Allowing for alfalfa to winterize before dormancy is a key factor in preventing winter kill across a stand. Traditionally, the recommendation has been to time the last cutting for roughly six weeks before the first frost. At a minimum, plants need three uninterrupted weeks to complete the transfer of carbohydrates to the crown and roots, which is the winterization process. The additional three weeks give us a cushion in case of an early frost.
While this general guideline has proven its worth over the years, many producers would love to have a more accurate method to time last cuttings. One way to narrow the no-harvest window down is by utilizing growing degree days (GDDs). Researchers at the University of Wisconsin calculated winterkill risk by looking at GDDs at a base of 41 F until a killing frost of 25 F occurred. They noticed two GDD levels of importance for alfalfa stands: 500 GDDs and 200 GDDs.
By providing at least 500 base 41 F GDDs after harvest, research showed that there was sufficient time for alfalfa to winterize. If harvest occurred with fewer than 200 GDDs left, alfalfa plants did not have sufficient time to regrow, which depleted carbohydrate reserves to a level that would negatively impact winterization.
While other factors like ground cover and stress of the stand over the course of the year need to be considered in the decision for a late cutting, this gives us a more accurate calendar point to shoot for if forage is needed.
A tool like the High Plains Regional Climate Center’s CLIMOD can be used to look at the GDDs of past years and help us decide if we can accumulate 500 GDDs or fewer than 200 GDDs going forward. When we are between the two, wait to cut, but once the chances of surpassing 200 GDDs are low and extra hay is needed, it’s probably safe to take that final cutting. — Ben Beckman, Nebraska Extension educator
