my pet eat grass?” The truth is there isn’t one clear reason. What we do know is that eating small amounts of grass can be a harmless, albeit curious, thing for a perfectly healthy dog or cat eating great hunks of the stuff—and grassy vomit hits the kitchen tile—it’s time to wonder what’s going on.
http://www.vetstreet.com/care/my-pet-wont-stop-eating-grass-whats-going-on