Fall Harvesting Alfalfa

With the unseasonably warm weather we’ve been experiencing, we often get the questions “how late in the fall can I harvest my alfalfa, do I cut now or wait until we receive a killing frost, when is the average first frost date in my area, is plant material that important to help crown health over winter?” These are all important questions to ask your trusted advisors leading into that potential last alfalfa harvest of the year.

Location	Avg Frost Date
Salem, NY	Sept 21^st
Niverville, NY	Oct 5^th
Sprakers, NY	Sept 29^th
Oriskany Falls, NY	Oct 2^nd
Auburn, NY / Bernardston, MA	Oct 5^th

The typical first frost temperature is just below freezing. These first frosts of the fall are the start of leaf decay and trigger the alfalfa crowns to start hardening for dormancy. A killing frost is 24 degrees Fahrenheit or colder for a minimum of two sustained hours. On average, the severe or killing frost date is 14-21 days following the first frost of the fall.

Following the killing frost, the crown will start to go into dormancy. The carbohydrates that are stored in the tap root will help the plant survive the coming winter months and supply the nutrients used for the first growth in the spring until the soil nutrients become readily available. Top growth will no longer grow, and the stems and leaves will turn brown and start to drop.

Fall harvest management for alfalfa or mixed alfalfa/grass stands that are 25-50% alfalfa or more should be based on a 500 or 200 GDD plan (base 41 degrees Fahrenheit). If the alfalfa accumulates 500 GDD before going into winter dormancy, it will have the needed amount of carbohydrates stored within the tap root. If you have missed this window for harvest, but still need the additional forage, there is still an opportunity to harvest without having a large impact on your stand quality. A late fall harvest could be an option. If the plants accumulate less than 200 GDD, the reserves of carbohydrates will not be affected due to the limited growth.

If the harvest is completed and we acquire less than 200 GDD, the only concern for our stand will be the lack of top growth. This growth dies over the winter and lays down on top of the soil surface to allow for a blanket between snowpack and the crown of the alfalfa plants. This layer gives an oxygen base and stops snowpack from suffocating and killing the plants. Fields that are relatively flat and do not have pockets where water will pond or slightly sloped fields will be less likely to have over wintering issues when the top growth is removed during the late fall harvest.

The tricky part is getting the timing right. At this stage of our growing season, we have started into fall and our average first frost dates are well behind us with no frost in the forecast. Because of this, if you harvest now, the likelihood of acquiring more than 200 GDD before the first killing frost is high. With the extended forecast, we are currently accumulating a little over 20 GDD a day. If we cut haylage today, we would need about 25 more continuous days of this weather pattern to achieve the 500 GDD allowing us to replenish the carbohydrates for overwintering. The better plant would be to finish up corn, get cover crops planted, and manure applications and fall tillage completed. When the forecast starts to show less GDD accumulated, that last harvest can be planned.