As long as there are green plant parts in the alfalfa, there is the chance for bloat. At this point, we would suggest that you wait until the plant has gone completely dormant as indicated when the plant has turned completely brown. Another suggestion to consider is to wait to turn the cows onto the alfalfa field when the ground has firmed up and is frozen. In the fall of the year, we usually get some moisture, and the cows can tear up the field and also cause damage to the crown of the alfalfa plant. If you are going to renovate an old stand, then this isn't a concern.
There are products that you can offer to help counter the bloat. The most common is called "Bloat Guard" and is a free-choice block. Because it is offered free-choice, not all cows will likely consume the block, so there is still the chance for bloat in those cows that don't consume. There is also a product that can be added to the water, which should work better to control bloat, especially if cows drink water out of a tank.
One of the best methods to control bloat is to fill the cows up with forage and then turn them out. They will adapt, and rumen gas production will be reduced the first few days. Also, monitor the cows. If one cow seems to have a bellyache, pull her off, let her recover, fill her up with forage, and then turn her back out. We see most deaths due to bloat caused by alfalfa when sick calves get well and are never pulled out. When they get well, they are hungry and take in a big load of alfalfa and bloat.
The chances of bloat are reduced after the plant freezes and goes dormant, but we would still recommend that you fill them with forage before turning them out the first time.
