For Alfalfa, the timing of growth stage progressions through the season is dependent on both the variety and environmental cues such as day length, or temperature. The Alfalfa growing degree day model, developed by researchers, uses heat units and time as a method to estimate plant harvest time for each stage of the 3-stage Alfalfa cutting system.

Calculating Growing Degree Days

For this model, a lower temperature threshold of 38.3F is used to calculate growing degree day accumulation beginning at the time of planting for spring Alfalfa, with 45.5F for the early summer cutting and 50F for late summer cutting. Growing degree days can be calculated from daily high and low temperatures, or via the Pest Prophet app, which uses hourly temperature data for a given field. This can be used to predict harvest, or 10% flowering of each alfalfa cutting stage.

Setting the Biofix Date

The biofix date, or date from when to start accumulating growing degree days should be set at last occurrence of 27°F (-2.8°C) or lower for spring, and the previous harvest for the following summer cutting stages.

Harvest time for the spring cutting will occur at approximately 1053 GDD(F) after the biofix date. The next harvest will occur at approximately 765 GDD (F) after that for the later, summer, cuttings.

Source:

Sanderson, M.A., T.P. Karnezos, and A.G. Matches. 1994. Morphological development of alfalfa as a function of growing degree days. J. Prod. Agric. 7:239-242.