The use of accumulated growing degree days (GDD) to time the first alfalfa cutting likely will be more important than ever this year, says Craig Thomas, Michigan State University Extension educator.

It looks like the crop will be ready unusually early, and GDD are the most reliable tool for deciding when to cut for optimum quality, says Thomas.

Growing degree days are calculated by averaging each daily minimum and maximum temperature beginning Mar. 1, then subtracting the base temperature of 41oF to get the number of GDD for that day. GDD for days with an average temperature of less than 41aren’t counted.

For high-producing dairy cows, the goal should be to harvest alfalfa at 40% neutral detergent fiber (NDF), he says. Research has shown that, in the Upper Midwest, alfalfa averages about 40% NDF when 750 GDD are accumulated. It takes an additional 220 GDD to reach 45% NDF, so starting to cut at 750 GDD will give about a seven-day window to complete the harvest before 45% NDF is reached.

Michigan growers can track GDD numbers on the Michigan State University Agricultural Weather Office Web site. Click on Enviro-Weather to bring up a Michigan map showing the weather stations recording GDD data. Click on the station closest to your farm, then under Degree-Day Tools go to Degree Day Accumulations for Region (alfalfa and corn development). That will bring up a table of GDD accumulations. The correct GDD data for alfalfa are in the second column from the left with the heading Degree Day Base 41oF for Alfalfa.

For more information, Thomas recommends the Michigan State publication, Timing Spring Alfalfa Harvest – The Final Word?