How to Assess Alfalfa for Winter Injury

Extreme cold temperatures without snow cover can lead to cold soil temperatures contributing to winter injury in alfalfa. Additional factors include plant health going into fall dormancy, the amount and condition of fall growth to provide soil cover, soil potassium status, and stress from freeze-thaw cycles.

Evaluate Your Stand

Growers can evaluate alfalfa fields by digging plant crowns soon after a soil thaw. Crowns and roots that are firm and a healthy white or cream color have a good chance of survival. Mushy crowns and roots will likely not survive. Dig roots and look at several areas of each field to determine the crop’s status.

Two alfalfa plants have been split to show a healthy alfalfa crown (left) and alfalfa crown rot disease (right).
Two alfalfa plants have been split to show a healthy alfalfa crown (left)
and alfalfa crown rot disease (right). Note discoloration of the top part
of the root (crown) on the right.

Spring green-up provides a second opportunity to evaluate alfalfa and forage stands. See whether alfalfa crown buds are growing and stems are elongating. Weakened plants may grow but have only one or a few stems. Plant and stem counts help determine yield potential of a field. University of Wisconsin research shows 55 stems per square foot provides full yield potential. Consider rotating out of fields when the number of stems is below 40 per square foot.

Make an Early Decision to Rotate

If an alfalfa field shows early signs of winter injury, a grower can rotate directly to corn without waiting and fretting over slow or sparse green-up. An early decision lets a grower plant corn to take advantage of the residual nitrogen from alfalfa. Fields with a good stand of alfalfa going into winter can provide the entire nitrogen requirement for a following corn crop.

An early decision also allows the grower to locate and plant a new field of alfalfa and capture the full-season yield potential. First year alfalfa stands typically yield less than a robust established stand, so it's important to get newly seeded alfalfa off to a strong start, minimizing the slow early-growth period. Count on 7-10 weeks from planting to first cutting.

Is My Alfalfa Stand Thick Enough?

The best measure of alfalfa viability comes after plants start to green up in early spring. Check for bud and new shoot vigor. Healthy crowns are large, symmetrical and have many shoots. If the crop exhibits delayed green-up, lopsided crowns and uneven growth of shoots, investigate further by checking for root rots, broken roots or damaged crowns.

This alfalfa field contains an area of winter injury where alfalfa was clipped late in summer and no cover was available for crown protection.
This alfalfa field contains an area of winter injury where alfalfa was clipped
late in summer and no cover was available for crown protection.

When alfalfa is 4-6 inches high, use stem counts (stems per square foot) to determine density. Count only the stems expected to be tall enough to mow.

When alfalfa is 4-6 inches high, use stem counts (stems per square foot) to determine density. Count only the stems expected to be tall enough to mow. A stem density of 55 per square foot has good yield potential. Expect some yield loss with stem counts between 40 and 50. Consider replacing the stand if there are less than 40 stems per square foot, especially if crown and root health is poor.

Chart: Alfalfa Stem Count and Yield Potential (dry matter yield)
 

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