Alfalfa Winter Survival

Recent weather has raised great concern about survival of winter wheat and alfalfa.  It is known that alfalfa will generally be injured or die if the crown falls to 5 to 15^o F depending on variety and past management.  Research has shown increased electrolyte leakage and cell rupture of alfalfa taproot cells exposed to 17.6^o F for as little as 30 minutes.

The crown is generally not exposed to temperatures this low because, even though air temperature may fall below 15^o F for extended periods, the crown is insulated by snow cover, crop reside, and soil.  As the data at right from Michigan show, as little as 4 inches (10 cm) will drastically reduce day/night variations in soil temperature and will insulate allowing air/soil temperature differentials of as much as 10^o F.

Soil temperatures have fallen below 10^o F at 2 inches in bare soil of weather monitoring stations at Arlington, Hancock and Spring Green.  Spooner reported 16^oF soils on Jan 24. It is believed that all areas of the state without snow cover have had soil temperatures at four inches fall below 15^o F.  This would indicate that all alfalfa and wheat have suffered some degree of winter injury and may have died.  However not all alfalfa and wheat were necessarily killed.  The graphs below show soil temperature at several depths with and without sod grass cover at the University of Minnesota St Paul Campus.  Soil with sod cover is consistently 6 to 8 degrees warmer than bare soil.  This demonstrates the insulating ability of plant residue.  It would also indicate that those fields with significant crop residue going into the winter may be sufficiently insulated to avoid winterkill.

In summary, due to the low soil temperatures resulting from lack of snow cover, we believe that significant alfalfa and wheat acreage are at risk of winterkill.  Those stands that were planted to more winter hardy varieties, better fertilized, and have greater residue will be less affected.  We will need to wait until closer to spring to fully assess the condition of alfalfa stands.

                        Dr. Dan Undersander                                               Dr. Ken Albrecht

                        Extension and Research Forage Agronomist       Research Forage Agronomist

                        University of Wisconsin                                            University of Wisconsin