Each winter, alfalfa stands are at risk of being injured or killed by harsh winter conditions. Bitter temperatures, sheets of ice and heaving all have the ability to wreak havoc on alfalfa stands.
According to the University of Wisconsin Forage Team, a number of factors affect the likelihood of winter injury in alfalfa stands. Among them are:
- Stand age. Older stands are more likely to winterkill than younger ones.
- Variety. Varieties with superior winter-hardiness ratings and a high disease resistance index are less likely to experience winter injury.
- Soil pH. Stands growing on soil with a pH about 6.6 are less likely to experience winter injury.
- Soil Fertility. Stands with high fertility, particularly potassium, are less likely to experience winter injury than those with low fertility.
- Soil moisture. Alfalfa grown on well-drained soils is less pone to winter injury.
- Fall soil moisture status. As dehydration is the primary means of tolerating freezing temperatures, stands that go into winter with low soil moisture are better able to lose moisture and are less likely to winter kill.
- Cutting Management. Both harvest frequency and timing of fall cutting affect alfalfa winter hardiness. The shorter the interval between cuttings, the greater is the risk of winter injury. Stands in which a last cutting is taken between September 1st and October 15th are at greater risk, as plants are unable to replenish root carbohydrate reserves before winter.
- Snow cover. Snow is an excellent insulator. The figure below shows soils temperatures under 0, 10 cm (4 inches), or 20 cm (8 inches) of snow. Temperature fluctuations are much less under snow cover. As little as 4 inches of snow can result in a 10°F difference in soil temperatures. Stands which have not been cut after September 1st or which have at least 6 inches of stubble left will be able to retain more snow cover and be less susceptible to winter injury.
If you suspect Old Man Winter took a tool on your forage stand, here are a few warning signs to look for:
- Slow Green Up. One of the most evident results of winter injury is that stands are slow to green up. If other fields in the area are starting to grow and yours are still brown, it is time to check those stands for injury or death.
- Asymmetrical Growth. Buds for spring growth are formed during the previous fall. If parts of an alfalfa root are killed and others are not, only the living portion of the crown will give rise to new shoots resulting in a crown with shoots on only one side, or asymmetrical growth.
- Uneven Growth. During winter, some buds on a plant crown may be killed and others may not. The uninjured buds will start growth early while the killed buds must be replaced by new buds formed in spring. This will result in shoots of different height on the same plant, with the shoots from buds formed in spring several inches shorter than the shoots arising from fall buds.
- Root Damage. The best way to diagnose winter injury is by digging up plants (4 to 6 inches deep) and examining roots. Healthy roots should be firm and white in color with little evidence of root rot. Winter killed roots will have a gray, water-soaked appearance early, just after soils thaw. Once water leaves the root, the tissue will become brown, dehydrated and stringy. If the root is soft and water can be easily squeezed from it, or is brown, dry and stringy, it is most likely winter killed. Also, if 50% or more of the root is blackened from root rot, the plant will most likely die during spring green up or later in the year.
If winter injury or kill is present, the University of Wisconsin provides the following steps to take:
- Determine yield potential. Potential yield of an alfalfa stand may be estimated by determining the number of stems in a square foot area. Once stem number is determined use the following formula to calculate yield potential of that stand:
Yield (tons/acre) = (Stems/ft2 x 0.1) + 0.38
- Allow alfalfa plants to mature longer before cutting. Allowing plants to mature to early, mid or even full bloom will help the plants restore needed carbohydrates for subsequent production. How long and during which cutting depends on the extent of winter injury.
- Increase cutting height. This is particularly important when allowing plants to flower before cutting. As this time, new shoots may be developing at the bas of the plants. It is important to not remove these shoots as it will further weaken the plant to have to produce new one.
- Fertilize. It is particularly important that winter injured stands have adequate fertility. Soil test and apply needed fertilizer prior to first cutting if possible.
- Control Weeds. Herbicide applications to control weed competition will help the stand by eliminating weeds that compete for moisture, light and nutrients.
- No Late Fall Cutting. Do not cut winter injured stands after Sept 1st to allow for the buildup of food reserves prior to winter unless the intent is to plow down the stand.
