Alfalfa assessment: Factors leading to winter injury
A number of factors can contribute to winter damage of an alfalfa stand. These are all important to consider as spring assessments are made. The following elements may all play a role in winter survivability.
The ability of a stand to overwinter starts with proper management decisions. Varieties must be selected by regional and climatic specificity to ensure the appropriate genetics are present for winter hardiness and disease resistance. Proactive fertility management, as well as pest management, are also critical, as stressed plants are weaker and are not efficiently synthesizing carbohydrates for winter storage. Although not a controllable factor, droughty fall conditions also reduce the storage of root reserves.
Additionally, the timing of harvest events, especially in the fall, can serve to either maintain or deplete root reserves critical for winter survival. No cutting is recommended between September 1 and October 15 ("critical harvest period"), as this is a critical period of carbohydrate accumulation. The critical harvest period dates will vary depending on how far north you are located, but regardless of the date, it is critical that final harvest occurs 4 to 6 weeks before the average date of the first killing frost. Cutting during this period interferes with accumulation of food reserves because new growth is produced at the expense of winter reserves. It is also recommended to maintain a minimum 6 inches of standing growth or stubble to serve as a snow catchment for insulation.
Lethal temperatures at crown depth
A minimum 4 inches of snow is considered adequate to insulate the soil and prevent direct freezing damage to alfalfa. With little–to–no snow cover in 2015, soil temperatures (at 2 inches) reported from Waseca dropped as low as 2°F on four days in late February (Figure 1). Temperatures of 15–5°F at the crown are capable of causing winter injury. Depending on regional snow cover and minimum air temperatures, as well as degree of fall hardening, plant health, nutritive status, and soil moisture, stressed or vulnerable alfalfa stands may have suffered this form of winter injury this year.
Spring weather (dehardening)
In some years (2013 for example), precipitation and freeze–thaw cycles in late winter and early spring can lead to formation of a layer of ice at the soil surface, or "ice sheeting". If prolonged by the 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 7 days can cause significant injury and even death.
Breaking dormancy early
Warm soil temperatures (>40°F) can cause alfalfa to deharden and break dormancy. If followed by extreme cold or icy weather, severe winter injury may result. In Waseca, soil temperatures (at 2 inches) reached 50–55°F in mid–March, followed by air temperatures as low as 16°F later in the month.
Assessing winter injury
Regardless of winter conditions, it is always recommended to make a close assessment of stand health each spring. Winter injury may not be immediately apparent. It may be indicated by slow or uneven spring growth, or could go undetected until after the first cut.
The most direct assessment of 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 should be off-white in color and turgid (firm and hydrated as shown on the left). Damaged or winterkilled roots will be dark, dehydrated, and "ropey" (as shown on the right).
Early assessments of overall stand health and production potential are achieved through stem counts per square foot:
- >55 stems per square foot = Density is not limiting to production.
- 40–55 stems per square foot = Some reduction has occurred, but adequate production is still likely.
- <40 stems per square foot = May need to consider termination or supplementary options.
If winter injury is a concern, it is also important to watch for slow or uneven regrowth, and monitor regrowth closely following the first cut. Reduced stem count or plant vigor may occur as a result of mild winter injury. Depending on plant health and severity of the damage, production may decrease throughout the year, or recover. Recognize that every stand and every field is different and could require specific assessment and management planning.
We will make alfalfa survival assessments by the end of April. Currently our alfalfa stands on the St. Paul campus are greening up nicely and given the mild winter, we are not expecting injury in our plots that were managed for maximum persistence. As always, we will know more by May.
Management options: What to do?
As management options are considered, remember that injured alfalfa stands can exhibit delayed regrowth, but may be capable of recovering. Be careful not to rush into alternative options if the stand can be maintained for acceptable production. If action is required, carefully consider the cost and expected benefit of alternatives with regard to the situation. Supplemental forages such as teff, annual ryegrass, and small grains can be interseeded into a thin stand or used to cover the "bad spots" if present. If a large percentage of the stand has been damaged, termination and planting of silage corn or BMR sorghum may be more appropriate. See: Maximizing forage in winter injured and killed stands, Seeding in an Existing Alfalfa Stand (University of Wisconsin), and Emergency forages: Warm season grasses for details on alternative options.
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