I have not found reference to this anywhere, so I want to post this for the world to know: grass is not enough for ruminant animals. These days we see reference to grass fed beef, grass fed pork... and so on. While that is definitely an improvement over feedlot or factory conditions, it is not sufficient to supply the full range of nutrition for animals. There needs to be a mixture of forest, swamp, meadow, and riparian areas to provide the full range of nutrition for cows, sheep, pigs, etc. Animals, like humans, are very resilient. Human beings can exist on Oreo cookies and pasteurized milk with some mac and cheese thrown in... likewise, animals can exist on crappy rations and artificial light and come out looking like meat - but the nutrition isn't there.
Have you ever heard or experienced that grass fed beef (or whatever) is lean and a bit tougher than feedlot beef? Well, that's not a good thing. It's because the animal didn't have the best nutrition.
I'm not saying that grass fed is a gimmick, but ultimately people desire good tasting, well developed, easy to chew meat. And generally speaking, grass fed cannot provide that. Ever had wild venison or elk? You'll know what I'm talking about.
Grass is not enough.