Will alfalfa fields be green and growing or brown and dead?  That is the gut-wrenching question we ask each spring while an entire farm's cropping system hangs in the balance.  However, keep in mind that there is also an intermediate outcome for spring alfalfa fields........winter injury.  In this case, plants may not be dead but they're also not firing on 8 cylinders.  Winter injury to alfalfa stands is caused by a number of factors.  These include weather, previous cutting management, soil fertility, and stand age.  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 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 (see Figure 1).

·      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 (Figure 2).  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 (Figure 3), the plant will most likely die during the spring growth cycle or later in the year.

       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.  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. 

A keeper or not?

        Deciding whether to keep or terminate a stand in the spring is based on several factors.  If most root systems look healthy and there are over 40 stems per square foot, then a stand is generally thought to have high enough yield potential to keep.  Conversely, if stem number is less than 40 per square foot and many root systems are discolored, yield potential is significantly compromised.  In years where many stands are winter injured, the cutoff point for what might be considered an acceptable stand may have to be lowered.