## Field Scouting: Using Growing Degree Days and Crop Heat Units
| Corn
| Soybeans | Forages
| Cereals | Dry
Edible Beans | Pub
811: Agronomy Guide > Field
Scouting > Using Growing Degree Days and Crop Heat Units
## Table of Contents
- Growing Degree Day
- Growing Degree Day Equation
- Crop Heat Units (CHUs)
- Calculating Daily CHU
- Field Record Forms
## Using Growing Degree Days and Crop Heat Units## Growing Degree DaysGrowing 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 EquationThe GDD equation used by OMAFRA is calculated as follows: Daily GDD = ((Tmax + Tmin) ÷ 2) - Tbase T max = the daily maximum
air temperature
Pest: European
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. -
**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. **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.**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.**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) |

Author: | OMAFRA Staff |
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Creation Date: | 30 April 2009 |

Last Reviewed: | 30 June 2011 |