Growing Beef Newsletter

November 2024,  Volume 15, Issue 5

Should I take a late fall alfalfa cutting? Is it too late or too warm?
Shelby Gruss, ISU extension forage specialist

In the past few weeks, I've received several questions from farmers about whether it’s too late or too warm to harvest alfalfa. The general rule of thumb is to allow 5 to 6 weeks (900 Growing Degree Days, or GDD) between the final harvest and a killing frost, or less than 2 weeks (360 GDD) before the killing frost. Alternatively, harvesting after a killing frost is possible, but it should be done soon afterward to limit nutrient leaching. Alfalfa GDDs are calculated the same was as corn, but with a lower base temperature of 41°F. GDDalfalfa = ((tempmax + tempmin) / 2) - 41.

This 5-6 week window is crucial for allowing alfalfa to rebuild the root reserves for winter survival. Initially, after a harvest, the plant uses its root reserves to fuel regrowth. However, once the plant reaches about 8-10 inches of growth, it starts to photosynthesize enough to replenish these reserves and support continued growth. This timeline typically aligns with the 5-6 week (900 GDD) guideline, as supported by research.

Additionally, during this time the plant starts preparing for winter by winterizing due to the cooler and shorter days. A killing frost for alfalfa generally occurs when temperatures drop to 24°F (-4°C) or lower for several hours. At this point, the plant's top growth is damaged, and active growth stops if temperatures remain low. After this, harvesting can be considered but should happen quickly to avoid nutrient leaching and loss.

If you are cutting within 360 GDD (roughly two weeks) before a killing frost, it is generally safe because the plant won’t have enough time to grow significantly, meaning its root reserves won’t be depleted. This year’s relatively dry fall may slow regrowth, potentially expanding this safe window. Still, I recommend sticking to the <360 GDD guideline to be cautious. If a killing frost hasn’t occurred yet, late October is usually within this safe range. However, if warm temperatures persist, the risk increases, and cutting now can become questionable. To help assess your current situation, you can use a tool developed by the University of Wisconsin that uses past weather data to evaluate the risk of cutting alfalfa now.

However, before deciding on a final harvest, it’s worth considering whether it’s necessary. The yield from a late cutting is often low, but the quality tends to be high. If you need high-quality hay, this might make the cutting worthwhile. On the other hand, leaving the remaining material in the field can help collect snow, which forms an insulating layer that helps prevent winterkill.

In conclusion, we are likely in the safe range for a final cutting, but as we all know, weather can be unpredictable. It's essential to consider your specific forage needs. Leaving stubble in the field can provide added benefits by promoting snow accumulation and creating an insulating layer that aids in winter survival.


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