What defines a “killing freeze” in alfalfa?

Bruce Anderson, Extension Forage Specialist, University of Nebraska

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