It is not uncommon for dogs to eat grass and is generally un-harmful to the dog providing the grass has not been chemically treated. Your pooch might vomit the chewed grass with frothy saliva, not long after eating it. The definitive answer to the question "Why do dogs eat grass?" has not been found. But experts have some interesting theories.
According to this theory, modern-day domesticated dogs eat grass because, in their evolutionary past, they were scavengers. Sometimes that included grass
Dogs are omnivores
Some dogs will graze on fine grass and may even digest it to provide roughage in the diet. As omnivores, dogs may simply have a natural craving for grass. Some dogs may eat more grass than others.
One thought for dogs doing this is to relieve an excess of digestive juices that accumulate in the stomach when it is empty. Experts still aren't entirely clear if dogs eat grass because they feel a need to cleanse their stomachs, or simply because they want to!
When should you be concerned?
If the grass eating is accompanied by prolonged or persistent vomiting, and particularly if the vomitus contains blood, veterinary advice should be sought. The use of pesticides, herbicides, fertilizers, and other lawn treatments is another cause for concern. Make sure he doesn’t snack on any lawn that's been treated. When in doubt, play it safe and keep him away.