Dr. Mark Sulc, Forage Specialist, Ohio State University
Early September is ideal for taking that last yearly cutting of alfalfa. The timing of this last cutting can be very important to the long-term health of the stand. In this article, we will review what is known about fall cutting management of alfalfa.
Alfalfa should not be cut during the 5 to 6 week period before a killing frost. During this critical period, cold resistance and energy reserves for winter survival are built up. A killing frost for alfalfa occurs when temperatures drop to 25oF or less for several hours. So the period from mid-September through October is the critical fall rest period in our region. Harvesting during this period disrupts accumulation of energy reserves and development of cold hardiness.
Producers often harvest alfalfa during the critical fall period despite the increased risk of winter injury. This year rainy weather has delayed cutting schedules throughout the growing season, pushing back the time when the crop will be ready for a last harvest. The tonnage expected from a fall cutting and the need for the forage should be high before considering a cutting during the critical fall period.
When harvesting alfalfa during the critical fall period, several factors can moderate the risk of winter injury:
- Young, healthy stands are less susceptible to winter injury from fall harvesting than older stands. On the other hand, more future production potential is lost if a younger stand is injured from fall cutting.
- Forages in well-drained soils will be at lower risk of injury than those with marginal drainage. Fall cutting should not be attempted on soils prone to heaving! Removal of the top growth cover increases the potential for heaving injury.
- Length of harvest interval during the growing season is often more important than the actual date of fall cutting. Making a 3rd cutting during the fall is less risky than making a 4th cutting in the fall, because a 3-cut schedule allows longer intervals for plant recovery between cuttings compared with a 4-cut schedule. Likewise, a growth interval of 45 days BEFORE a fall harvest will reduce the risk of injury compared with a pre-harvest growth interval of 30 days. The longer growth period allows more energy buildup before the fall harvest, lessening the amount of energy reserves needing to be built up after harvest.
- Fields with optimal soil fertility levels (pH, P, and K) are at less risk than where fertility levels are lower.
- Disease resistant and winter hardy varieties lessen the risk of injury from fall cutting.
- Alfalfa that was protected from potato leafhopper injury during the summer will be at lower risk than where leafhoppers caused severe injury. Any stress (like wet soils) that weakened the crop during the year can increase the risk of damage from fall cutting. This is the case in many of our alfalfa fields this year.
- Cutting AFTER a killing frost (25oF for several hours) in late October or early November can be an option for well-drained soils. Leave a 6-inch stubble after late fall cutting. Cutting this late in the year prevents regrowth that burns up energy reserves; however, late removal of plant cover increases the risk of frost heaving! Fall cutting should not be practiced on soils prone to heaving.