Every year I get questions about late-season harvest of forages including alfalfa and other legumes.
According to Forage specialists at Ohio State University, the best time to take a last harvest of alfalfa and other legumes is sometime in early September in Ohio, for the least risk to the long-term health of the stand.
These forages need a fall period of rest to replenish carbohydrate and protein reserves in the taproots that are used for winter survival and regrowth next spring.
Harvest now will be ideal if this is indeed the last harvest of the season. But some growers might try to squeeze out another late cutting, and others have fields that are not quite ready for harvest right now.
There are trade-offs and risk factors to consider when making a fall harvest of forage legumes after the first week of September.
Forage specialists note that the decision of when to take the last harvest with the least risk to the stand 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 prior to winter dormancy.
Cutting in-between those times (mid-September to mid-October) means more risk to the stand. Factors such as previous cutting management, age of stand, soil fertility, variety, and soil moisture affect the level of that risk.
The recommendation in the 15th edition of the Ohio Agronomy Guide is to complete the last regular harvest of alfalfa by Sept. 7 in northern Ohio, Sept. 12 in central Ohio and by Sept. 15 in southern Ohio. The corollary is to delay final harvest until a killing frost (25°F for several hours) has occurred.
According to some research, another approach to fall harvest management uses growing degree-days (GDD) rather than calendar dates. Research conducted in Canada showed that alfalfa needs 500 GDD (based on degrees Celsius and base 5 C for alfalfa growth) between the last cutting and a killing frost to generate sufficient regrowth to provide good winter survival and yield potential the following year.
Researchers at the University of Wisconsin suggest we must wait only 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 accumulations of 200 to less than 500 GDD is a no-cut period. This GDD approach provides more exact timing for the date of last harvest, but it involves more risk because the grower must consider the probability of either accumulating enough GDD for energy replenishment or GDD not accumulating to enough to trigger regrowth that uses up energy reserves.
Historic weather data, like that available from the OSU weather stations (http://www.oardc.ohio-state.edu/weather1/), is useful to calculate those probabilities.
Based on this GDD approach, Ohio State researchers studied 5 years (2013-2017) of weather data at Wooster, Ohio. The date of a killing frost (25°F for several hours) ranged from Nov. 3 to 22. The no cut zone of 500 to 200 GDD prior to the killing frost was Sept. 17 to Oct. 13 for three of the five years, but Sept. 4 to 30 in 2014 and Sept. 10 to Oct. 4 in 2013.
This agrees 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 result in little to no regrowth and no significant depletion of root reserves.
Previous harvest management should be a part of the risk assessment for fall cutting. The cutting frequency of 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, because the regrowth uses root reserves leaving little time for the plant to restore this energy for winter survival and spring regrowth.
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, 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.
Alfalfa on well-drained soils tolerates late fall cuttings better than on moderately or poorly drained soils.
Forage specialists warn, removing the top growth of alfalfa plants going into winter on heavy, poorly drained soils increases the risk of spring frost heaving. This would be a concern when cutting very late after the 200 GDD threshold date.
Finally, consider the economics of a fall harvest. Costs to harvest last cutting tonnage can be very high on a per ton basis.
Does the cost out-weigh the tons harvested and the potential risk to the crop for the future?
There are lots of factors to consider, however forage specialists urge producers to observe the fall rest period for forage legumes.
Tony Nye is the state coordinator for the Ohio State University Extension Small Farm Program and has been an OSU Extension Educator for agriculture and natural resources for over 30 years, currently serving Clinton County and the Miami Valley EERA.