I've been grazing alfalfa and grass for a number of years now and you've received some good advice. This is how I graze my alfalfa (up to 90%) and I've never had a case of bloat.
1) Never turn them on with empty stomachs.
2) Never turn them on when the alfalfa is wet.
3) I always wait until there is some bloom on the alfalfa, even 10%. As was mentioned, the cattle will often hit the alfalfa first so your risk of bloat is a little higher.
4) Throw some roughage bales out in the pasture. The cattle will go to these when they need to. Toss them out in a few places and don't worry about the waste. It'll fertilize your crop next year.
5) If you're still concerned, pull them off the pasture at night and put them in a paddock with straight grass. Do this for 2 or 3 days, then give them the boot out into the alfalfa.
I've seen second cut grazed (i've personally never grazed second cut) without having to use rumensin using the above rules and never saw a case of bloat. Your cows will do very well on it, especially if you're in a wetter area like we are. The grass just never has quite enough kick in it, especially later in the summer. The alfalfa really juices them up
Rod