Herbicides can have replanting restrictions. If you breakup a hayfield and reseed it in the spring, you run the risk of autotoxicity from any existing alfalfa plants. This means that they won't let the new seed germinate. A rule of thumb is three weeks dead, before you try to re-establish alfalfa. It is also hard to make a good seedbed and to get adequate weed control in fields where alfalfa is recently killed and the field spring plowed.
Annual forages can be an option to produce dry matter tonnage.
Each one has benefits and drawbacks. Comparisons of production costs should be made. Equipment needed to handle them is necessary as many cannot be harvested in a dry form. You should decide what animals you will be feeding this material to and determine if the feed quality will be adequate. Ration balancing will be especially critical if this is a new feed for you to use.