Alfalfa growers have to balance the need for tonnage, forage quality and winter survival when deciding whether to take a late cutting, say Dan Undersander, University of Wisconsin Extension forage agronomist, and Bill Bland, Extension climatologist.

They point out that alfalfa must either be cut early enough in fall to regrow and replenish root carbohydrates and proteins or so late that the alfalfa doesn’t regrow and use root carbohydrates. This has resulted in the recommendation of a “no-cut” window from Sept 1 to killing frost for Wisconsin and several other states. However, research in Quebec has helped define that window by indicating that alfalfa needs 500 growing degree days (GDD) after the last cutting to regrow sufficiently for good winter survival and yield the next year.

On the other extreme, the Quebec research showed that cutting later in fall is acceptable if less than 200 GDD accumulate after the cutting.

“It is important to remember that we do not need to wait for a killing frost to take the last cutting,” says Undersander. “We must only wait until it is so cool that little or no regrowth will occur.”

So the last cutting should be taken early enough so that regrowth and root replenishment occur or so late that little to no growth occurs. Calculating the sum of those two probabilities reveals the risk of winter injury or kill due to harvesting at different dates during September. That data was calculated for eight Wisconsin sites where 30 years of weather history were available.

At Lancaster in southwestern Wisconsin, for example, 500 GGD have accumulated after Sept. 8 94% of the time, but waiting one more week reduces the probability to 61%.

Graphs showing Wisconsin data are at http://ipcm.wisc.edu/WCMNews/tabid/53/EntryId/416/Late-Summer-Cutting-Management-of-Alfalfa.aspx. Similar graphs for various Michigan locations are at http://www.agweather.geo.msu.edu/agwx/articles/article-09.html

Undersander says to assume that the graphs are for very winterhardy varieties (winter survival score of 2 or less) and that less winterhardy varieties would be at more risk.

“Forage quality of alfalfa changes little during September, so harvesting vs. delaying cutting should be based on the likelihood of winter injury or survival if the stand is to be kept,” he says.