Diagnosing and managing winter injury in alfalfa -

Diagnosing winter injury

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.

Dan Undersander

Forage Professor Emeritus / University of Wisconsin

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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 injured 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 problems

Probably the best way to diagnose winter injury is by digging up plants and examining roots. Healthy roots should be firm and white in color with little evidence of root rot. Winter-injured roots have a gray, water soaked appearance or a brown discoloration due to root rot. If the root is soft and water can be easily squeezed from the root, it is most likely winter killed. If the root is firm but showing signs of rot, it may still produce, depending on the extent of injury.

If over 50 percent of the root is damaged, the plant will most likely die that year. If less than 50 percent is injured, the plant will likely survive for one or maybe two years, depending on management and the subsequent winter. Table 1 below may be helpful in determining the likelihood for survival into the next season.

Winter injury Table 1

Managing winter-injured stands

Winter-injured stands require different management than healthy stands if they are to stay in production for one or more seasons. If winter injury is evident, consider the following:

Determine yield potential
Potential yield of an alfalfa stand may be estimated by determining the number of stems in a square foot area. Once the 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
For example, an alfalfa stand with 50 stems per square foot would have a yield potential of 5.38. Remember, this is potential yield. Soil factors, nutrient deficiency, insects, diseases and many other things may affect the actual yield.

Use Table 2 below, in conjunction with Table 1 above, to aid in deciding whether to keep an existing, winter-injured stand.

Winter injury Table 2

Allow 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 the winter injury. For severely injured stands, allow plants to go to nearly full bloom in the first cut and to early flower in subsequent cuttings. This will give these stands the best chance at survival. Stands with less injury could be harvested somewhat earlier depending on the extent of the injury. Stands with only mild injury could be allowed to go to 10 to 25 percent bloom at some time during the season. It may be best to choose the second or third cutting with these stands, as the first crop is usually largest.

Increase cutting height

This is particularly important when allowing plants to flower before cutting. Now, new shoots may be developing at the base of the plants. It is important to not remove these shoots, as it will further weaken the plant to have to produce new ones.

Fertilize

It is particularly important that winter-injured stands have adequate fertility. Soil test and apply needed fertilizer prior to the first cutting if possible.