Protect your alfalfa investment: Management tools that minimize the risk of alfalfa winterkill

As published in of NAFA News, July, 2022

Like any crop, planting an alfalfa field involves a substantial investment. An investment in terms of land preparation, labor, fertilizer, seed, and the expense involved to get the alfalfa stand established and productive. All this comes with the grower’s expectation that this investment will provide forage for 3-5 years. Sometimes a severe winter can result in production losses and in some cases complete stand losses due to winterkill. This can result in lost revenue and subsequent replanting costs to replace the lost production.

Not all alfalfa winterkill can be prevented. However, there are several management tools available to the alfalfa producer. These tools can help reduce the risk of winter injury and/or the severity of the stand loss due to winterkill. The following are some management options to consider when growing alfalfa in production regions that are prone to alfalfa winterkill.

Variety Selection

Selecting an alfalfa variety adapted to your production region is critical. Beyond selecting a variety for disease resistance, pest resistance, and desirable agronomic traits, there are two main varietal factors that should be first considered. These varietal traits are the Fall Dormancy Rating (FDR) and the Winter Survival Rating (WSR). If used together in variety selection they can increase yield potential while reducing the risk of winterkill.

For years producers selected alfalfa varieties for winter survival based solely on their FDR (1-10). The lower the varieties FDR number the less fall growth. This “fall dormancy” protected plant root reserves by allowing the plants to avoid repeated topgrowth losses due to fall freezing. These varieties with lower Fall Dormancy Ratings however tended to have less yield potential in contrast to the higher the FDR varieties which had more growth in the spring and fall.

States in the northern tier of the U.S. or high elevation regions have historically used alfalfa varieties with FDRs of 2, 3, and 4. However, in recent years there has been a trend by producers to plant a higher Fall Dormancy (i.e., FD 4 vs 3 or FD 5 vs 4) to gain yield potential. These higher dormancy varieties are now being selected combining the FDR with a low WSR value of 2.3 or lower. This practice gives some assurance of adequate winter survival and capture higher yields.

 

 

Reducing the risk of Winterkill: New Alfalfa Plantings

Whether you plant alfalfa in the spring or fall there are certain precautions you should take to make sure winter conditions don’t damage your new planting.

Spring planting: Alfalfa is relatively tolerant to cold temperatures as it germinates. Therefore, it is generally considered relatively safe to plant alfalfa any time after the fear of frost has passed. However, as it starts to grow and enters into the second trifoliate leaf stage, the plants tolerance to cold injury declines and is more susceptible to a late spring frost. If freezing damage occurs in a new planting, the damaged field can be replanted without fear of autotoxicity anytime within the first year.

• To minimize the risk of winterkill, spring planted alfalfa should be managed to minimize any stress factors (insects, poor fertility, etc.) during the first growing season.
  • Avoid cutting the first crop early. Allow the first growth to bloom before cutting. This will increase root growth which will contribute to higher root food reserves for optimal plant growth before winter.
• Timing the last cut before winter: Final cut of an alfalfa stand before winter should be 4-6 weeks before a killing frost (<26°F) to ensure adequate root reserves to survive winter conditions.
  • Avoid a late cut that may waste root reserves on short regrowth that freezes.

 

Fall planting: Some regions of the U.S. prefer late summer/early fall plantings of alfalfa. This avoids many annual weed problems and allows the stand to establishment enough in the fall to provide a full production year the following spring.

The main thing to remember about fall plantings is to plant 5-6 weeks before the historical killing frost date: This will allow the plants to have enough growth to minimize the risk of winterkill.

• Plants need to reach a growth stage of 2-3 trifoliate leaves before the killing frost.
• For alfalfa seedlings the killing temperature is four or more hours at 26° F or lower.

 

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Reducing the Risk of Winterkill: Established Alfalfa

We can’t control the weather. Some years the potential for alfalfa winter injury to an established alfalfa stand does exist. Alfalfa under most conditions is a very durable crop. However, under the right conditions can suffer significant winter damage. Often the damage is due to the severity of the winter. In some cases our management decisions and/or harvest schedules can be a contributing factor. You can use some management tips to “Winterize Your Alfalfa” and give it the best chance of surviving harsh winter conditions.

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 during the growing season.

Management Tips for Severe Winter Weather

Four weather related conditions are generally responsible for alfalfa winterkill;

• Cold soil temperatures,
• Ice sheeting,
• Plant heaving, and
• Warm mid-winter periods followed by sudden cold temperatures.

 

The following is a description of those conditions and some management tips that have been shown to minimize the potential winterkill risk related to each factor.

• • Cold soil temperatures: Alfalfa winterkill can occur if soil temperatures (at 2-4 Inches) reach in the range of 12-13 degrees F or lower. Snow cover of at least four 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: Adjust cutting height of the last fall cut to leave 4-6 inches of stubble. The stubble can be very beneficial in collecting adequate snow cover to provide some insulation to the alfalfa plant from the 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. the lack of oxygen under the ice results in plant death. Plant death may occur more often in low lying areas of fields where water can pool and freeze. Management tip: Leave 4-6 inches of stubble on last cut. It has been reported that a high stubble that sticks above an ice sheet can 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 the pushing of a portion of alfalfa root out of the ground. The top 2-4 inches of the crown and root may be exposed above the soil surface in severe cases. Plant death results when these exposed crowns are cut off during the first harvest. Management tip: a. Tilling fields to improve drainage can be beneficial by removing excess moisture that contributes to heaving. b. 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 mid-winter periods followed by sudden cold temperatures: Plants can lose their ability to survive cold temperatures if there is a mid-winter warm period where the alfalfa plant breaks dormancy. This break in dormancy followed by a sudden change back to cold temperatures can result in winter injury. Management tip: Very little can be done to prevent this type of winterkill. More dormant varieties are less likely to break dormancy during these warm Mid-winter periods, and therefore are less likely to be injured by a sudden drop in temperature.

 

Management Tips: During the Growing Season

Any practice that increases plant stress can affect the alfalfa’s ability to survive harsh winter conditions. The following is a list of stress factors and some management tips to minimize the risk of winter injury and/or winterkill.

• 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: Fields need to be cut early enough in the fall to regrow and replenish root reserves before winter. Or late enough where there is no short regrowth that depletes the root reserves. Timing the last cut of the season should occur 6-8 weeks before the killing frost date (<25 degrees F) to allow adequate regrowth time to recharge the root reserves.
• 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 the alfalfa fields winter survival. Management tip: If several harvest intervals have been aggressive (pre-bud or bud) during the season, it is best to allow a later cut to reach ~50% flowering before winter, preferably the last cut. Avoid cutting older stands aggressively in the fall.
• Plant stress factors: Plants under stress are more likely to winterkill. Management tips: a. Maintain a soil pH in the range of 6.5-7.5 by adding lime if needed. b. 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 ppm) reduces risk of winterkill. c. Control pests during the growing season as needed to minimize plant stress.

 

Variety selection and what you do during the growing season can greatly affect the ability of your alfalfa stand 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. The timing of the last cut is probably the most important factor in preventing winterkill.

Additional Resources:

• Evaluating Alfalfa Stands for Winter Injury & Winterkill
• Fall Management to Reduce Alfalfa Winterkill
• High Yield Potential Alfalfa with Less Winterkill