Timing Last Alfalfa Harvest With GDD

Allowing for alfalfa to winterize before dormancy is a key factor preventing winter kill across a stand.  Traditional practice is to time the last cutting for roughly 6 weeks before the first frost. This gives plants time to fill up root reserves and gives a bit of wiggle room in the case of an early cold snap.

While this general guideline has proven its worth over the years, many producers would love to have a bit more accurate method to time last cuttings.  One way to narrow the no-harvest window down is by utilizing growing degree days (GDD).  Work from Dr. Dan Undersander with the University of Wisconsin calculated winterkill risk looking at GDD at a base 41°F accumulating until a killing frost of 24°F.  The two GDD levels of importance for alfalfa stands were 500 and 200. 

By providing at least 500 base 41°F GDD after harvest, research trials showed that there was sufficient time for alfalfa to winterize.  If harvest occurred with under 200 GDD left, alfalfa plants did now have sufficient time to regrow and deplete carbohydrate reserves to a level that would negatively impact winterization. 

While GDD can help a lot with understanding the risk of a late cutting, other factors need to be considered as well.  Stress of the stand over the course of the year, from drought, old age, pest damage, or an intensive cutting schedule, will mean the stand needs more time in the fall to recover and store up reserves. We also need to keep in mind that late cuttings may have a difficult time drying down in a timely fashion.  Finally, regrowth on a late cutting may be minimal, meaning the potential of catching and holding snow over the course of the winter may be reduced.

Right now we are sitting right on the edge of our 500 GDD threshold.  To decide where you stand, the High Plains RCC CLIMOD can be used to look at past years GDD. You can reach that resource at climod.unl.edu. Select Daily Degree Days on the left side menu, change the base to 41 (not base 50 you would use with corn), select your date range, and pick a station in your area. Remember this uses historical data, so you’ll have to use date ranges from past years you feel like match our current circumstances.

The final piece of this puzzle is figuring a freeze date to calculate to.  The National Weather Service in Valley has average freeze date maps at weather.gov/oax/freeze.  If you look at the map labeled First Hard Freeze, you can get a date that gets fairly close.  For most of northeast Nebraska, that date falls around the middle of October.

Now, using accumulated 41 GDD for past years and your expected first freeze dates, we can avoid the 500-200 GDD no harvest zone and plan a late alfalfa cutting if needed. 

-Ben Beckman is a beef systems Extension Educator serving the counties of Antelope, Cedar, Knox, Madison and Pierce.  He is based out of the Cedar County Extension office in Hartington.  You can reach him by phone: (402) 254-6821 or email: ben.beckman@unl.edu