The growth rate of many biological organsims is controlled primarily by temperature. Figure 1 demonstrates a generalized response of growth rate to increasing temperature. Growth begins at some minimum (in this case 50°F). The rate of development increases with rising temperature until it reaches a plateau at some optimum temperature (86°F in the figure). As temperature increases above the optimum the growth rate declines. The shape of this response curve is similar among many organisms; however, the minimum, optimum, and maximum temperature will vary.
When an organism's growth rate is predominantly under temperature control its rate of development can be characterized using a system called growing-degree days (GDD) or heat units (HU). This GDD concept assumes that:
GDD are calculated by determining the mean daily temperature and subtracting it from the base temperature needed for growth of the organism. Limits are usually set on the low and high daily temperature so only temperatures that result in additional growth rate are considered. The organism in figure 1 can be used to demonstrate this. The mimimum temperature in the GDD calculation for this organism is set at 50°F since very little growth occurs at temperatures below this. Because the growth rate slows under conditions warmer than 86°F, it is used as the maximum temperature. The GDD value for one day for this organism is then represented by the following equation:
GDD = (Tmax +Tmin)/2 - Tbase
where Tmax is maximum daily temperature and is set equal to 86°F when temperatures exceed 86°F
Tmin is the minimum dail temperature and is set equal to 50°F when temperatures fall below 50°F
Tbase is the base temperature for the organism
Growing degree days have many applications in crop management. Accumulated GDD, calculated by summing GDDs for each day during a period, are useful in tracking the development of several important crops and insect pests. One of the original uses of GDD was characterization of corn development and classifying corn hybrid maturities. Corn has a base temperature of 50°F and each corn hybrid has a certain GDD requirement to reach maturity. Those grown in the central Corn Belt require anywhere from 2100 to 3200 GDD depending on the hybrid. Table 1 lists the growing-degree days needed to reach different growth stages for a typical 2700 GDD hybrid.
| Phase | Development Stage | GDD |
| Planting | 0 | |
| Vegetative | Two leaves fully emerged | 200 |
| Four leaves fully emerged | 345 | |
| Six leaves fully emerged (growing point above soil) | 476 | |
| Eight leaves fully emerged (tassel beginning to develop) | 610 | |
| Ten leaves fully emerged | 740 | |
| Twelve leaves fully emerged (ear formation) | 870 | |
| Fourteen leaves fully emerged (silks developing on ear) | 1000 | |
| Sixteen leaves fully emerged (tip of tassel emerging) | 1135 | |
| Reproductive | Silks emerging/pollen shedding (plant at full height) | 1400 |
| Kernels in blister stage | 1660 | |
| Kernels in dough stage | 1925 | |
| Kernels denting | 2190 | |
| Kernels dented | 2450 | |
| Physiological maturity | 2700 |
Growing-degree days are also useful in Integrated Pest Management (IPM). The primary use of degree days in IPM is to time scouting for pest insects. Scouting efficiency can be increased by knowing the number of degree days required for a pest to reach a destructive growth stage. Alfalfa weevil is a good example of this use of GDD. Table 2 lists the degree day requirements for this economic pest of alfalfa.
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Begin degree day accumulation on January 1 |
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| Stage | Activity | Degree Days |
| Egg hatch | 300 | |
| 1st - 2nd Instar | Light leaf feeding | 301-438 |
| 3rd - 4th Instar | Major leaf feeding | 439-595 |
| Pupa-adult | Feeding stops | 596-810 |
Alfalfa weevil scouting should begin once 300 degree days have accumulated from January 1. Economic thresholds are then used to determine management practices for this pest.
1. May 15 is the target date for completion of corn planting in Iowa. An average day for Ames, Iowa on this date has a minimum temperature of 50°F and a maximum termpature of 72°F. July 19 is the warmest day of the year in Ames, with a average minimum temperature of 65° and an average maximum of 86°F. How many more growing-degree days (GDD) will accumulate for corn (base temperature and minimum threshold = 50°F, maximum threshold = 86°F) in Ames on the average July 19 when compared with May 10? Express your answer as a multiplication factor of the GDD available on May 15.
GDDMay 10 = (72 + 50)/2 - 50 = 11
GDDJuly 19 = (86 + 65)/2 - 50 = 25.5
25.5/11 = 2.3 times as many GDD available on July 19 when compared to May 15
2. You are tracking degree days for stalk borer (minimum threshold and base temperature = 40°F). Three consecutive days have the following minimum and maximum temperatures.
| Day | Mimimum (°F) | Maximum (°F) |
| 1 | 38 | 57 |
| 2 | 40 | 64 |
| 3 | 44 | 70 |
What is the accumulated degree days for this three day period?
GDDDay 1 = (57 + 40)/2 - 40 = 8.5
GDDDay 2 = (64 + 40)/2 - 40 = 12
GDDDay 3 = (70 + 44)/2 - 40 = 17
GDDaccumulated = 8.5 + 12 + 17 = 37.5