This time of year there are always questions on when that last cutting of alfalfa could be made—or should be made—and still allow enough time for the plants to build root reserves of carbohydrate before the first killing frost.
The answers to these questions are usually something like… "It's fine to harvest alfalfa through the first week of September", and… "we recommend harvesting alfalfa at least 6 weeks before the killing frost." On average, a typical killing frost (25oF) in northeast Iowa occurs in the third week of October, says Brian Lang, Iowa State University Extension field agronomist at Decorah in northeast Iowa.
While those "good old answers" still work, the more correct answer actually deals with growing degree days or GDD, not the calendar, says Lang. Researchers now define a risk assessment of fall harvest of alfalfa based on alfalfa GDD. The research basically says as long as the plants accumulate at least 500 GDD from harvest to killing frost, the crop will have stored enough carbohydrate in the roots to survive the winter.
Latest research says you need to accumulate at least 500 growing degree days from harvest until time of first killing frost
A summary of this alfalfa research is available at the following web site from the University of Wisconsin. The data from Lancaster and Beloit (southern Wisconsin) would apply quite nicely to northeast Iowa, says Lang. Here's the website.
The Lancaster & Beloit data suggest alfalfa harvested through the first week of September is very low risk of winter injury, having plenty of time to replenish root carbohydrates going into the winter. A September 15 harvest could start providing some risk, and a September 21 harvest even more risk.