Many factors can contribute to winter damage of an alfalfa stand. All are important to consider when making spring assessments and managing injured stands.
What affects alfalfa's ability to survive winter
A stand’s ability to overwinter starts with proper management decisions, including:
Variety selection. Select varieties specific to your region and climate to ensure the appropriate genetics for winter hardiness and disease resistance.
Proactive fertility management and pest management. Stressed plants are weaker and don’t efficiently synthesize carbohydrates for winter storage.
Harvest event timing, especially in the fall. This can serve to either maintain or deplete root reserves critical for winter survival.
Harvest event timing: Recommendations
No cutting is recommended between Sept. 1 and Oct. 15 (critical harvest period), as this is an important period of carbohydrate accumulation. Critical harvest period dates will vary depending on how far north you are.
Regardless of the date, it’s critical that final harvest occurs four to six weeks before the average date of the first killing frost. Cutting during this period interferes with food reserve accumulation because new growth is produced at the expense of winter reserves.
It’s also recommended to maintain a minimum of 6 inches of standing growth or stubble. This serves as a snow catchment for insulation. While not a controllable factor, droughty fall conditions also reduce the storage of root reserves.
At least 4 inches of snow is considered adequate to insulate the soil and prevent direct freezing damage to alfalfa.
As an example, consider the winter of 2015. With little to no snow cover, soil temperatures (at 2 inches) reported in Waseca dropped as low as 2 degrees Fahrenheit on four days in late February (Figure 1). Temperatures of 15 to 5 at the crown are capable of causing winter injury.
Stressed or vulnerable alfalfa stands may suffer winter injury in similar years, depending on:
Minimum air temperatures.
Degree of fall hardening.
In some years (2013, for example), precipitation and freeze-thaw cycles in late winter and early spring can cause a layer of ice to form at the soil surface. This is known as ice sheeting.
If prolonged by weather, these conditions can deplete soil oxygen and create toxic levels of carbon dioxide, ethanol and methanol beneath the ice.
Alfalfa can endure short periods of ice sheeting. However, periods lasting longer than seven days can cause significant injury and even death.
Warm soil temperatures (more than 40 degrees) can cause alfalfa to deharden and break dormancy. If followed by extreme cold or icy weather, severe winter injury may result.
In one Waseca spring, soil temperatures (at 2 inches) reached 50 to 55 in mid-March, followed by air temperatures as low as 16 later in the month.
Assessing winter injury
Regardless of winter conditions, it’s always recommended to closely assess stand health each spring. Winter injury may not be immediately apparent. You may be able to tell if there’s slow or uneven spring growth, or it could go undetected until after the first cut.
Root color and turgidity
The most direct way to assess spring plant health is root color and turgidity. Dig a few plants from representative areas of the field, and split the taproot down the center as in Figure 2.
Healthy roots will be off-white and turgid (firm and hydrated), as shown on the left. Damaged or winter-killed roots will be dark, dehydrated and ropey, as shown on the right.
For an early assessment of overall stand health and production potential, count the number of stems per square foot:
More than 55 stems: Density isn’t limiting to production.
40 to 55 stems: Some reduction has occurred, but adequate production is still likely.
Less than 40 stems: You may need to consider termination or supplementary options.
If you’re concerned about winter injury, watch for slow or uneven regrowth and closely monitor regrowth following the first cut. Mild winter injury may cause reduced stem count or plant vigor.
Depending on plant health and the severity of the damage, production may decrease throughout the year or recover. Recognize every stand and every field is different and could require specific assessment and management planning.
Managing injured alfalfa stands
As you consider management options, remember injured alfalfa stands can exhibit delayed regrowth, but may be capable of recovering. Be careful not to rush into alternative options if you can maintain the stand for acceptable production.
Even when best management practices (BMP) are observed, there still can be significant alfalfa injury and potential kill. If action is required, carefully consider the cost and expected benefit of alternatives with regard to the situation. You can interseed supplemental forages such as teff, annual ryegrass, and small grains into a thin stand or use it to cover any bad spots.
If a large percentage of the stand has been damaged, it may be more appropriate to terminate and plant silage corn or BMR sorghum instead. Use the decision tree below to determine your next steps and read about other alternative options:
Reviewed in 2018