Producers often ask if the old rule “no alfalfa harvest in September” still applies. Most know that an untimely harvest in the fall can reduce stand vigor the following spring, but some may have tried harvest in early to mid-September without any negative effects to the crop the following spring. Research from the 1980s suggested that alfalfa spring regrowth rate was largely a function of temperature and carbohydrate root reserves available when alfalfa dormancy was induced in the fall. More recent research has shown that nitrogen root reserves play a greater role in re-growth and levels increase more rapidly than carbohydrates.
Regardless of the form of the energy, it is important that these reserves are available in sufficient quantities in the spring. Alfalfa has little photosynthetic (leaf) area to draw from in the early spring, so it relies on energy reserves in the crown and roots. When alfalfa reaches 8 inches in height, the minimum level of energy reserves for maximum spring re-growth is present in the root and crown. Therefore, producers only want to harvest in September if they are reasonably certain that the alfalfa will have enough growing degree days (GDD base 41°F) to attain an 8-inch height. The number of GDD (base 41°F) to attain this height under normal soil moisture conditions is 500. Producers can also maintain energy reserves of alfalfa plants by waiting until they are sure that the alfalfa will not re-grow more than 2 inches following a fall harvest. To limit growth to less than this height, less than 200 GDD’s should accumulate.
Like so many actions that we take as producers, the outcome is largely dictated by the weather. We can do an excellent job of managing our crops, but abnormal weather conditions turn the hard work and preparation into an exercise in futility. We can, however, utilize regional historical weather data to make an informed decision. Meteorologists and agronomists at MSU developed region-specific probability charts to aid producers in making the decision of when to harvest. In these charts, the probability of accumulation of 500 or 200 GDD’s are graphed. A grower must decide what level of risk they are willing to accept (probability of accumulation of greater than 500 or less than 200 GDD’s) and then look at the range of dates in the chart that the “no cut window” would apply.
This tool can be found on the MSU Forage Information Systems website.