In established stands, it will take at least that much time to determine if the top growth was damaged or whether the stems will recover. The growing point is the initial development source of new leaves and stem on the main stem of alfalfa. The growing point is located inside the dense cluster of unfolded leaves at the top of the main stem.
When the growing point is frosted off, that stem will die and new growth must come from new shoots at the crown. Although the plant itself is not dead, the new growth will be delayed. Cutting off damaged plants will hasten recovery.
If the growing point was not frosted off, the current growth may wilt for a few days and then regain its upright stature once it gets warm again.
New alfalfa seedlings are generally very tolerant of cold temps, partially from heat from the soil and partially from natural plant tolerance. Seedlings no older than first trifoliolate growth stage will probably handle temps in the low 20’s. As they advance in growth, cold tolerance lessens. Seedlings at the 3rd or 4th trifoliolate stage can be difficult to diagnose. If leaf tissue is just singed by frost, they probably will recover slowly.
If your new seeding is frozen to the ground…it’s dead. Reseed or plant to another crop ASAP.
Last years’ late summer planting will probably respond similar to an established stand, although recovery will probably be a lot slower. It’s only early April, so give these plants a little time before you decide to cut, shred, or reseed.
Sounds like more cold temps are on the way...we'll see what happens.
Thank You and Have a Great Day!!