Yes, it's ok to feed alfalfa. If you're grazing them on an alfalfa field you need to make sure they are full prior to turning them out the first time, and have enough bloat blocks out that the cows have ready, and easy access to them. If feeding hay, I would start them out on a small amount at first and build them up so they have the opportunity to get used to it. Depending on the quality of the hay, alfalfa carries a much higher protein content, and is more easily digested than grass, so it has a much higher potential for bloat. I would also be very careful how much 4th cutting you fed(especially to calves), because it has a higher potential for bloat than 1st, 2nd, or 3rd. We supplemented with baled hay during storms or when the snow cover was deep. The baled hay was fed at a rate of 2-3 grass bales for every alfalfa bale. I hope this helps.
Alfalfa's fine as long as you don't have to buy it - expensive beef feed. I have read that you don't want to feed more than 50/50 to breeding cows due to the phytoestrogens in the hay - not sure how much truth there is to that. Most people don't feed it heavy due to expense and most grown animals just don't need the protein levels. We feed some because we don't have a lot of hay acres and need more than one good cutting.