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Photo courtesy of Minnesota Extension

Evaluate winter injury in alfalfa stands

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ST. PAUL, Minn. -- They may look fine from the road, but winter may have done some damage to your alfalfa stands. You'll want to get a close-up look at any plant injury, so you can make good decisions going forward into the growing season.

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Alfalfa will likely break dormancy in the next week to 10 days with forecasted temperatures 10 to 20 degrees warmer than normal, according to University of Minnesota Extension climatologist Mark Seeley.

A number of factors affect the likelihood of winter injury in alfalfa stands, including stand age, variety, soil pH, soil fertility and cutting management. Snow is an excellent insulator and temperature fluctuations are much less under snow cover. The lack of snow this winter is concerning, but, the relatively mild temperatures have helped reduce injury potential.

Alfalfa has a greater potential for winter injury or kill if the temperature near the crowns falls below 15 degrees. Spring-like temperatures in January and February followed by brief cold spells increased the injury potential. Current soil temperatures range from 20 to 30 degrees and will increase quickly.

Winter injury is difficult to predict and is usually a result of a combination of several environmental and plant stress factors, including cold soil temperatures, lack of snow cover, and alternating warm and freezing temperatures. Newer alfalfa stands that are well-fertilized, and varieties with superior winter-hardiness, are less susceptible.

Diagnose winter injury by digging up plants and examining roots after the spring thaw. Healthy roots are firm and white in color. Roots with winter injury are gray and appear water-soaked. If 50 percent or more of the root appears injured, the plant will most likely die during spring green-up or later in the year.

To estimate yield potential in injured stands, count the number of stems in a square-foot area. If more than 55 stems are found, yield is not likely to be affected; however, if fewer than 40 steams are found, yield will likely be severely limited.

Winter-injured stands require different management than healthy stands. If you decide to keep an injured stand, allow alfalfa plants to mature longer before cutting, increase cutting height, ensure fertility is adequate, control weeds, and do not take a late-fall cutting.

Krishona Martinson is a forage crops educator and an equine specialist with University of Minnesota Extension. Craig Sheaffer is a University of Minnesota professor in the Department of Agronomy and Plant Genetics.Media Contact: Catherine Dehdashti, U of M Extension, (612) 625-0237, ced@umn.edu

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