Check your alfalfa's risk for winter injury

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Will your alfalfa stands make it through the winter? As temperatures drop and the snow begins to fall, this question probably pops into your mind frequently. And, it may make you especially nervous if you didn't assess your alfalfa's risk of winter injury this fall.

September was a good time to assess your risk of alfalfa winter injury, the degree to which all alfalfa buds formed in the fall are able to grow in the spring. However, September may have slipped by, leaving you with alfalfa fields left unchecked. That could set your fields up for winter injury and leave you with poor quality forage and lower yields next spring. Take the following quiz to determine your risk of alfalfa winter injury.

For each section, determine the point value. Then, add up the points from each section to determine your final score. That will help you assess your alfalfa's winter-injury risk.

1. Determine alfalfa stand age
The older an alfalfa stand is, the greater the risk for winter injury.

"A new seeding is the most winterhardy," says Dan Undersander, extension agronomist at the University of Wisconsin. As the stand ages and plants become larger, they become more diseased, and more susceptible to injury. That, in turn, makes the plants more susceptible to winter injury.

  • A. Is your alfalfa stand greater than three years old? (4 pts)
  • B. Is the stand two to three years old? (2 pts)
  • C. Is the stand less than one year old? (1 pt)

2. Determine your alfalfa variety's disease resistance
Make a list of the diseases which have occurred in your fields in the past few years. Then, refer to your seed company's product literature to determine if your alfalfa varieties protect against these diseases. It's important to know this, since alfalfa weakened by disease becomes more susceptible to winter injury.

Most improved alfalfa varieties have high resistance to these five major diseases: bacterial wilt, Phytophthora root rot, Fusarium wilt, Verticillium wilt and anthracnose, Undersander says. However, if your variety doesn't exhibit high resistance, and one of these diseases lurks in your fields, your alfalfa stands a greater chance of suffering winter injury.

  • A. Is your stand moderately resistant to bacterial wilt only? (4 pts)
  • B. Is it moderately resistant to bacterial wilt, plus either Phytophthora root rot, Fusarium wilt, or anthracnose? (3 pts)
  • C. Is the stand moderately resistant to all the diseases mentioned? (1 pt)

3. Assess your alfalfa variety's winterhardiness score
Check your seed company's product literature to determine your alfalfa varieties' level of winterhardiness and compare the score to the "Alfalfa winter survival index" below. If your varieties don't have winter survival scores which match your needs, your alfalfa stands have a greater chance of suffering winter injury. Winter survival indexes will vary, depending on your location and alfalfa management practices. (See the map, "Winterhardiness zones,")

  • A. Does your variety have a winterhardiness score one point higher than that recommended for your region (Such as a score of 3.0 when the suggested winterhardiness score is 2.0)? (3 pts)
  • B. Does your variety have a winterhardiness score equal to that recommended for your region? (2 pts)
  • C. Does your variety have a winterhardiness score one point less than that recommended for your region (Such as a score of 2.0 when the suggested winterhardiness score is 3.0)? (1 pt)

4. Determine soil potassium levels
Potassium influences your alfalfa's risk of winter injury more than any other nutrient, says Neal Martin, extension agronomist, University of Minnesota. In other words, alfalfa with sufficient levels of potassium can withstand winter injury better than alfalfa with a potassium deficiency, explains Martin.

Although soil potassium tests vary between states, they will indicate if the levels of potassium in the soil are sufficient, explains Michael Schmitt, extension soil fertility specialist, University of Minnesota. Generally, the results of a soil potassium test indicate if soil exchangeable potassium levels are low (less than 80 parts per million), medium (81 to 120 ppm), optimum (121 to 160 ppm), or high (greater than 160 ppm).

If your most recent soil test indicates that potassium levels are less than 160 ppm, and you didn't fertilize your alfalfa stands this fall, the likelihood of winter injury increases. Contact your state or county extension office to determine the amount of potassium fertilizer needed on your fields.

  • A. Is your soil exchangeable potassium level low – less than 80 parts per million? (4 pts)
  • B. Is your soil exchangeable potassium level medium –  81 to 120 parts per million? (3 pts
  • C. Is your soil exchangeable potassium level optimum – 121 to 160 parts per million? (1 pt)
  • D. Is your soil exchangeable potassium level high – greater than 160 parts per million? (0 pts)

5. Assess available soil moisture

  • A. Is your soil saturated? (Contains low lying areas where water pools)  (3 pts)
  • B. Is your soil moisture above normal? (Is it wetter than what you normally see at this time of year?)  (2 pts)
  • C. Is your soil moisture dryer than what you normally see at this time of year? (1 pt)

6. Analyze harvest frequency
The more frequently you harvested alfalfa during the summer, the more likely it could experience winter injury. And, if you took your last cutting less than six weeks before the first killing frost in your area, the likelihood of winter injury increases. Give yourself points as follows:

  • 30 days harvest internal:
    Last cutting: Three to six weeks before a killing frost (5 pts)
    After a killing frost  (4 pts)
    Prior to six weeks before a killing frost (3 pts)
    (In other words, a last cutting is taken prior to Sept. 1 in states where the first killing frost occurs six weeks later around Oct. 15.)
  • 30 - 35 days harvest internal:
    Last cutting: Three to six weeks before a killing frost (4 pts)
    After a killing frost (2 pts)
    Prior to six weeks before a killing frost (0 pts)
  • Greater than 35 days harvest interval:
    Last cutting: Three to six weeks before a killing frost (2 pts)
    After a killing frost (0 pts)
    Prior to six weeks before a killing frost (0 pts)

7. Evaluate the stubble
If you harvested alfalfa three to six weeks before a killing frost, without leaving a 6-inch stem and leaf stubble, your risk of winter injury increases. That's because a 6-inch stubble catches snow and acts as insulation for alfalfa.
Did you leave a 6-inch stubble? A. Yes (1 pt) B. No (0 pts)

Now, add up the points from all seven categories and place them here:   

Determine your risk as follows:
Score: 3-7 points. Your risk for winter injury is: Low/below average
Score: 8-12 points. Your risk for winter injury is: Moderate/average
Score: 13-17 points. Your risk for winter injury is:High/above average
Score: 18 or more points. Your risk for winter injury is: Very high/dangerous

Did your alfalfa management practices pass the winter-injury test? If they didn't, you could find yourself with winter-damaged fields in spring. If that's the case, make yourself a note to improve your alfalfa stand management next fall so you don't run into winter-injury problems again.


Alfalfa winter survival index

Score: 1  Winter hardiness rating: Superior
Score: 2  Winter hardiness rating: Very good
Score: 3  Winter hardiness rating: Good
Score: 4  Winter hardiness rating: Adequate
Score: 5  Winter hardiness rating: Low
Score: 6  Winter hardiness rating: None

Environmental factors

Sometimes you don't have any control over winter injury. The following environmental factors contribute to winter injury of alfalfa:

  • Sudden temperature changes and mid-winter thaws.
  • Lack of snow cover causing soil temperatures to drop.
  • Two weeks or more of temperatures below 15 F.
  • Rainfall on frozen ground causes the formation of ice sheets which smother plants.

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