# Growing Degree Days

The growth rate of many biological organisms is controlled primarily by temperature. As a result of this relationship between temperature and growth, it is possible to predict when specific developmental events should occur by calculating the average heat accumulation. Growing Degree Days (GDD) is defined as the number of degrees above a certain threshold base temperature at which growth begins for the particular organism. Below this base temperature, growth is zero. GDD are calculated by determining the mean daily temperature and subtracting it from the base temperature needed for growth of the organism.

GDD = (Tmax + Tmin)/2 - TBase

One degree day is one day when the average daily temperature is at least one degree above the lower developmental threshold; the base temperature. For example, if the low for the day was 35oF and the high was 65oF, then the day had an average temperature of 50oF. If an organism has a temperature base of 32oF (for example, wheat), then that day counts as 18 degree days. Accumulated GDD are calculated by summing GDD for each day during a period and are useful for tracking the development of crops and insect pests.

Growing Degree Days have many applications in crop management. An example is the characterization of crop development in order to estimate growth stages, including maturity and harvest dates. Growing Degree Days are useful in Integrated Pest Management programs where they are used to predict insect life stages, which in turn help to improve the efficiency of insect scouting efforts. Growing Degree Days can also be used to assess the suitability of a region for production of a particular crop, predict the best timing of irrigation, fertilizing or pesticide application. Growing Degree Days can also estimate heat stress on crops, and plan spacing of planting dates to produce separate harvest dates.

The following tables contain accumulated GDD and the corresponding developmental stages for the Orange Wheat Blossom Midge (Table 1) and phenological development of wheat (Table 2.)

Table 1. Midge Degree Day Model, 40° Base Temperature
Event GDD °F
Midge breaks the larval cocoon and moves close to soil surface and forms the pupal cocoon. 450
About 10% of the females will have emerged 1300
About 50% of the females will have emerged 1475
About 90% of the females will have emerged 1600

Table 2. Phenology Calculations Using 32°F Base Temperatures for Hard Red Wheat Grown in Montana. Stages shown are Universal Growth Stages.
Event Description Stage GDD °F
Emergence Leaf tip just emerging from above-ground coleopty 1.0 257-320
Leaf Development Two leaves unfolded 1.1 336-406
Tillering First tiller visible 2.1 696-789
Stem Elongation First node detectable 3.1 1097-1218
Anthesis Flowering commences; first anthers of cereals are visible 6.1 1484-1653
Seed Fill Seed fill begins. Caryopsis of cereals watery ripe (first grains 1/2 final size). 7.1 1954-2145
Dough Stage Soft dough stage, grain contents soft but dry, fingernail impression does not hold 8.5 2613-2832
Maturity Complete Grain is fully mature and drydown begins. Ready for harvest when dry 8.9 2800-3092