If you have a dog, there's a good chance you've seen it out in your yard, at one time or another, grazing. Sometimes one of my dogs can't get out of the house fast enough. I'm thinking she has to go to the bathroom really bad. But no, she doesn't have to go at all but starts eating grass immediately. All of our dogs occasionally eat grass, some eat it more than others. When Kitsu runs out and starts eating grass, I have connected it to an upset stomach- her stomach had been growling and grumbling all through the night. Eating grass can induce vomiting in dogs.
Some dogs eat grass because of a deficiency. Try adding some greens and vegetables to your dog's diet, like spinach, broccoli, cauliflower, kale, or parsley. Also, a variety of fruits will help, just make sure that the fruits and the vegetables are steamed and finely chopped and then mixed with their regular food.
And then other dogs may eat grass because they are in a restricted environment and they are just looking for some stimulation. Give your dog some appropriate chew toys to engage him in a healthy activity.