Fall Harvest Considerations for Alfalfa

Rory Lewandowski, OSU Extension Educator, Wayne County: Mark Sulc, OSU Extension Forage Specialist

A popular question as we enter September is: What is the last date to harvest alfalfa? Unfortunately, there is not one single right answer to this question. It really depends upon a number of risk factors and how comfortable the alfalfa grower is with varying degrees of risk. Based on risk, there are guidelines that can be used to help determine a last fall harvest date. According to the Ohio Agronomy Guide, the last regular season harvest dates vary depending upon the area of the state and are September 7 for northern Ohio, September 12 for central Ohio and September 15 for southern Ohio. These guidelines are based on avoiding a fall harvest during the late September through October period. This period is termed the critical period because this is when forages are actively storing carbohydrate reserves in the crowns and roots. Following this recommended last harvest schedule is a least risk, time-tested model.

There certainly are alfalfa growers who believe the Ohio Agronomy Guide fall harvest dates are too conservative. Cherney and Kilcer from Cornell University wrote a newsletter article in 2000 on the topic of fall forage management. In the article they say that research indicates the length of the harvest interval prior to fall cutting is more important than the actual date of fall cutting in terms of risk to an alfalfa stand. Their advice is to provide a rest period of at least 6 weeks between the last two cuts of the season. In a July 2012 Cornell newsletter, J. Cherney, D. Cherney and P. Peterson write about alfalfa fall harvest guidelines. They review some research from Canada that links the carbohydrate reserves accumulated between a second and third cutting to growing degree day (GDD) accumulation. That research showed that as long as there was a sufficient interval to accumulate 500 GDD between the fall harvest and the previous harvest, alfalfa could be cut during the critical period. Put in terms of weeks, this matched up once again with a cutting interval of 6 to 7 weeks. How dependable is this concept of harvest interval? How relevant is this concept of cutting interval to last fall harvest in Ohio?

Last winter, alfalfa growers across the Upper Midwest suffered the worst winter stand loss of alfalfa in recent memory with millions of acres of alfalfa lost. So, what went wrong? University of Wisconsin Extension forage specialist Dan Undersander chalks up the stand losses to poor energy reserves going into the winter. The winterkilled fields had been managed quite intensively the previous year (2012) when above-normal temperatures and drought stress caused the crop to flower sooner than normal and consequently, stands were cut early before carbohydrate reserves were adequately replenished. Many fields were cut on only three-week harvest intervals. The intensive cutting schedule made it hard for the alfalfa to survive the very long winter that followed. There simply was not enough energy for plants to overcome the weather conditions and have enough reserves to resume normal growth when sustained growing conditions finally arrived in May this year.

The harvest interval concept as a guideline for fall alfalfa cutting management has not been tested further south where fall regrowth may be more significant. So in addition to the uncertainty of winter and succeeding spring conditions, what happens in the fall after alfalfa is cut? If regrowth starts it must continue until the carbohydrate reserves used to generate that regrowth are replenished. So, while the harvest interval concept may have some merit, it is not without risk and even in those articles from New York the authors stated that the other key factor besides a long enough cutting interval is that the alfalfa stand must be healthy if it is cut during the critical period.

Factors that help to determine stand health include disease resistance, protection from insects, age of stand, cutting management and fertility. In the Cherney and Kilcer article, they state that alfalfa varieties with improved disease resistance enables those varieties to be more tolerant to negative effects of a fall cutting because there is less total stress on the plant. Any of the newer varieties of alfalfa have good resistance to the major alfalfa diseases. Research has shown that alfalfa is very sensitive to soil fertility. High soil potassium can increase plant health and plant tolerance to fall cutting effects. A high soil pH of 6.8 to 7.0 will also reduce the risk of fall cutting. Stands under 3 years of age are more tolerant of fall cuttings as compared to older stands. Protection from potato leafhopper stress during the year is also important for maintaining strong and vigorous plants that will be better able to withstand a fall cutting stress.

A significant risk factor that must be considered when making a decision about a final fall harvest is the number of times the stand has been harvested. Alfalfa that has been cut three or more times before a fall harvest has a higher risk factor for injury due to fall harvest. A final factor is soil drainage. Alfalfa stands on well drained soils tolerate later fall cuttings better than alfalfa on moderately or poorly drained soils.

The bottom line is this: Most years in Ohio growers can probably get away with cutting during the fall without severe injury, but it is very probable that fall harvest robs yield next year and it always increases risk of winter damage if the cutting date is such that alfalfa regrows without sufficient time to recharge what it used for that fall regrowth. Taking all risk factors into consideration, the worst time to harvest in Ohio is most likely between September 25 and October 15. If you do decide to cut during the fall critical period, leave several strips of alfalfa uncut in different parts of the field so you can compare the vigor of cut and uncut plants next year.

For more information about fall alfalfa harvests contact a member of the OSU Integrated Forage team.