Field Scouting: Using Growing Degree Days and Crop Heat Units

| Corn | Soybeans | Forages | Cereals | Dry Edible Beans |
| Spring and Winter Canola | Other Crops | Soil Management |
| Soil Fertility and Nutrient Use | Field Scouting |
| On-Farm Stored Grain Management | Weed Control |
| Insects and Pests of Field Crops | Diseases of Field Crops | Appendices |

Pub 811: Agronomy Guide > Field Scouting > Using Growing Degree Days and Crop Heat Units

Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Table of Contents

• Growing Degree Day
• Growing Degree Day Equation
  • Table 10-4. Daily Crop Heat Unit Accumulations Based on Maximum and Minimum Temperatures
• Crop Heat Units (CHUs)
• Calculating Daily CHU
• Field Record Forms

Using Growing Degree Days and Crop Heat Units

Growing Degree Days

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.

Growing Degree Day Equation

The GDD equation used by OMAFRA is calculated as follows:

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

T max = the daily maximum air temperature
T min = 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 (ECB)
Base Temperature for ECB: 10°C

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. The ECB GDD model was based on measurements in Celsius.

Crop Heat Units (CHUs)

Crop 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.

Calculating Daily CHU

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

Daily CHU = (Ymax + Ymin) ÷ 2

where:

Y max = (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)
(Accuracy should be <0.25°C)

Y min = (1.8 x (T min - 4.4)) (If values are negative, set to 0)

Tmin = Daily minimum temperature (°C)

Field Record Forms

There are many methods of recording field data, ranging from paper to electronic. Examples of various forms can be found on the OMAFRA website or on the OSCIA website.