My dogs can't seem to resist the first tender shoots of grass in the spring. They aren't sick and just seem to enjoy the taste.
Grass is also part of a carnivore's diet, usually consumed when they feed on smaller prey and consume the entire animal; including grassy stomach contents.
Veterinary researchers have asked this question too, only to find that "grass eating is a common behavior in normal dogs unrelated to illness and that dogs do not regularly vomit afterward. Vomiting seems to be incidental to, rather than caused by, plant eating."
Good Feeding Practices
Most dogs do best with 2 or more small feedings a day rather than one large feeding. Cats optimally would feed several small meals throughout the day. Whenever possible I have fed my dogs and cats free choice -- food available at all times. This aids in weight control (food is no longer a "big deal" and helps maintain digestive health.) This is not possible with many pets and busy lifestyles, but for some pets, it works great.
A commercial diet or well-prepared home diet should not be lacking in essential nutrients. If they have an empty stomach, bile may reflux (flow back up in to the stomach from the intestine) and this is irritating. This can cause vomiting of clear, yellow fluid.
Good Health Care
Any time that your pet experiences lack of appetite, eating odd items (pica), or vomiting, it is always best to check with your vet.
Gastrointestinal blockages are emergencies. Changes in appetite or vomiting can also indicate internal disease (http://vetmedicine.about.com/od/diseasesandconditions/tp/Kidney-Disease-In-Dogs-And-Cats.htm">kidney failure, hyperthyroidism, etc.), toxin ingestion, or infectious disease, just to name a few possibilities. Always best to check things out with your vet.