Field Scouting: Using Growing Degree Days and Crop Heat Units
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811: Agronomy Guide > Field
Scouting > Using Growing Degree Days and Crop Heat Units
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Using Growing Degree Days and Crop Heat Units
Growing degree days (GDDs) are used to estimate the growth and development of plants and insects during the growing season. Insect and plant development are very dependent on temperature and the daily accumulation of heat. The amount of heat required to move a plant or pest to the next development stage remains constant from year to year. However, the actual amount of time (days) can vary considerably from year to year because of weather conditions.
Each organism has a minimum base temperature or threshold below which development does not occur. These base temperatures have been determined experimentally and are different for each organism. GDD information can be very useful for predicting crop and insect development. Some Ontario crops still use the GDD system while others have moved to the crop heat unit (CHU) system described in the next section. Field crops that still use the GDD system are cereals (Base: 0), alfalfa (Base: 5) and canola (Base: 5).
To calculate GDD, first determine the mean temperature for the day. This is usually done by taking the maximum and minimum temperatures for the day, adding them together and dividing by 2. The base temperature is then subtracted from the mean temperature to give a daily GDD. If the daily GDD calculates to a negative number it is made equal to zero. Each daily GDD is then added up (accumulated) over the growing season.
Growing degree days are sometimes referred to as "degree days" or the "degree days averaging method." Some jurisdictions also use the term "heat units" interchangeably with "degree days." In Ontario, the terms "growing degree days" (GDD) and "crop heat units" (CHUs) are used independently since they represent two very different, temperature-dependent, development models.
The GDD equation used by OMAFRA is calculated as follows:
Daily GDD = ((Tmax + Tmin) ÷ 2) - Tbase
T max = the daily maximum
corn borer (ECB)
Daily GDD = ((28 + 15) ÷ 2) - 10 = 11.5
Therefore: 11.5 growing degree days were accumulated for that day for the European corn borer GDD model.
There are four factors to consider when comparing GDD accumulations from
various sources or regions.
heat units (CHU) are based on a similar principle to growing degree days. CHUs
are calculated on a daily basis, using the maximum and minimum temperatures; however,
the equation that is used is quite different. The CHU model uses separate calculations
for maximum and minimum temperatures. The maximum or daytime relationship uses
10°C as the base temperature and 30°C as the ceiling, because warm-season
crops do not develop at all when daytime temperatures fall below 10°C, and
develop fastest at about 30°C. The minimum or nighttime relationship uses
4.4°C as the base temperature and does not specify an optimum temperature,
because nighttime minimum temperatures very seldom exceed 25°C in Ontario.
The nighttime relationship is considered a linear relationship, while the daytime
relationship is considered non-linear because crop development peaks at 30°C
and begins to decline at higher temperatures. Daily crop heat units are calculated
by using the average of the two daily values from the equations below or can be
read from the matrix in Table 10-4, Daily Crop Heat Unit
Accumulations Based on Maximum and Minimum Temperatures. Figure
1-1, Crop Heat Units (CHU-M1) Available for Corn Production, gives a map view
of season total CHU(M1) accumulations for Ontario.
Producers who record high and low temperatures can use Table 10-4, Daily Crop Heat Unit Accumulations Based on Maximum and Minimum Temperatures, to calculate CHUs for their own farm. As of 2009, CHUs accumulate from May 1st at all locations and end with the first occurrence of -2°C in the fall. Corn development is driven primarily by temperature, and this is especially true during the planting-to-silking period. Unlike soybeans, day length has little effect on the rate at which corn develops. The Ontario Crop Heat Unit System has been developed to calculate the impact of temperature on corn development.
The following equation is used to calculate a daily CHU for a site:
Daily CHU = (Ymax + Ymin) ÷ 2
= (3.33 x (T max-10)) - (0.084 x (T max-10.0)2) (If values are negative,
set to 0)
Tmax = Daily maximum air temperature (°C) (measured from
midnight to midnight)
= (1.8 x (T min - 4.4)) (If values are negative, set to 0)
Tmin = Daily minimum temperature (°C)
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