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What defines a “killing freeze” in alfalfa?

Bruce Anderson Published on 16 September 2009

In its simplest form, a killing freeze is when temperatures get cold enough to kill all the top growth on the alfalfa plant.

The plant will wilt, turn a tannish color and its leaves will fall off. However, alfalfa tops don’t die at any set temperature.

In fact, as we get later and later into the fall without a killing freeze, it takes colder and colder temperatures to actually kill alfalfa tops.

That’s why we still see green alfalfa in many areas even after several hard freezes have occurred. In fact, only rarely do we get a freeze that actually kills alfalfa tops suddenly. Instead of worrying about a killing freeze, consider why we look for one.

Once alfalfa tops die, yield no longer increases and winterizing ends. Thus, a killing freeze can signal when we can harvest in the fall without increasing the risk of winter injury.

Experience in our region (Nebraska) shows us that alfalfa that has had at least six weeks of regrowth in mid-October since the previous cutting will have developed enough winterhardiness for all but the most severe winters.

And by mid-October, alfalfa begins to go dormant naturally because of shorter days and cooler temperatures.

As a result, harvest in mid-October or later is not likely to jeopardize stand persistence.  FG

—Excerpts from University of Nebraska Crop Watch Newsletter, November 2007

Bruce Anderson
Extension Forage Specialist
University of Nebraska

 

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