there's also a difference between digestion and ingestion. whereas a cow will ingest a diet that's no more than 5% crude fat, that cow will end up digesting, and thus absorbing, almost 70% of its energy from volatile fatty acids that were produced in microbial fermentation. gorillas accomplish the same result, but they have an enlarged cecum because they use post-gastric fermentation. therefore, both cows and gorillas digest their food into a composition whereby, on average, 75% of it is saturated fat.  once enough of these micro-organisms proliferate in size and number by consuming the plant fibres ingested by the animal, a portion of them is dumped into a hydrochloric acid chamber (stomach) within the herbivore, where they are digested. the bacteria farm within the bovine's rumen is the primary source of protein for the animal, because these microbes in question are the only things on earth that can digest plants, specifically the fibre and starch that plants are made of. humans can't do this because humans don't have rumens or cecums, which means we cannot internally farm the necessary fatty acids and amino acids to stay healthy like herbivores can after ingesting plants. note that grain makes their rumen become acidic, and it harms the bacterial culture within the rumen. so, not even the most extensive "herbivore" mammalian of all can handle grains and yet we consider it food.
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