That is not going to be easily done. My reasoning would be that it would create unwanted competition with the more expensive alfalfa. In my experience with alfalfa it does quit growing after the first hard frost, but the deer dont quit eating it till it is mowed off or it is buried under snow.

You could possibly plant a cold hardy plant or plants around the outside edge of the alfalfa. This is just my opinion. These other guys could have some ideas.