Predicting alfalfa winter injury is difficult, but a period of extremely cold weather this winter without snowcover has led us to conclude that a significant acreage of alfalfa in Minnesota is at risk of winter injury. This advisory can also be applied to other winter sensitive species such as red clover and ryegrass. A look at the factors involved in potential winter injury this year follows:

Our 2002-03 winter weather:

Let’s review some of the unique weather events that have occurred so far this winter:

• Arctic cold air masses drove air temperatures throughout Minnesota below 0^oF for consecutive days beginning in mid-January and this trend continued into February. These low air temperatures coincided with periods without adequate snowcover to provide insulation. The impact of these environmental conditions is shown in Figure 1 for bare soil at St. Paul. On January 23rd, soil temperatures in the crown region of alfalfa (0.5-2 inch depth) dropped to between –3 and 3^oF. Throughout the state, soil temperatures were in the single digits to mid-teens at depths as great as 4 inches. These patterns are similar to those seen in the late 1980’s when winter injury occurred with a high frequency. Monoculture alfalfa fields that were harvested last fall to increase short supplies of high quality forage may be at relatively greater risk of winter injury since they lack insulating vegetative cover. However, based on soil temperature data under grass sod, alfalfa-grass mixtures may have relatively less risk of winter injury compared to alfalfa monocultures because of the insulation provided by the grass sod.

• The February 2-3, 2003 Groundhog Day snowstorm provided as much as 10 inches of snow to the central portion of the state, but portions of southern Minnesota still lacked adequate snow cover as of mid-February to insulate the soil. From 4 to 6 inches of snow is considered a minimum to insulate alfalfa crowns from the temperature extremes we’ve seen. Current snow depths are shown in Figure 2. For an update on snow depth in your area see: http://climate.umn.edu/doc/snowmap.htm.

• Above normal temperatures without snowfall occurred throughout December. Remember that brown Christmas! On January 7 and 8, 2003, many areas of the state had high temperatures in the 40-50^oF range. In Fairmount, 60^oF was recorded. Although 40-50^oF temperatures can stimulate alfalfa regrowth and break dormancy, Dr. Paul Li, a cold tolerance researcher in the Department of Horticulture at the University of Minnesota, feels that these temperatures likely had little effect on alfalfa because their duration was insufficient (it takes 7-10 days) to break alfalfa dormancy.

• Despite a wet early fall, below normal precipitation from mid-October till February dried out the top several inches of soil. Desiccation (drying) of plant crowns may have occurred on some soils that lacked plant stubble.

The winter weather ahead

Additional stress may occur to already injured alfalfa during late winter and early spring. This has traditionally been a high-risk time for alfalfa winter injury because of air temperature fluctuations. At that time, alfalfa dormancy is more easily broken as temperatures frequently rise to 50^oF for several days, but periods of freezing weather often follow. Buds and leaves that have broken dormancy have very little cold tolerance and are susceptible to freezing. In addition, melting and freezing of water can cause ice sheeting that may literally smother alfalfa plants.

Figure 1. Average soil temperatures under bare soil at depths of 0.5 (1 cm) and 2 inches (5 cm) from January 19 to 24, 2003 at St. Paul, MN. Soil temperatures under sod showed considerably less reduction in soil temperatures as air temperatures plunged.

Alfalfa management impacts the severity of winter injury

Crop management will interact with weather conditions to lessen or intensify winter injury. A risk assessment approach of predicting winter injury is provided in Table 1. This approach considers the impact of the level of important management factors (i.e. controllable factors) on winter injury. More points signify relatively greater risk of winter injury. This winter, an above average score combined with a fall harvest may increase your risk.

Table 1.  Calculate your risk of alfalfa winter injury (more points = greater risk).

Explanation of terms:

1. Stand age: Because of cumulative stress of plant diseases and physical injury, older stands are more susceptible to winter injury than   younger stands.

2. Variety: Varieties with greater winter hardiness and disease resistance are less susceptible to winter injury than varieties with less winter hardiness and disease resistance. For detailed descriptions of variety disease resistance and winter hardiness, see Minnesota Variety Trial Results MP108-2003. or http://www.agro.agri.umn.edu/forage/

3. Soil K level: Soil potassium (potash) is very important in enhancing alfalfa tolerance to winter injury. However, plants stressed by low pH (<6.5) or deficiencies of other nutrients such as P, B, and S will also be more susceptible to winter stress.

4. Soil drainage: Poorly drained soils are more likely to lead to plant diseases and ice sheeting.

5. Harvest management:

Harvest number: Although growing seasons vary throughout the state, normally, more frequent cutting will cause more plant stress. That’s why in general three cuts are less risky that four cuts in southern Minnesota.

Last harvest date: For least risk in a three-cut system, make the final cut by September 1^st. This allows time for alfalfa to regrow, accumulate carbohydrate reserves, and undergo the normal changes associated with fall dormancy. For four-cut systems, the cutting frequency required to obtain four cuts by September 1^st often creates more risk than delaying the 4^th cut until around October 15, EXCEPT perhaps when the winter that follows (eg. this winter) combines extreme cold with no snow cover. Fall cutting on 15 September causes alfalfa to regrow, but the regrowth period is inadequate to replenish carbohydrate reserves and for the plant to obtain a high level of dormancy. Cutting on 15 October about the time of the first killing frost lessens the risk of winter injury relative to cutting on September 15 because there is normally a minimal chance for regrowth.

6. Stubble: Stubble from unharvested plant residue insulates the soil, catches snow for insulation, and by shading the soil surface from sunlight can minimize freezing and thawing cycles. Therefore, harvesting in the fall can enhance the risk of winter injury especially during winters with minimal snow cover.

The alfalfa plant: a hibernating perennial

    The alfalfa plant is somewhat like a hibernating bear. It is over wintering and will regrow in the spring from crown buds formed in the fall.

• In the fall, decreasing temperatures and shortening daylengths cause biochemical and morphological changes that allow the plant to tolerate low temperatures until the spring.
• The plant has a low level of metabolic activity but still relies on respiration of carbohydrate reserves throughout the winter.
• While temperatures below 25^oF can kill alfalfa during the summer; during the fall, very dormant and winter hardy varieties can withstand temperatures as low as –5^oF. Most commonly grown hay varieties will undergo injury at temperatures below 5-15^oF.
• Prolonged exposure (7-10 days) to temperatures above 40-50^oF can cause alfalfa to break dormancy and becomes susceptible to freezing injury. Dormancy is most easily broken in late winter/early spring.
• Of the over-wintering structures, cold tolerance is greatest for crown buds and least for the roots. The crown buds are the source of vegetative regrowth in the spring. If not completely dead, winter-injured plants display very uneven growth due to death of some crown buds.
• Alfalfa plants are protected from freezing temperatures in the winter by the soil, plant residue, and snow. The first line of defense for the plant is to have a crown deeply buried by the soil. Varieties vary in this trait; winter hardy, grazing tolerant entries often have the deepest crowns. Soils vary in their cooling properties.
• Plant residue, including stem stubble and dried leaves, helps catch and hold insulating snow, but also protects the soil from rapid freezing and thawing. That’s why if alfalfa is cut in the fall, the risk of winter injury can be increased.
• Dependable long-term snow cover is one of the most important environmental factors affecting winter injury. Snow cover has insulating properties that protect the soil. A minimum snow depth of 4-6 inches is needed to insulate the soil from low temperatures.