Manage Alfalfa Based on Winter Injury

Our calendars show that spring has arrived.  Although March temperatures have been cool across the Midwest, the snow has finally begun to melt.  Soon plants will emerge from their winter dormancy, so growers will need to visually evaluate grass regrowth and the vitality of their alfalfa plants.

Usually at this time of year, our alfalfa is waking up and ready to stretch from its long winter nap.  But, I believe this season could be quite different as last summer’s drought could have slowed the plant’s ability to adequately build carbohydrates needed to maintain the plant through the winter months.  Patience will certainly be key.

How our unusual winter weather will affect the alfalfa crop is anyone’s guess at this point.  Snow cover and residual vegetative cover help insulate the soil and stabilize soil temperatures.  As temperatures warm through March, plants typically “break dormancy” and regrow.  But, our weather has been anything but typical!  Last Sunday a snow storm hit the nation’s midsection, stretching from D.C. to Denver.  Now 46.6% of the nation is snow covered, according to a WeatherNational blog post.  At this same time last year, only 7.4% of the country had snow cover.

What Causes Winter Injury

courtesy of Joe Murphy, Iowa Soybean Association

Our hope is that, although daytime temperatures fluctuated between above normal to freezing throughout the winter, night-time temps were cold enough to prevent plants from breaking dormancy.  When plants break dormancy early, they’re more susceptible to cold crown temperatures.  We’re also hopeful early season snow melt didn’t submerge alfalfa plants in what then became frozen ponds in low-lying areas.

In addition to temperature and soil moisture, University of Wisconsin extension agronomists say a number of factors affect the likelihood of winter injury in alfalfa stands:

  • Stand age. Older stands are more likely to winterkill.
  • 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 soils with a pH above 6.6 are less likely to experience winter injury.
  • Soil fertility. Stands with high fertility, particularly potassium, are less likely to experience winter injury.
  • Cutting management. Both harvest frequency and timing of fall cutting affect winter hardiness. The shorter the interval between cuttings, the greater is the risk of winter injury.

Evaluating Stands

When evaluating alfalfa in late winter for winter injury, consider both the number of plants per square foot and the age of the stand.  Winter-injured plants are often slow to recover in spring, so avoid making a quick decision to destroy a winter-injured stand.

Stephen K. Barnhart with ISU’s Department of Agronomy suggests waiting until the spring regrowth is about 3 to 4 inches high, and then selecting random stand count sites. Check at least one, 1-square-foot site for every 5 to 10 acres.  Dig up all of the plants in the 1-square-foot area.  Inspect for new growth and the crown and buds to determine if the tissue is still alive. Then count the number of live plants per square foot. Use the following table, developed by Iowa State University, to begin your rating of the stand.