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Growing Degree Days For Field Crops
4/5/2002
Perry Brewer
Area Extension Agent (Technology Education/Youth)
Colorado State University Extension
Golden Plains Area		 Perry Brewer, Area Extension Agent
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The growing degree day index (or heat unit index) is the accumulation of degrees above a threshold daily mean temperature. This reflects the rate of chemical reactions inside plant cells, and the rate of cell division and cell enlargement.

Follow these steps to calculate the degree day index for your crop:
  1. Record the daily maximum temperature in degrees F. (ex: 85 degrees F.)
  2. Record the daily minimum temperature in degrees F. (ex: 53 degrees F.)
  3. Calculate the daily mean temperature:
    (Max. Temp. plus the Min. Temp. divided by 2 equals the daily mean temperature) (ex: (85+53)/2= 138/2= 69 degrees F.)
  4. Determine the threshold temperature for the crop.
    Corn and Sunflowers have a threshold temperature of 50 degrees F. Wheat has a threshold temperature of 40 degrees F. Field beans have a threshold temperature of 60 degrees F.
  5. To determine the growing degree day index for that day, subtract the threshold temperature from the mean temperature. (ex: 69-50(threshold temperature for corn)=19 degree day units). If the daily mean temperature is less than or equal to the threshold temperature then the degree day value for that day is zero (0). (ex: 48(mean temperature)-50(corn threshold)=-2 degrees. The degree day accumulation for that day would be 0.
  6. Add the daily degree day values to produce a cumulative degree day index.
  7. For days with a daily mean temperature above 86 degrees F., the degree day units are also considered to be zero because of heat stress on the plant.
 
Page Created and Maintained by: Perry D. Brewer, Area Extension Agent (Technology Education/Youth)
1/16/2003
 		  
 
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