By Mark Sulc and Rory Lewandowski, CCA, Ohio State University Extension
The best time to take a last harvest of forages is this week and next in Ohio, for the least risk to the long-term health of the stand. This is especially true for alfalfa and other legumes that need the fall period to replenish carbohydrate and protein reserves in the taproots that are used for winter survival and regrowth next spring. This fall rest period is particularly important this year, because our surviving stands have suffered a great deal of wet soil stress this year. Adding the stress of fall cutting will be like adding insult to injury, in our opinion carrying a higher degree of risk this year than normal.
Unfortunately, many fields this year may not be at a reasonable harvest stage during the next two weeks, because the rainy weather early this season blew apart our normal harvest schedules. Many producers are faced with the choice between harvesting lower yields at a less mature stage now or waiting to harvest when yields will be higher. Like most farming decisions, there are trade-offs and risk factors to consider when making a fall legume harvest. This article reviews best management practices under the conditions we face this year.
The decision of when to take the last harvest of alfalfa to ensure good winter survival and yield potential for the following year can be boiled down to two choices: 1) cut early enough in the fall (generally early September) to permit alfalfa to regrow and replenish carbohydrate root reserves, or 2) cut late enough so that alfalfa does not regrow and use up root reserves for that limited regrowth prior to winter dormancy. Cutting in between those times means more risk to the stand; however, there are factors such as previous cutting management, age of stand, soil fertility, variety, and soil moisture that affect the level of that risk.
For those who are risk adverse, following the last cutting date recommendations offers the highest probability of promoting 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 fall harvest, Dan Undersander, University of Wisconsin Extension retired forage specialist, wrote in a 2012 article “…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 period (GDD calculated from degrees Celsius scale with base 5C). This GDD approach provides flexibility in date of last harvest, but it 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 this GDD approach, we studied 5 years (2013-2017) of weather data at Wooster, OH. 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. So, the period of most risk for cutting alfalfa based on this GDD criterion agrees well with past recommendations to not cut alfalfa from early September to mid-October. Therefore, cutting in late October prior to a true killing frost of forage legumes, is likely to not result in little to no regrowth and no significant depletion of root reserves. However, there is still the risk of frost heaving with the late removal of forage cover (discussed more below).
Previous harvest management should be 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 short regrowth period just prior to the fall harvest can be especially risky if that fall harvest occurs between mid-September and early October, because the regrowth uses root reserves and there won’t be enough growing weather remaining for the plants to accumulate a high level of root reserves before cold weather shuts down the plants. This lower root reserve status may limit winter survival and spring regrowth, depending on the winter and early spring growing conditions.
Variety selection may also affect the risk assessment of fall cutting. Today’s top varieties have genetics selected to better withstand intensive cutting schedules. 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 generally more tolerant of fall cuttings than older stands where root and crown diseases are setting in. However, you have more productive stand life to lose if younger stands are harmed by fall cutting.
Consider soil drainage and soil moisture. High soil moisture content slows 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. But a word of CAUTION – Removing the top growth of alfalfa plants going into the winter on heavy soils and poorly drained soils increases the risk of damage from spring frost heaving, which is a significant risk on many Ohio soils with higher clay content. This would be a concern when cutting very late after the 200 GDD threshold date.
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 as we saw this spring and alfalfa stand health and survival will suffer when early freezes, open and very cold winters, early springs with ice, and/or extreme rainfall and temperature variations occur. Unfortunately, forage supplies are short this year, and some producers may be forced into taking more risk than they would like to take with fall cutting. But if at all possible, we urge producers to observe the fall rest period for forage legumes this year.