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. Is there any evidence that herbivores derive essential parts of their diet from sources other than cellulose? Or that they do not?
2$\begingroup$ Cellulose alone is just a sugar source. Plants do contain minerals and vitamins. They also contain protein, though the amount can be extremely low - thus the need for a lot of grassland to feed cattle. I'd expect the amount of protein intake from hapless bugs and spiders would be low, though, compared to the overall intake from plant sources. $\endgroup$– MCMNov 3, 2012 at 23:36
$\begingroup$ As a fraction of diet it must be low, yes. $\endgroup$– danielNov 3, 2012 at 23:46
$\begingroup$ Deer have been known to eat bird eggs or young birds possibly to supplement calcium for antler growth. I can't image it's a large part of their diet though. $\endgroup$– RioRaiderNov 4, 2012 at 6:21
$\begingroup$ @RioRaider: interesting, do you have a source for that? $\endgroup$– nicoNov 4, 2012 at 10:35
2$\begingroup$ @nico Here is an article in National Geographic. Here is an interesting YouTube video of a deer eating a bird. Here are two USGS webpages, 1st and 2nd, that discusses nest predation by ungulates. Also, take a look at this Journal of Zoology article. $\endgroup$– RioRaiderNov 4, 2012 at 17:15
One key nutrient worth thinking about in the context of this question is vitamin B12. Herbivores such as cows and sheep will derive this directly from the bacteria in their rumen. Rabbits produce two types of feces - soft and hard - and they eat the soft variety, rich in gut bacteria, as a source of nutrients like vitamin B12. Gorillas are primarily herbivores, but are thought to derive their vitamin B12 from insects, eaten purposefully or inadvertently.
So yes, herbivores derive this particular nutrient from non-plant sources.