Concern always arises in cold periods over winter about the effect of the low temperatures on alfalfa winter survival. This is of some concern because certainly the alfalfa plant will die if exposed to cold enough temperatures.
However, generally alfalfa survives the winter and its periodic cold spells. The reasons are:
1) Alfalfa can survive temperatures of -10 to -15°F.
2) This is the temperature of the crown not the topgrowth.
3) As little as 4 inches of loose snow will insulate against up to 16°F of air temperature.
4) The crown is insulated by soil as well; therefore the crucial temperature is the temperature at 2 to 4 inches below the soil surface.
The soil temperature on Jan 8 (after the last cold spell) is generally above 0°F and most of the Midwest. The higher temperature than the air the last few days is due to insulating ability of both snow (most of Midwest had at least 4 inches of snow cover) and the insulating ability of the soil. This situation should indicate little to no injury or kill of alfalfa.