We’ve once again reached the that time of year when the sight of green, growing alfalfa plants is as welcomed as greeting the Publisher’s Clearinghouse people on the front doorstep.  In the recent past, East Central Wisconsin has been fortunate not to have experienced widespread alfalfa winter injury or winterkill. Such was not the case in northern Wisconsin last year.  If and how much winter injury has occurred this year has yet to be determined and often will vary from field to field because of a number of factors.  Among them are: 

·      Stand age.  Older stands are more likely to winterkill than younger ones.

·      Variety.  Varieties with superior winterhardiness ratings and a high disease resistance index are less likely to experience winter injury. 

·      Soil pH.  Stands growing on soils with a pH above 6.6 are less likely to experience winter injury.

·      Soil fertility.  Stands with high fertility, particularly potassium, are less likely to experience winter injury than those with low fertility.

·      Soil moisture.  Alfalfa grown on well-drained soils is less prone to winter injury.

·      Fall soil moisture status.  As dehydration is the primary means of tolerating freezing temperatures, stands that go into winter with low soil moisture are better able to lose moisture and are less likely to winter kill.

·      Cutting management.  Both harvest frequency and timing of fall cutting affect alfalfa winterhardiness. The shorter the interval between cuttings, the greater is the risk of winter injury.  Stands in which a last cutting is taken between September 1 and October 15 are at greater risk, as plants are unable to replenish root carbohydrate reserves before winter.

·      Snow cover.  Snow is an excellent insulator and temperature fluctuations are much less where snow is present.  As little as 4 inches of snow can result in a 10 degree difference in soil temperature.

Look, then look again..... 

        Perhaps one of the more important lessons to learn when evaluating alfalfa injury is to check stands more than once.  Often, plants will begin to green-up and grow but later die-off.  This can be caused by numerous factors including low root carbohydrate reserves, severe frost, and disease.   

        Plants that green-up and continue growing may also have suffered winter injury.  These plants may exhibit the following characteristics: 

·      Slow Green-Up.  One of the most evident results of winter injury is that stands are slow to green up.  If other fields in the area are starting to grow and yours are still brown, it is time to check those stands for injury or death.

·      Asymmetrical Growth.  Buds for spring growth are formed during the previous fall.  If parts of an alfalfa root are killed and others are not, only the living portion of the crown will give rise to new shoots resulting in a crown with shoots on only one side.

·      Uneven Growth.  During winter, some buds on a plant crown may be killed and others may not.  The uninjured buds will start growth early while the killed buds must be replaced by new buds formed in spring.  This will result in shoots of different height on the same plant, with the shoots from buds formed in spring several inches shorter than the shoots arising from fall buds.

·      Root Damage.  The best way to diagnose winter injury is by digging up plants (4 to 6 inches deep) and examining roots.  Healthy roots should be firm and white in color with little evidence of root rot.  Winterkilled roots will have a gray, water-soaked appearance early, just after soils thaw.  Once water leaves the root, the tissue will become brown, dehydrated and stringy.  If the root is soft and water can be easily squeezed from it, or is brown, dry and stringy, it is most likely winterkilled.  Also, if 50% or more of the root is blackened from root rot, the plant will most likely die during the spring growth cycle or later in the year. 

If stands are winter injured..... 

        Winter injured stands are often more difficult to evaluate from the standpoint of what to do next than winterkilled stands.  Often, it comes down to accepting something less than full yield potential but more than what might be obtained from a new seeding.  The following table was derived from Wisconsin research and offered here as guide to evaluate yield potential based on the number of stems per square foot. 

Using Stem Density to Evaluate Alfalfa Stands 

        Cutting management on winter injured stands needs to be somewhat more conservative than on healthy stands.  If possible, delay first-cut until flowering and cut fields higher than normal.