Martin, depends on where you are.  Grazing turnip tops will work, but more growth will be seen if sown earlier than labor day.  That is HERE.  Central Mo.  The cattle will graze the bulbs after they eat the greens, and do well on them.  Oats sown here, are a spring oat, and if sown in the fall will not overwinter.  Some of the deer hunters are using a winter oat, and if sown in the fall will overwinter, and can be grazed in the spring as well. 
Good suggestions above,  mixes are good, probably best, the more in the mix the better.