Evaluating Alfalfa Winter Kill

As alfalfa breaks dormancy, it is important to evaluate the condition of your post-winter alfalfa stand. You can start to evaluate as soon as the frost is out of the ground and monitor all the way to spring green-up. A few things to consider when looking for winter injury or kill include living individual plants, plant health and stand density.

Evaluation of living plants: Start the evaluation process when the frost leaves the ground and through spring green-up. It is recommended to dig up a few plants throughout the field approximately 4 to 6 inches deep to evaluate the condition of the tap root.1 If the tap root is turgid or firm and white in color, it is most likely alive and healthy. Signs of winter kill include roots that appear grayish, have a water-soaked appearance just after the soil thaws, appear ropey and become dehydrated.

Evaluation of plant health: In preparation for spring growth, alfalfa plants form buds in the fall. If these buds don’t survive the winter, the plants must develop new springtime buds. This can result in shoots of different heights on the same plant, longer shoots from buds formed in the fall, and buds that are several inches shorter from spring buds. Forming new buds can delay growth and reduce yield for the first cutting before plants can recover and produce for subsequent cuttings. Management factors to improve plant health over the winter could include pH, soil fertility management and use of winter hardy varieties.

Evaluation of stand: The final step to evaluating winter injury or kill is to determine the stem density in your stand. An adequate stem density such as greater than 55 stems/square foot can still produce well after suffering some winter losses. It is likely some reduction in yield can occur if a density of fewer than 40 stems/square foot occurs.  

Table 1. Using stem density to evaluate alfalfa stands.1

Density (stems/ft2)

Action

Over 55

density is not limiting yield

40-55

density limiting yield potential

Under 40

density severely limiting yield

What causes winter kill or injury? Alfalfa plants, through biological changes, prepare for winter when nighttime temperatures drop below 40˚ F. Biological changes include:

·      Cell membrane adaption to allow the retention of fluids

·      The cellular freezing point is lowered by the accumulation of sugars

·      The loss of cellular water so they can tolerate freezing

These changes allow alfalfa plants to tolerate temperatures of 5˚ F to 15˚ F (depending on variety).2 When temperatures fall below a tolerable range, the ice crystals that form within frozen plant cells can puncture the cell walls and cause them to leak upon thawing. Another form of winter injury or death can occur when large ice sheets develop over the plants, causing toxic metabolites to accumulate.2

Many factors can affect the probability of winter kill/injury on alfalfa stands such as stand age, variety, soil pH and moisture. It is important to evaluate stands in the spring to determine if winter kill or injury can be yield limiting.

Sources:
 

1Cosgrove, D. and Undersander, D. 2003. Evaluating and managing alfalfa stands for winter injury. Vol 5: No. 8. Team Forage. University of Wisconsin-Madison. https://fyi.extension.wisc.edu/. 

2Undersander, D., Grau, C., Cosgrove, D., Doll, J., and Martin, N. 2011. Alfalfa stand assessment: Is this stand good enough to keep? A3620. Team Forage. University of Wisconsin-Madison. https://fyi.extension.wisc.edu/. 

Web sites verified 12/20/19    

Performance may vary, from location to location and from year to year, as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and years whenever possible and should consider the impacts of these conditions on the grower’s fields.

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