I would not feed them without first taking a nitrate test. The cornstalks in our part of the world are notorious for high nitrates. That being said, have seen them fed just fine free choice. If it is swathed and baled all the better, you get some corn, a little cob, and most of the leaves. If it is baled behind the combine you lose some of the more palatable parts, but can still be a good filler.