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What Are Growing Degree Days?

Jun 30, 2011

Question: What are growing degree days, and how can I use them to determine how my corn crop is developing?

Answer: You can use heat measurement standards called growing degree days (GDDs), or growing degree units, to assess and project corn development stages.  Knowing the GDDs for your geography can also help equip you to select a package of hybrids for your farm that includes a variety of relative maturities.  Many seed corn companies readily provide such information for that purpose.
 
The amount of GDDs a specific corn hybrid requires to reach each development stage during the growing season remains constant from year to year. However, the amount of time that hybrid needs to accumulate those heat units can vary considerably from year to year due to planting date, weather conditions and temperature. Contact your local or state Extension service for specific information on GDDs for your geography. University Extension resources also often provide a GDD calculator to make tracking heat units an easier process.
 
GDDs are calculated for each day starting the day after planting. To calculate GDDs, review the basic equation, direction and examples provided here.
 
GDD= [(high °F + low °F)/2]-50
If the high is above 86°F, use 86°F in the equation.
If the low is below 50°F, use 50°F in the equation.
 
Examples:
High: 81°F Low: 63°F
GDD= ((81+63)/2)-50= 22 GDD
 
High: 68°F Low: 44°F
GDD= ((68+50)/2)-50= 9 GDD
 
Be aware that while most companies rate hybrids based on the timing between planting and maturity, some rate them from emergence to maturity.  If the latter is what you find, add 150 to get the GDDs from planting.
 
This year’s class of Corn College graduates will leave the event with take-home-to-the-farm techniques to raise their corn yields higher.
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