I've had a 4 acre field of Whitetail Institute "Alfa Rack" for several years now that I planted principally to feed deer but we do also cut it regularly for hay. This is a mix of alfalfa, imperial clover and chicory. Mostly alfalfa. The problem I've observed is that unless you can somehow strike a balance of deer numbers vs. alfalfa production so that the alfalfa never matures, you will end up with a tall plot of mature alfalfa that falls over on itself and is too tough for the deer to eat. It needs to be cut about once a month. Another negative for the WI alfalfa when cut for hay is that it can't tolerate cutting as short as you have to do for hay. "Cut it higher" you say? Sounds easy but in the real world rain will inevitably keep you from cutting the hay when you need to. If you wait another week or so beyond the ideal time to cut, the alfalfa falls over. Once it does that you have to drop the mower down to the dirt to cut it. Over time this kills the alfalfa. This means that every year you need to re-seed the field at about half the normal rate.
Again, it might work ok with a smaller plot that the deer can keep eaten down better. It is a great protein producer for sure.