If you look at a dogs mouth and musculature, you will know that dogs are in fact, carnivores.

They have verystrong necks and jaws for ripping and tearing meat, teeth designed for ripping and tearing as opposed to all flat teeth like a horse or combinaton teeth like humans. That alone proves that dogs are carnivores. If you look at the enzymes and bacteria in a dogs system. In a canines saliva, they lack enzymes to break down vegetable matter. The stomach acids and linings, unike ominvores and herbivores, have beneficial bacteria to kill harmful bacteria like staph, salmonella and e.coli. Carnivores have short digestive tracts to prevent fermentation of meat.

Some carnivores to intentionally eat plants to act as an emetic. This mostly includes cats, but some dogs have been known to eat grasses or plants because of indgestion, and they try to alleviate the pain. On the other hand, some dogs like to graze because they enjoy it....not because they're looking for nutrition.

Dogs fed on a strict vegetarian diet can develop diseases like dilated cardiomyopathy from lack of L-carnitine.