This question has been answered a million times. The experts don’t know why dogs eat grass. They suggest various possibilities, which is the best they can do.
Folic acid – Vitamin B9
The most recent possibility is that dogs eat grass as a means to ingest folic acid which is a vitamin supplement to their meat diet. The problem with this explanation is that most dogs eat a balanced diet and therefore they don’t need a vitamin supplement.
Inherited from the wolf
Perhaps a better reason is that the domestic dog is simply following what their wild ancestor, the wolf, did. In examining the scats of wolves experts find that between 11 and 47% of them eat grass. This is because there is grass in the stomachs of prey animals that are killed by wolves. Perhaps grass is a necessary ingredient to a fully balanced diet which harks back to the first point that grass is a vitamin supplement or the roughage argument below.
The classic reason given is that they eat grass to make themselves sick because they are feeling ill. But this is a chicken and egg problem. Are they eating grass to make themselves sick or are they sick because they ate grass? And in any case, studies have shown that only 10% of dogs were ill before they ate grass and less than 25% of dogs vomit after eating it. These statistics undermine this theory.
Technically, a dog eating grass is a form of pica, which is eating a non-food item. It is a condition not uncommon in domestic cats when they eat wool for example. This applies to Siamese cats more often than not. But, once again I’ve got to return to the original point that it may be a dietary supplement and therefore it cannot be treated as a non-food item.
Another suggestion is that dogs eat grass because it adds roughage to their diet. This seems a more reasonable suggestion. Roughage helps a dog to pass faeces through the lower intestines. This may be why wolves eat the stomach contents of prey animals. They are adding roughage to their diet. I find this suggestion the most plausible together with the vitamin supplement proposal which is often put forward by Dr Desmond Morris who is a world-renowned cat and dog behaviourist.
Dogs eat grass for roughage and vitamin B9 in following their wolf ancestors.