Dairy producers recognize the importance of high quality alfalfa in dairy cattle rations. High quality alfalfa in the ration can increase milk production and lower feed costs. 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. For a more thorough discussion see the publication mentioned above.
In my years of following alfalfa GDD accumulations in Michigan’s Thumb we rarely see any significant GDD accumulation in the month of March. However, 2012 has been a very unusual year. Table 1 shows the alfalfa GDD accumulations for several weather stations in the Thumb along with historical data for comparison. Nearly every other weather station in Michigan is also recording an unusually warm spring.
The bottom line is this: We are on course this year to reach 40% NDF in the alfalfa crop at an unusually early calendar date. Thus, it is even more critical for dairy producers to follow the GDD accumulation at a weather station as near as possible to their farm to time this year’s first cutting at an optimal date. The Michigan State University Agricultural Weather Office maintains a website that allows you to track GDD accumulations at nearby weather stations. Go to the website and click on “Enviro-Weather.” This will bring up a map of Michigan showing the weather stations recording this important data. Click on the weather station closest to your farm, then under the heading “Degree-day tools” go to “Degree Day accumulations for Region (alfalfa and corn development).” This will bring up a table of GDD accumulations. The correct GDD accumulation data for alfalfa is in the second column from the left with the heading “Degree Day Base 41oF for Alfalfa.”
It is impossible to forecast exactly what weather we will experience between now and time for first cutting. However, if the weather this winter and so far this spring is any indication it will be extremely important to follow alfalfa GDD accumulations very closely to optimally time this year’s potentially early first harvest.
1GDD equals growing degree days calculated using base 41 method
2Growing degree day accumulation as of 3/25/12
3Years include 2007, 2008, 2009, 2011
4No data in 2007 or 2008 for Richville/Frankenmuth station
Source: Craig Thomas, Michigan State University Extension