Two major factors can contribute to winter injury and/or winterkill in alfalfa fields. The most obvious is severe winter weather, and the second is crop management decisions made during the growing season.
Management practices can help minimize the risk. If implemented correctly, these practices can, in a sense, “winterize your alfalfa” and give it the best chance of surviving harsh winter conditions.
Four weather-related conditions are generally responsible for alfalfa winterkill:
• cold soil temperatures
• ice sheeting
• plant heaving
• warm midwinter periods followed by sudden cold temperatures
Here are several management tips that have been shown to the potential winterkill risk related to each weather-related factor:
Cold soil temperatures. Alfalfa winterkill can occur if soil temperatures (at 2 to 4 inches deep) reach in the range of 12 to 13 degrees F or lower. Snow cover of at least 4 inches is considered adequate to insulate alfalfa from these extreme temperatures. The lack of snow cover in open winters is a major factor in alfalfa winterkill and/or winter injury.
Management tip No. 1: Adjust the cutting height of the last fall cut to leave 4 to 6 inches of stubble. The stubble helps collect adequate snow cover to provide some insulation to the alfalfa plant from cold winter temperatures.
Ice sheeting. This condition occurs when melting snow or rain refreezes during the winter, completely covering plants. This type of winterkill can be quite severe, especially if the ice covers the alfalfa for more than 30 days. Plant death is attributed to the lack of oxygen under the ice and may occur more often in low-lying areas of fields, where water can pool and freeze.
Management tip No. 2: Leave 4 to 6 inches of stubble on last cut. A high stubble that sticks above an ice sheet has been reported to help reduce the amount of damage. However, it is no guarantee.
Plant heaving. Heaving occurs on heavy soils that have high moisture content. The repeated freezing and thawing of these wet soils can result in pushing some alfalfa roots out of the ground. In severe cases, the top 2 to 4 inches of the crown and root may be exposed above the soil surface. These exposed crowns are often cut off during the first harvest, resulting in plant death.
Management tips Nos. 3 and 4: Tiling fields to improve drainage can be beneficial by removing excess moisture that contributes to heaving. Leaving some crop residue on the soil surface can reduce winter heaving by insulating the soil and reducing the number of times freezing and thawing occurs.
Warm midwinter periods followed by sudden cold temperatures. Plants can lose their ability to survive cold temperatures if there is a midwinter warm period, where plants break dormancy. This break in dormancy followed by a sudden change back to cold temperatures can result in winter injury.
Management tip No. 5: Very little can be done to prevent this type of winterkill. However, more dormant varieties are less likely to break dormancy during these warm midwinter periods. Therefore, they are less likely to be injured by a sudden drop in temperature.
Crop management during the growing season
Any practice that increases plant stress can affect alfalfa’s ability to survive harsh winter conditions. Here is a list of stress factors and some management tips during the growing season to minimize the risk of winter injury and/or winterkill.
Untimely last cut. The timing of the last cut is probably the most important factor in preventing winterkill. An untimely last cut can result in plants with low root reserves, which are more likely to winterkill.
Management tip No. 6: Fields need to either be cut early enough in fall to regrow and replenish root reserves before winter, or late enough that there is no short regrowth that depletes root reserves. Timing the last cut of the season should occur six to eight weeks before the killing frost date (24 degrees) to allow adequate regrowth time to recharge root reserves.
Aggressive cutting schedules. Beware the consequences of aggressive cutting schedules. Frequent short harvest intervals during the growing season without a period of full recovery, especially on the last cut before winter, can be detrimental to alfalfa fields’ winter survival.
Management tip No. 7: If several harvest intervals have been aggressive, such as cutting at pre-bud or bud during the season, allow a later cut to reach about 50% flowering before winter — preferably the last cutting. Avoid aggressively cutting older stands in fall.
Consider plant stress factors. Plants under stress are more likely to winterkill.
Management tips Nos. 8, 9 and 10: Maintain a soil pH in the range of 6.5 to 7.5 by adding lime if needed. Maintain soil fertility. Potassium is especially important for winter survival and acts as a plant’s natural antifreeze within the cells. High exchangeable soil potassium (greater than 160 parts per million) reduces risk of winterkill. Control pests during the growing season as needed to minimize plant stress.
What you do during the growing season can greatly affect the ability of your alfalfa to avoid winter injury. Properly managing plant stresses and alfalfa root reserves throughout the season can pay big dividends to producers in the form of improved quality, yield and stand life.
Note that of all the management tips to minimize the risk of winterkill, timing of the last cut is probably the most important factor in preventing it.
Miller is director of product development of Alforex Seeds. A full version of this article appeared in the Aug. 26, 2021, issue of the Midwest Forage Association's Clippings newsletter.