The decision of when to take the last harvest of alfalfa to insure good winter survival and yield potential for the following year can be boiled down to two choices. Either cut early enough in the fall to permit alfalfa to regrow and replenish carbohydrate root reserves or cut late enough so that alfalfa does not regrow and use carbohydrate root reserves. Matching calendar dates to those choices is tricky and is where uncertainty and talk of probability enters. In addition, there are factors such as previous cutting management, age of stand, soil fertility, variety, and soil moisture that affect the level of risk with fall cutting.
For those who are risk adverse, following last cutting date recommendations offers the highest probability of promoting alfalfa stand winter survival and vigorous green up and growth the following spring. The recommendation in the 15th edition of the Ohio Agronomy Guide is to complete the last regular harvest of alfalfa by September 7 in northern Ohio, September 12 in central Ohio and by September 15 in southern Ohio. The corollary is to delay final harvest until a killing frost (25F for several hours) has occurred.
Another approach to fall harvest management uses growing degree-days (GDD) rather than calendar dates. Work done by Belanger et al. and published in 1998 in the Canadian Journal of Plant Science, indicates that alfalfa needs 500 GDD (based on degrees Celsius and base 5 C for alfalfa growth) between a late season cut and a killing frost to generate sufficient regrowth to provide good winter survival and yield potential for the following year. With regard to taking a late, no regrowth harvest, Dan Undersander, University of Wisconsin Extension forage specialist, now retired, in a 2012 article entitled “Late Summer Cutting Management of Alfalfa” wrote “…we do not need to wait for a killing frost to take the last cutting. We must only wait until it is so cool that little or no regrowth will occur. Thus harvesting in late fall, when less than 200 GDD will accumulate, minimizes winter injury…” The period between an accumulation of more than 200 GDD and less than 500 GDD is a no cut zone. This GDD approach provides flexibility in date of last harvest, but involves more risk because the grower must predict or consider probability of either accumulating enough GDD or GDD not accumulating. Historic weather data, like that available from the OARDC weather stations (http://www.oardc.ohio-state.edu/weather1/), is useful to calculate those probabilities.
Based on the last 5 years (2013-2017) of weather data at Wooster, the date of a killing frost (25 F for several hours) ranged from November 3 to 22. The no cut zone of 500 to 200 GDD prior to the killing frost was September 17 to October 13 for three of the five years, but was September 4 to 30 in 2014 and September 10 to October 4 in 2013.
Previous harvest management is always a part of the risk equation and assessment of the probability of a fall harvest affecting stand survival and health. The cutting frequency during the growing season affects the energy status of the plant going into the fall. Frequent cutting (30 day intervals or less) results in the plant never reaching full energy reserve status during the growing season. A fifth cutting taken in the fall carries more risk than taking a fourth cutting, which is more risk than a third cutting during the fall.
Variety is a factor. Today’s top varieties are selected to better withstand intensive cutting schedules resulting in reduced fall cutting risk. Alfalfa varieties with high disease resistance and good levels of winter hardiness will be more tolerant of a fall cutting. Adequate fertility, especially soil potassium levels, and a soil pH near 6.8 will improve plant health and increase tolerance to fall cutting. Stands under 3 years of age are more tolerant of fall cuttings than older stands where root and crown diseases are setting in.
Consider soil drainage and soil moisture. High soil moisture content soils slow down the cold hardening/fall dormancy process, increasing the risk of winter injury. Alfalfa stands on well-drained soils tolerate later fall cuttings better than alfalfa on moderately or poorly drained soils. Removing the top growth of alfalfa plants on heavy soils and poorly drained soils going into the winter increases the risk of damage from spring frost heaving, especially with higher clay content soils.
Finally, consider the economics of a fall harvest. Often the height of the alfalfa is deceptive as an indicator of tonnage. The resulting windrow after cutting is often small or sparse. Thus, the cost of mechanically harvesting is high on a per ton basis.
Fall cutting risk can be reduced but not eliminated. Nature bats last and alfalfa stand health and survival will suffer in years with early freezes, open and very cold winters, early springs with ice, and/or extreme temperature variations. Who needs Las Vegas? Farmers are inherently gamblers and late season alfalfa management is another gamble.