"Dairy producers recognize the importance of high-quality alfalfa in dairy cattle rations. One of the most important factors determining alfalfa quality is its maturity at harvest. As alfalfa matures, its fiber content increases while its fiber and dry matter digestibility decrease. On the other hand, if alfalfa is harvested at an immature stage it becomes difficult to feed because its quality can be too high. Thus, timing of harvest is always a balance between these two nutritional factors, plus early harvest tends to lower the life of alfalfa stands.
When alfalfa is fed to high-producing lactating dairy cows the goal is to harvest it at 40% neutral detergent fiber (NDF) content. At this fiber content the dairy producer and dairy nutritionist are provided with a feed that gives the cow adequate fiber for normal rumen function and maintains high feed intake for good milk production. It also provides flexibility to keep the cow’s ration at a proper energy density, keep protein supplementation at a reasonable level, and maintain good overall animal health. For a more complete discussion of these very important issues I recommend reading an excellent MSU publication entitled “Timing Spring Alfalfa Harvest—The Final Word?”
Usually the yield of alfalfa dry matter is highest at the first cutting, thus, timing first cutting to get the proper NDF level is critical. Currently the best method to use for timing optimal first cutting is harvesting based on accumulated growing degree days (GDD). Growing degree days are a temperature-based index measuring the amount of accumulated heat the crop has been exposed to during the first part of the growing season. Growing degree days are calculated by averaging each daily minimum and maximum temperature beginning March 1, then subtracting the base temperature of 41oF to get the number of GDD’s for that day. Daily GDD for days with an average temperature of less than 41oF are not counted. 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, thus, beginning harvest at 750 GDD will give about a 7 day window to complete alfalfa harvest before 45% NDF is reached."
Source: MSU Extension News