Some possible reasons:
1) The dog feels sick and wants to induce vomitting by gulping the grass down quickly so that it can feel better.
It is kind of a natural remedy the dog seeks to self-treat for stomach upset, bloatedness or gassy tummy. You should bring your dog to the vet if it suddenly eats lots of grass when it has never done so previously.
However, if your dog eats grass and then proceeds to be sick more than once a week, it may have contracted intestinal parasites (many parasites lay their eggs in the lawn), which would need medical attention. If your dog is on heartworm preventives regularly, then it shouldn’t be a problem as heartworm meds protect against most common intestinal parasites.
Another reason the dog gets sick after eating grass could be that it may have consumed toxic chemicals sprayed on the grass. Visit the vet as soon as possible.
2) It may indicate that your dog has some type of nutritional deficiency, so it is trying to fulfill this unmet nutritional need which includes the need for fiber. If this is the reason you suspect your dog is eating grass frequently, you may want to start it on a high fiber diet to see if this habit changes.
Typically, dogs will chew and graze more when they are feeling well. The more they chew the grass, the more the blade becomes saturated with saliva, and in becoming so, it is less “tickly” as they swallow. That is why dogs don’t necessary vomit when they eat grass.
3) It is simply a sign of boredom, especially when practiced by puppies and younger dogs. Give your poor dog some toys and chewies and spend some quality time with your pet daily. It is your responsibility as a pet owner.
4) Your dog simply likes the way grass tastes or feels.
5) It may just be a natural way of improving digestion for the dogs.
Although most experts agree that grazing itself isn’t harmful, one thing dog owners ought to keep in mind is that certain herbicides and pesticides used on lawns can be quite toxic, especially if ingested. As a matter of fact, fertilizers were one of the top 10 causes of pet poisoning in 2008. In addition, quite a number of common house and garden plants are toxic, so it could possibly lead to problems if your dog munches on them along with the lawn.