If you grow alfalfa, you have experienced winter kill.
Alfalfa winterkill is a complex matrix of many factors: variety, fertility, general stand health, previous fall weather conditions, snow cover, winter temperatures, and spring weather patterns. My own experience has proven one type of winterkill is rarer, while the other is more common.
The rarer situation occurs when conditions are ideal for the alfalfa to overwinter, but temperatures warm up in late winter or early spring and are followed by several freeze-thaw events. The rapid thaw causes the snow to melt into and around the permeable alfalfa crown and tap root. The sudden, deep freeze crystalizes that moisture around the crown. I had a case where this type of weather caused the crown to pop two to four inches out of the soil! While this is an extreme example, there was nothing anyone could have done to save this alfalfa. It was an act of nature.
The other type of winter kill also results from Mother Nature but is more subtle and common. This type of winter kill is a combination of many factors, such as a poor and/or diseased stand, a stand with low fertility, a stand that has limited insulating snow cover throughout winter months, and a cheap alfalfa variety with a lower winter hardiness score.
The effects of winterkill can be quite variable in the field due to slopes, soil types, fertility and other factors. If you see a reduced stand after spring green up, walk around the field to evaluate whether it’s time to rotate the crop.
Tips for assessing spring stands:
- Watch for green up; be patient.
- If the alfalfa isn’t greening up in some areas of field, dig roots and split them. If the average stem density is low in some areas of field but not in others, consider interseeding something as a quick rescue like Italian ryegrass. It establishes quickly, provides tonnage, is good quality and allows you to capture the value of the alfalfa that is still alive.
- Calculate yield potential and consider rotating the crop. The University of Wisconsin – Madison Extension provides a calculator to help estimate yield based on the number of stems in a square foot. Once the stem number is determined, use this formula to calculate the yield potential of that stand: Yield (tons/acre) = (Stems/ft2 x 0.1) + 0.38
For example, an alfalfa stand with 50 stems per square foot would have a yield potential of 5.38 tons per acre. Keep in mind that soil, nutrient deficiency, insects, diseases and other factors may affect the actual yield.
