My dog eats grass when he has a chance and, to be honest, it's the least of my worries...I've also seen him eat turds...He also once ate a whole fence, a door handle on my BMW and a tyre, he ate a wall in my (rented) house and now can't be inside unsupervised...we never put anything in the back of the car with him now.
Before anyone asks, he is very well fed and yes, he is a Labrador.