You might want to introduce them to the alfalfa when they are full and after the dew is off. I would also just limit their hours on the alfalfa for the first two or three days. And as Msscamp says have them on bloat blocks. Good luck.
Yup, done it a whole lot, and have a friend that on her ranch grazes a bunch of it. The previous advice is good. Basically saying that you strip graze them on a limited amount per day for bloat reasons but also so they do not waist too much protein also. It is a good way to get great gains especially in the summer when the grasses have gone dormant or highly vegetative and high in fiber.