They don't get silage unless the farmer makes a point of it. Most of the time the stalks and chaff and spit out the back of the combine and tilled under. Furthermore they'd get penalized for it in weighing if its mixed in with the corn so there's little to no incentive for the extra work of it. If the farmer has a small enough herd though they may let them roam on the field afterward, but the big lots don't do that.