October 4, 2012
With the drought and unusual cutting schedule this year, many farmers need to take a late-fall cutting to build feedstuff inventories. But, as University of Wisconsin Extension Forage Agronomist Dan Undersander points out, remember that this year's alfalfa growing conditions have thrown cutting schedules way off.
You need not concern yourself about a killing frost. Just wait until the weather is sufficiently cold (less than 50 degrees Fahrenheit) so regrowth after cutting will be minimal, he adds. Waiting for a killing frost (25 degrees for alfalfa) often causes delays until much later in the fall when hay and haylage-making conditions are less advantageous.
Marvin Hall, Penn State Extension forage agronomist, has a slightly different opinion on waiting for a "killing frost" to take that final harvest. "This practice tends to be less risky to the alfalfa stand than harvesting between now and the killing frost.
"However, knowing when frost has stopped alfalfa growth is tricky. You don't want a warm period after harvesting and the alfalfa begins growing again."
New info on winterhardiness
For good winter survival and rapid spring greenup for good yields next spring, alfalfa must either be cut early enough in the fall to regrow and replenish root carbohydrates and proteins, or so late that it doesn't regrow or use any root carbohydrates. That's why Undersander recommends a "no-cut" window beginning about six weeks before your average killing frost date.
However, research in Quebec has helped define this 'no-cut' window. Alfalfa needs 500 growing degree days after the last summer cutting to regrow sufficiently for good winter survival, he adds.
Growing degree days is calculated with base 41 degrees Fahrenheit, and accumulates until a killing frost. That means you can cut as late as long as 500 GDD will still accumulate without hurting the winter survival.
That explains why some farmers have cut alfalfa in the "no-cut" window and not been hurt. Some years we have a warm fall and 500 GDD accumulates from later starting points than average.
A late fall cutting is a good method to obtain more forage if needed. Open winters will tend to cause more alfalfa winter injury. Just must weigh the risk of injury against the need for forage.
Also keep in mind that alfalfa in fields with less than adequate soil pH (6.5 or less), low soil potassium or less winterhardy varieties (winter survival score of more than 2) are at more risk of winter injury or kill. And, grassy fields should be cut or grazed before winter. Grasses tend to make a mat and cover disease underneath.
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