Using Grower Degree Days and Crop Heat Units

Excerpt from Agronomy Guide for Field Crops (Chapter 1)

Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Table of Contents

1. Growing Degree Days
   • Growing Degree Day (GDD) Equation
2. Crop Heat Units
   • Calculating Daily CHU
3. Updates on Field Scouting
4. Related links...

Growing Degree Days

Growing degree days, or 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. As an organism develops over a growing season, its development is closely related to the daily accumulation of heat. A certain amount of heat is required to provide enough energy for the organism to move to the next development stage, for example, the emergence of a new leaf on a corn plant or the first flight of European corn borer adult moths. The amount of heat required to move to the next development stage remains constant from year to year, but depending on weather conditions, the amount of actual time can vary. 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.

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.

Some Ontario crops still use the GDD system while others have moved to the crop heat unit 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).

Confusion has been created over the years by the terminology used to describe growing degree days. In Ontario, the terms "growing degree days" (GDD) and "crop heat units" (CHUs) are used, but they represent two very different, temperature-dependent, development models. The crop heat units calculation now used in Ontario is identical to the traditional corn heat units calculation, with the name change reflecting its now broader application to other crops. 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."

Growing Degree Day (GDD) Equation

The GDD equation used by OMAFRA is as follows:

Daily GDD = ((Tmax + Tmin) ÷ 2) - Tbase

where:

Tmax = the daily maximum air temperature
Tmin = the daily minimum air temperature
Tbase = the GDD base temperature for the organism being monitored

Example:

Maximum Temperature: 28°C
Minimum Temperature: 15°C
Pest: European corn borer
Base: 10

Calculation: (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.

1. Are the base temperatures used in the equations the same?
   Different organisms have different base temperatures used to calculate GDDs: 150 GDD at base 10 does not equal 150 GDD at base 0.
2. Are the start dates for the accumulations the same?
   Generally, GDD accumulations start on April 1 each year, but some insect GDD models start at the emergence of a specific life stage. This is referred to as a biofix.
3. Are the equations used to calculate the daily GDD the same?
   Many modifications to the simple GDD calculation have been developed over the years and may be referred to generally as degree days.
4. Are the temperatures used in degrees Celsius or Fahrenheit?
   GDD accumulations will vary significantly, depending on whether they are being tracked in Celsius or Fahrenheit. GDD models have been designed specifically for use in one or the other and cannot be interchanged without making conversions.
   

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Crop Heat Units

Crop heat units (CHUs) are based on a similar principle to growing degree days. CHUs are calculated on a daily basis, using the maximum and minimum temperatures, yet the equation that is used is quite different. There are separate calculations for maximum and minimum temperatures. The maximum or daytime relationship uses 10°C (50°F) as the base temperature and 30°C (86°F) as the optimum 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 (40°F) as the base temperature and does not specify an optimum temperature because nighttime minimum temperatures very seldom exceed 25°C (77°F) in Ontario. The nighttime relationship is considered a linear relationship, while the daytime relationship is considered a non-linear one because crop development peaks at 30°C (86°F) and begins to drop 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 3-8, Daily Crop Heat Unit Accumulations Based on Maximum and Minimum Temperatures.

Calculating Daily CHU

The following equation is used to calculate a daily CHU for a site:

Daily CHU = (Ymax + Ymin) ÷ 2

where:

Ymax = (3.33 x (Tmax-10.0))-(0.084 x (Tmax-10.0)²) (If values are negative, set to 0)
Tmax = Daily maximum air temperature (°C) (measured from midnight to midnight) (Accuracy should be <0.25°C)

Ymin = (1.8 x (Tmin-4.4)) (If values are negative, set to 0) Tmin = Daily minimum temperature (°C)

For more information on crop heat units, refer to the OMAFRA Factsheet, Crop Heat Units for Corn and Other Warm-Season Crops in Ontario, or order this factsheet Order No. 93-119, or visit the OMAFRA Web site at www.omafra.gov.on.ca/english/crops.

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Updates on Field Scouting

No updates available at this time.

Related links...

• Ontario Climate

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