I found this. Hope it helps.
Q: Why do dogs eat grass?
A: I surfed the Internet to gather veterinarian opinion on this question and found that dogs eat grass for a variety of reasons.
Left: [Drs. Foster & Smith, Inc.] A dog eating grass.
"It's a normal part of their diet," says Holly Frisby, DVM, of Doctors Foster & Smith, Inc. in Wisconsin. Wild canine relatives of dogs--wolves and foxes--eat all of an animal when they catch it. They end up, therefore, eating grasses and plants in the intestines of these animals. Domestic dogs, eating prepared dog food, lack this part of their diet and so they eat grass.
"They just like it!" Frisby also says. She mentions a beagle that picks raspberries faster than his owner.
Another reason: dogs may eat grass when they have an upset stomach in order to cause vomiting. "The grass will bind with the offending material. Then the grass acts as an irritant and causes the dog to vomit, bringing up the poisonous material at the same time," says Celia Feiler, DVM, of Winston Veterinary Hospital in North Carolina.
Finally, dogs may appear to eat grass, says Feiler, when they are just running the blades through their mouth to gather information. Their sense of smell and taste may act together to detect if other animals have walked through their area or urinated on the grass.
And then there's the city dwelling dog-deprived of grass. For a mere $8 a three-ounce bottle, he can eat: Barley Dog, the "Original" Barley Grass Supplement for Dogs.
Further Surfing:
Eating Grass: A Common Behavior by Holly Frisby, DVM, of Drs Foster & Smith, Inc. PetEducation.com
A Dog Eat Grass World by Barry Smith, Irrelativity.com
http://www.wonderquest.com/DogsGrass.htm