I was planning on hollowing out some fields deep underground. I just got done reading the latest Discworld book and decided deeper is better, so instead of bringing the magma to me, Urist is going to the magma, down at z-60 or so. I guess I will have to wall off the deepest cavern level and keep my cave sheep there instead. That sounds like more fun anyway.
Cave fungus grows great on underground soil, though. One ten by ten plot underground easily supported the two yaks I arrived with, but I find yaks get cantankerous if there are more than two adults in the same ten by ten square. I brought some sheep, they seem to take crowding better, so the yak family is going in the stewpot. Urist is getting out of the yak business, all yak products fifty percent off. And because of the way animals feed, putting a bunch in a big pasture still leads to fighting, I even saw yak infanticide once. Dividing up the same 20x20 pasture into four 10x10 pastures and putting two or three animals per pasture stopped the fighting.
I started with a walled in 20x20 pasture above ground. For aesthetic reasons and security purposes, I've moved everything underground. The same ten by ten plots underground have more bare patches than the grass above did. I think fungus grows slower than grass. One full grown yak alone in a 10x10 pasture keeps about a quarter of it cropped down to bare sand all the time. Two adult sheep in the same size plot don't even eat half that much. Plus, the sheep breed quicker and grow faster.
I wonder if heaver animals trample the grass more, and that is part of the problem? It seems like sheep, goats, alpacas and the like are much more economical than yaks, cows, horses and other bigger animals, especially trying to raise them underground. That seems right, in the real world cows do take far more land than smaller critters.