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Assess alfalfa stands now

Paul Peterson, Extension Forage Agronomist

April 14, 2007

With April comes the anticipation of how alfalfa stands will look as they green up. Though much of the region was without snow cover for extended periods last winter, temperatures during those times generally didn’t dip low enough to be of major concern. But winter injury to alfalfa is difficult to predict because it is influenced by so many interacting factors. Fields that look okay on a drive-by during initial green-up can fool you. While it’s easy to recognize the potential yield loss due to dead plants, it’s less easy to spot reduced yield potential associated with winter injury. So regardless of what winter was like, it’s best to take a close look at your alfalfa fields now.

Diagnosing Winter Injury BEFORE Green-up

Dig up a few plants in different areas of the fields, cutting the roots off at 4-6” below the soil surface. Inspect the crown, crown buds, and root. Split the crown and root lengthwise.

Healthy roots are firm and white inside, with little evidence of root rot. Winter-injured roots are gray and water-soaked or brown due to root rots. If the crown/root is soft and water-soaked, it is most likely dead. If the crown/root is firm but showing some rot, it may produce this year, depending upon the severity of winter injury. If over half of the root is damaged, the plant will likely die. Crown buds are the most cold-tolerant belowground structures of alfalfa; so if they appear gray, withered, and/or water-soaked, the plant may be dead.

Diagnosing Winter Injury AFTER Green-up

If other alfalfa fields in your area are starting to grow and yours are still brown, check your stand for injury or death. During winter, some crown buds may be killed and others may not. Uninjured buds will start growth early. Killed buds must be replaced by new buds formed in spring, resulting in shoots of different height on the same plant. Once the alfalfa stand is at least 6 inches tall, determine the average number of stems per square foot in at least five representative areas per field. Wisconsin research has shown that stem density is a far better determinant of yield potential than plant number since plants that are injured can still survive but have significantly reduced stem number and, thus, yield potential. Stands with an average stem density >55 stems/ft2 are in good shape and have potential for continued high production. Stands with <40 stems/ft2 should be replaced or inter-seeded. Stands with between 40 and 55 stems/ft2 are probably in their last year of production.

Factors Affecting Alfalfa Winter Survival

  • Variety Selection. Modern varieties with high levels of disease resistance and good winter hardiness are less susceptible to winter injury.
  • Stand Age. Because of the cumulative stress of plant diseases and physical injury, older stands are more susceptible to winter injury.
  • Soil Fertility. Soil potassium is important for enhancing alfalfa tolerance to winter injury, especially with aggressive harvest schedules. Plants stressed by low pH or deficiencies of other nutrients are also more susceptible to winter stress.
  • Cold Temperatures and No Snow Cover. Michigan research demonstrated that as little as 4” of snow cover was sufficient to protect alfalfa varieties from winter injury due to cold exposure.  Anywhere from 5 to 15°F is often cited as the critical low soil temperature for alfalfa survival, but this is far from an absolute range.
  • Fluctuating Winter-Spring Temperatures. When alfalfa breaks dormancy, it loses its cold temperature tolerance. After that, if temperatures drop to frigid levels again, these plants are particularly sensitive, and can experience “winter” injury or even death.
  • Soil Moisture. Low fall soil moisture favors good winter hardening. Wet soils favor ice sheeting and heaving.
  • Cutting Management During 2006. Stands cut more frequently are generally more susceptible to winter injury than those cut less frequently. Also, stands cut during the “fall critical period” (September) are at greater risk. Typically, taking a fall cutting of healthy stands of good varieties in or after mid-October is relatively safe; but during open winters, stands that go into winter with minimal residue can be at greater risk of winter injury.

So, if after checking your alfalfa fields you decide to terminate a stand either now or after first cutting, follow the terminated stand with a crop that can take advantage of all that free fixed N left behind by the alfalfa (e.g., corn, small grains, summer annuals, grass forage). Establish new thick stands of proven varieties in fields with adequate fertility and drainage for alfalfa. Consider seeding with a nurse crop to boost seeding-year yield and/or a perennial grass or red clover to provide some insurance against future injury.  Perennial grasses have lots of digestible fiber and red clover will improve silage quality.

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