Warmer than average weather makes gains on Growing Degree Days
Early May’s warm weather in Michigan led to fast accumulation of Growing Degree Days resulting in a predicted timely to slightly delayed alfalfa harvest.
A colder than normal April 2013 resulted in lower than normal values for Growing Degree Days (GDD) in Michigan. However, above average temperatures during the first week of May brought the heat accumulation closer to normal.
GDD provide a measure of heat unit accumulation. Because early season growth of alfalfa is related to the sum of heat units accumulated, monitoring GDD is an easy and effective way to determine when to harvest to optimize forage quality of first cutting alfalfa. The measure of fiber most commonly used to balance diets of lactating dairy cows is neutral detergent fiber (NDF). The optimum concentration of NDF for alfalfa fed to lactating dairy cows is 40 percent. Michigan State University Extension recommends that dairy hay should be harvested at the mid-bud stage of growth which will normally provide 40 percent NDF to maximize the yield and quality of feed to lactating dairy cows. Alfalfa containing 40 percent NDF allows reasonable grain concentrations in the diet while maintaining adequate NDF concentrations. Research at MSU demonstrated that both GDD (growing degree days, base 41 degrees Fahrenheit) and PEAQ (predictive equations for alfalfa quality) provide good estimates of NDF for first cutting alfalfa in normal years.
Recommendations are to begin cutting alfalfa at 750 GDD for upright silos and 680 GDD for horizontal silos. According to data collected in the upper Midwest over several years, alfalfa typically averages 38 percent NDF at 680 GDD and 40 percent NDF at 750 GDD. Filling horizontal silos with layers of alfalfa of increasing maturity will allow harvest to begin a little earlier because the layers of alfalfa are blended as the alfalfa is removed from the silo.
For alfalfa, GDD is based on the minimum and maximum daily temperatures beginning March 1, using a base of 41 F. The daily calculation is [(maximum temp + minimum temp)/2] - 41. The GDD accumulation is the sum of the daily GDD values across days beginning March 1. Producers can get a good idea of how the season is progressing by comparing accumulated GDD to previous seasons.
As of May 6, most of Michigan has made great gains in GDD accumulation, with some areas even ahead of normal (1981-2007). It is important to note that these maps are using a degree day base of 50 F, and a base of 41 F is used for alfalfa, but the relative change between dates is the same. It is important to keep close track of the GDD as we progress this spring. Depending on where you are located in the state and the weather the following two weeks, harvest could be in the normal timeframe, or a week delayed.
It is important to monitor alfalfa growth in growing seasons that are atypical so that forage yield and quality are maximized for the dairy herd. Tracking GDD or taking measurements using a PEAQ (predictive equations for alfalfa quality) stick will result in better quality feed than by going from previous calendar dates of when alfalfa is typically ready for harvest. With hay inventories short after the drought of 2012, it may be tempting to take hay early, but it will pay in the long run to wait until the GDD indicate it is the proper time to harvest. In addition, scout fields to determine weed pressure and damage from disease and insects that can make timely harvesting important.