What is a killing freeze for alfalfa? In its simplest form, a killing freeze occurs when temps get cold enough to kill all the top growth on the alfalfa plant -- the plant wilts, turns tannish in color, and leaves fall off.

However, alfalfa tops don't die at any set temperature, points out Bruce Anderson, University of Nebraska-Lincoln agronomy professor.

"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 today in many areas [of Nebraska] even though several hard freezes have occurred," he says.

He says a freeze that actually kills alfalfa tops suddenly is rare. Thus, rather than worry about a killing freeze, he says it might be wiser to think about why producers look for a killing freeze.

"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. So what we really need to consider is when fall harvests are safe," he says.

Experience in the Northern Plains indicates alfalfa that's had at least six weeks of regrowth in mid-October since the previous cutting will have developed enough winter hardiness for all but the most severe winters. 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 isn't likely to jeopardize stand persistence.

"You may not get a true killing freeze very often, but you still can take advantage of what it can cause by monitoring the calendar and your harvest timing," Anderson says.
-- Bruce Anderson, University of Nebraska-Lincoln