Not sure which herbivores you are thinking of, but herbivores like ruminants, equids, camelids, etc have completely different digestive systems, both anatomically and in many ways functionally. Great apes such as gorillas have a more extensive large intestine, where bacterial digestion and fermentation of plant fiber occurs, than humans. It is much Jess efficient than ruminants or equids, but their diets are more varied than grazing or browsing hindgut fermenters.