Doesn’t even need to be green, just any sort of plant or really any sort of organic matter. Eating goats that have lived off of old trash is probably not the best idea though

Which neatly raises the point of how modern large monoculture does a lot less of that kind of use of agricultural products unusuable by humans.

Absolutelly, the whole of a cow slaughtered in a slaughterhouse is famously used (down to the hoves) and nothing thrown out, however you don't see goats being raised on the unusable parts of a corn plant (whilst wheat straw is actually used as feed, for corn the silage for cattle made from it uses the whole plant including kernels not just the left-over unusable by humans parts).