Many theories abound as to why dogs eat grass. Some veterinarians believe it is because their prepared diets are lacking in greens. Another theory suggests that a dog that is fed an unbalanced diet may eat grass to compensate for a lack of roughage.
Another theory suggests that the dog is performing a behaviour similar to his ancestor, the wolf. Wolves and other carnivores will eat the entire animal they catch on a hunt. Interestingly, carnivores tend to go for the contents of the stomach and intestine first. Since they primarily hunt herbivores, the contents of the stomach are usually fermented grasses and plants; therefore, eating grass is a perfectly normal canine activity.
The most popular and accepted theory is that dogs may eat grass when they have an upset stomach in order to cause vomiting. The grass binds with the offending material and then acts as an irritant and causes the dog to vomit, bringing up the poisonous material at the same time.
No matter what the reason, you need not get unduly worried over these grazing habits, unless the dog eats grass that has been treated with fertiliser, pesticides or herbicides.
This could cause stomach upset or even worse problems for your dog.