A lot of people may be thinking about a late cutting this year, not because they planned one but because they never got their last traditional cutting off. The rain came, the fields stayed soft, and the window closed. Now there is standing forage that was supposed to be in storage weeks ago, and the question becomes whether to go after it late or write it off.
The basic timing rule
The general rule of thumb is to allow 5 to 6 weeks (900 growing degree days) between the final harvest and a killing frost, or to cut less than 2 weeks (360 GDD) before the killing frost. A killing frost for alfalfa generally occurs when temperatures drop to 24°F (-4°C) or lower for several hours. At that point top growth is damaged and active growth stops, as long as temperatures stay low. Harvesting after a killing frost is also an option, but it should be done soon afterward to limit nutrient leaching.
Alfalfa GDDs are calculated the same way as corn, but with a lower base temperature of 41°F:
GDD alfalfa = ((temp max + temp min) / 2) – 41
That 5-to-6-week window matters because it is what the plant needs to rebuild root reserves for winter survival. Right after a harvest, the plant draws on those reserves to fuel regrowth. Once it reaches about 8 to 10 inches, it is photosynthesizing enough to start replenishing them and support continued growth. That timeline lines up with the 900 GDD guideline. During this same period, the plant is winterizing in response to cooler temperatures and shorter days.
Cutting within 360 GDD — roughly two weeks — of a killing frost is generally safe because the plant will not have time to put on significant regrowth, so root reserves are not depleted. The University of Wisconsin has a tool that uses past weather data to evaluate the risk of cutting alfalfa now: https://connect.doit.wisc.edu/alfalfa-tool/
What a wet fall changes
A wet fall shifts this in a few ways, and not all of them point the same direction.
Regrowth may be faster than you expect.
In a dry fall, moisture stress slows regrowth and effectively widens the safe window. This year is the opposite, if soil moisture is good and temperatures stay warm, that stand can put on growth quickly and start pulling down reserves. Do not assume the window is more forgiving than the GDD numbers say. If anything, be stricter with the <360 GDD guideline this year.
The window may have already moved.
Many people intended to take a cutting weeks ago and could not get equipment in the field. If you are now sitting between the two safe windows — more than two weeks out from a likely killing frost, but without the full 5 to 6 weeks of recovery time ahead — the honest answer is that waiting is usually the better play.
Drying conditions are poor.
By late fall you are working with short days, heavy dews, and cool temperatures. Dry hay is hard to make and easy to ruin. If you are committed to getting this cutting off, baleage or chopping is the more realistic route. Wide swathing helps, but it will not overcome unforgiving weather conditions.
Driving on wet fields
This is the part that is easy to underweight. Wheel traffic on saturated soil does real damage to alfalfa crowns, and that damage shows up as a thinner stand next spring — not as anything you can see the day you cut.
- Crown damage from traffic is worse on wet soils. Crowns get crushed or torn rather than pressed, and those wounds are entry points for crown and root rot. Wet conditions favor those pathogens anyway.
- Ruts are a multi-year problem. They damage the stand where they form, they make every future operation rougher, and they hold water.
- Compaction limits root growth going into the season when the plant most needs a functioning root system.
- Older stands have less to give. If this is a fourth- or fifth-year stand, or one already showing thinning, the traffic cost is higher and the payoff is lower. That is a stand where leaving it alone is easier to justify. Now if this is your last year, winter survival may not be a concern.
If the field will not carry equipment without rutting, that answers the question. A marginal tonnage of high-quality forage is not worth a compromised stand.
Is the cutting worth taking at all?
Even setting aside the field conditions, it is worth asking whether you need this cutting.
Yield from a late cutting is usually low, though quality tends to be high. If you need high-quality forage in the barn, that can justify it. But leaving the material standing has real value too — it catches snow, and that snow cover forms an insulating layer that helps prevent winterkill, in which is at a higher risk in wet soils. If you decide to take a late cutting leaving 4-6 inch stubble height can be very beneficial during ice sheeting. This is crucial for gas exchange in the plant having stem outside of the ice, to avoid plant suffocation.
Bottom line
Run the GDD numbers using the Wisconsin tool for your location to check where you actually stand rather than guessing. But this year, run a second check before you decide: can you get across that field without rutting it, and can you get the forage dry enough to store? If either answer is no, leaving the stand standing is a legitimate choice, not a failure to get the job done.
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- Assistant Professor
- Forage Extension Specialist