Farm Progress

Alfalfa roots offer clues to the extent of winterkill.

March 31, 2017

3 Min Read
THIN STANDS: Yield losses in this thin stand of alfalfa warrant replanting.

How much alfalfa died this winter?

“The main factor that kills alfalfa is ice,” says Marisol Berti, North Dakota State University forage specialist. “It chokes the plants, taking the oxygen away. We did have a lot of ice. When I took off from Fargo recently, most fields looked like ice-skating rinks, not good, but we won’t know the extent of the damage yet.”

In a few weeks, the roots will offer clues to how much winterkill has occurred, says Karla A. Hernandez, South Dakota State University Extension forages field specialist.

Winterkilled roots will have a gray appearance. If the root is soft and water can be easily squeezed from it, or it has a brown color, it is a possible sign of a winter cold-related death.

Asymmetrical growth and uneven growth are also indicators of winter injury. Compare the shoots on the same plant, and if you notice that one set of shoots seems to be drastically outperforming another in terms of growth, it could be that winter cold damaged the bud structure of your plants, she says.

Top 6 risk factors
The top six factors that Hernandez says could affect alfalfa winterkill are:

• Stand age. Older stands are more likely to winter-kill than younger plants.

• Soil pH. Soils with a pH above 6.6 are less likely to experience winter injury.

• Soil fertility. Stands planted in soils with high natural fertility are less likely to experience winter injury than those with low fertility.

• Variety. Alfalfa varieties with superior winter-hardiness ratings and a high disease resistance index are less likely to experience winter injury.

• Cutting management. Harvest frequency and timing of fall cutting will affect alfalfa winter-hardiness. The general trend shows that the shorter the interval between cuttings during the growing season, the greater risk of winter injury. An aggressive harvest schedule prevents the plant from storing carbohydrates, which it will need to maintain health as it regrows, in its root structure. Stands in which last cutting is taken between Sept. 1 and the middle of October are at greatest risk, as plants did not have enough time to accumulate adequate carbohydrate levels in the root system before winter.

• Snow cover. Snow provides insulation to the plants and the crown. The crucial temperature region is 2 to 4 inches below the soil surface where a large part of the root structure is located. Stands that have at least 6 inches of stubble left will be able to retain more snow cover and be less susceptible to winter injury.

Stem density guidelines
Wait until your crop is growing vigorously to take your stand counts. The recommended time is typically when alfalfa is 2 to 4 inches tall. To get an accurate assessment, throw an alfalfa square and count the number of plants and stems that fall within the border of the square.

If your field is relatively consistent, take one count per acre. Otherwise take two per acre. Remember to only count plants that are healthy and will survive. Any winterkilled plants should not be counted so as to avoid an inaccurate reading and incorrect plan for your crop moving forward. A good rule of thumb is that plants with only one to three stems growing out of one side should not be counted.

If you have more than 55 stems per square foot, stem density won’t limit yield.

Some yield reduction can be expected at 40 to 55 stems per square foot. You can keep the stand, but plan to replace the stand next year.

If you have less than 40 stems per square foot, you can expect significant yield reduction. Eliminate or replace the stand as soon as possible.

Winter injury management
If you do need to keep winter-injured crops for the coming season, you’ll want to be sure to pay special attention to them.

• Delay first cutting until first flower, giving the crop more time to recover.

• Top-dress damaged stands with potash and/or phosphorus according to soil test.

• Do not treat winter-injured crops with spring herbicides that are known to stress or stunt plant growth.

Not all crop injury can be avoided, but taking these steps to assess damage and establish a plan to move forward will help you get the most out of your crop heading into summer.

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