Minotaurs, if anything like their brahman brethren, can get nutrition from all kinds of roughage that us puny primates can't. So while we're scrambling around for nuts and goodberries, they can make a meal of all the weeds sprouting in terrible soil, and the odd hay bale lying around to feed someone's horses.

TL;DR: They could feasibly turn what we consider indigestible garbage-plants into calorie-rich milk.