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Extension > Local Extension Offices > Meeker > County Agriculture Educator > Articles > Checking Alfalfa for Winter Injury

Checking Alfalfa for Winter Injury

Source: Dave Nicolai, Extension Educator, Crops, UMN Extension

Alfalfa had started to green up and grow in many areas of Minnesota. When growth begins again, stands need to be evaluated soon. Alfalfa usually comes through winter in good shape but this has been an unusual winter.  The lack of snow cover during cold temperatures this winter may have permitted cold injury or more likely, enabled dry winter winds to dry out and kill some exposed plants.

When assessing your fields, check plant densities in several areas when the early shoots are 4 to 6 inches tall.  Since some shoots begin growing later than others, stands with enough plants but slightly low shoot density may be all right, especially if shoot height and distribution is fairly uniform.  But, if plant density is low, or shoot growth is not uniform, yields probably will be lowered.

Recently Dan Martens, who is a local extension educator in Sterns, Morrison and Benton counties, published some guidelines for evaluating winter injury in alfalfa fields in central Minnesota which is excellent information for alfalfa growers to utilize when evaluating their own alfalfa stands:

“There is probably a lot of variability around the area with how soils are warming up and drying out based on soil conditions, old crop residue, north/south slopes, low spots, varieties and other factors. Be thoughtful and patient. Sometimes roots turn soft and decay and dead plants are quite obvious. Sometimes alfalfa will grow to be 4 to 6 inches and before they give up. Sometimes fall growth buds were damaged and it takes an extra time to get up and going. Plants that are 3 to 4 years can have a brown core with enough firm tissue around the core to grow fine. This can be normal wear and tear from harvest, wheel traffic and other damage. The dark brown core you see now is NOT from injury during this past winter.”

For alfalfa that was seeded a year ago we might like to have 15 to 25 plants per square foot this spring. For older stands we might like to have 4 to 6 plants per square foot for good yield potential. Obviously this varies a lot with how well plants develop.

Stem counts might be more useful in the spring, if you can wait to make a decision long enough for stems to develop well. They don’t always start at the same time. A common scale for stem counts is that greater than 55 stems per square foot could have 100% yield potential; 45 to 50 stems at 80 to 90%; 30 to 40 stems at 50 to 70%; and less than 30 stems at less than 50%. A chart in the North Central Alfalfa Management Guide shows that alfalfa with 55 stems per square foot might average 5.5 tons per acre; at 40 stems 4.5 tons; at 30 stems 3.5 tons. We tend to lose about one tenth of a ton in yield potential for every shoot below these numbers.

For counting purposes, a square foot is obviously12”x12”. A circle of stiff wire that is 27 inches long makes a square foot. Add what you need to tie the ends. A square 17”x17” is 2 square feet. A circle with a circumference of 60 inches is close to 2 square feet.

Internet users can find more University of Minnesota Extension information concerning the diagnosis of alfalfa winter injury by doing a search for z.umn.edu/alfalfawinterinjury.

Contacts

David Nicolai
Extension Educator, Crops & Institute for Ag Prof Coord
(651) 480-7706
nico0071@umn.edu
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