Problem:
Giving minimal credence to estimates in popular media of the average biomass of insects/arachnids, etc. in an acre of land, it seems that a "free-range" cow (I don't mean to pick on cows) might be something of an omnivore.
Question: Is there any evidence that herbivores derive essential parts of their diet from sources other than cellulose? Or that they do not? Please justify your answer.