Fall Alfalfa Harvest – Cut now or wait till after a killing frost?

How late can you make the last cutting of alfalfa? Do you cut now or wait until a killing frost? This ICM Blog will provide some guidelines to help you determine when you should make that last cutting and other management considerations when taking the last cutting for the year.

Rule of Thumb

The old-old-old rule of thumb for fall alfalfa harvest is to “either harvest at least 5 to 6 weeks before the killing frost or to wait until the killing frost and then harvest”. A killing frost is considered to be somewhere in the ballpark of when temperatures drop down to the 24 to 25^o F range for some significant period of time, maybe an hour or two. It’s not actually a killing frost for the plant. Rather just the top growth has froze, which will result in the plants to wilt and drop their leaves.

Harvest 5-6 Weeks Before a Killing Frost

The rule of thumb of harvesting at least 5 to 6 weeks before a killing frost works quite well, but this decision is made at a time when we have no idea when the killing frost will occur. To help take the guess work out of making that decision, one can base the decision on average killing frost dates, which are reported by Crop Reporting District in Table 1.

Table 1. Average alfalfa killing frost dates and suggested harvest date deadlines for late-summer cut alfalfa.

Another way to look at this is with alfalfa growing degree day (GDD) research. Fall harvest management research on alfalfa is based on a window of 500 to 200 GDD (base 41^oF). As long as fall cut alfalfa accumulates at least 500 GDD before going into the winter, it will have good carbohydrate reserves. This data matches well with the rule-of -thumb to “harvest at least 5 to 6 weeks before the killing frost”.

Harvest Near a Killing Frost

If you missed the opportunity to harvest the 5 to 6 weeks prior to a killing frost, there is still a window to harvest before a killing frost. A late fall cutting can be taken if the alfalfa does not accumulate more than 200 GDD before a killing frost. This will still allow the alfalfa to enter the winter with good carbohydrate reserves.

The tricky part is to guess when a killing frost might occur and what dates will fall within 200 GDD prior to the killing frost date. Using the AccuWeather 30-day outlooks in 2019 for some towns in northern, central, and southern Iowa, we can get a better idea on the best timing to take a late fall cutting prior to a killing frost (Table 2).

At this time, the cooler weather outlook in northern Iowa suggests cutting alfalfa anytime that is convenient. The warmer weather for central and southern Iowa means that more than 200 GDD may still accumulate. Therefore, it is suggested to wait on the last cut until mid- or late October.

Table 2. Predicted alfalfa killing frost dates and suggested harvest window prior to the killing frost.

Fall Stubble Height

Whether you harvest near a killing frost or after a killing frost, it is important leave a good stubble height (about 6 inches) to help trap snow and insulate the plants since there will be minimal to no regrowth. If your equipment cannot be set that high, set it as high as you can.

Alfalfa, Frost, and Bloat

One last issue that often comes up in fall is the old wives’ tale that alfalfa becomes toxic after a frost. There are no ‘toxic’ compounds produced in alfalfa following a frost. However, there is a greater chance of bloat for cattle grazing alfalfa early morning following a frost, and likely also with the presence of a heavy dew.

Hold cattle off from grazing an alfalfa field following a night-frost until after the dew subsides. The concern is the combination of the frost bursting plant cells that then readily release soluble proteins into the rumen, and the dew adding excess water intake that can elevate the chance for bloat to occur.