AMES, Iowa — This time of season there are always questions on when that last cut could be made and still allow enough time to build root carbohydrate before the killing frost. The answers to these questions are usually something like… “its fine to harvest through the first week of September,” and… “we recommend harvesting at least 6 weeks before the killing frost.” On average, the alfalfa killing frost (25oF) in northeast Iowa occurs in the 3rd week of Oct. So there is minimal risk harvesting alfalfa through about Sept. 10 in northeast Iowa. FYI killing frost for the last few years: Oct. 31, 2014; Oct. 17, 2015; close on Oct. 13, 2016, definite on Nov. 12, 2016; Oct. 28, 2017.
While those “good old answers” still work, the more correct answer actually deals with growing degree days (GDD), not the calendar. Researchers now define a risk assessment of fall harvest based on alfalfa GDD. The research basically says as long as the plants accumulate at least 500 GDD from harvest to killing frost, plants should have stored enough root carbohydrate to survive the winter. A nice summary of this research is available at the following web site. The data from Lancaster and Beloit, WI (southern WI) would apply quite nicely to northeast Iowa. https://fyi.uwex.edu/forages/
What if you chose to harvest “after the killing frost”? The GDD research says as long as the plants do not accumulate more than 200 GDD from after harvest to before the killing frost, the plants should still overwinter just fine. This means that you do not have to wait for the actual killing frost to occur as long as you are close enough to it when you harvest. i.e. Oct. 15 is a good cut-off date where if the killing frost has not occurred yet, it likely will soon, and the weather in late Oct. is usually cold enough that 200 GDD will not accumulate in the time remaining in the fall. A critical issue with harvesting after a killing frost is that little to no regrowth will occur following the harvest, so you want to cut high, leaving a good stubble height (~6 inches) to help trap snow and insulate the plants.
— Brian Lang, Agronomist, Iowa State University Extension and Outreach
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