Growing Degree Days

Crop Production & Quality Forecasting And Growing Degree Days

The Gilliam County Wheat Quality Initiative includes development of a regional crop forecasting system.   This system is intended to provide growers and their customers with ongoing estimates of grain production and quality prior to harvest and storage of the crop.   These estimates can provide growers with a competitive edge in providing customers with the amount and quality of grain they require.   One component of a crop forecasting system is the calculation and tracking of growing degree days (GDD).

Accumulated heat from daily temperature drives all plant development.   Growing degree days (GDD) are the units used to measure this heat accumulation over time.   GDD are calculated by taking the average of the daily minimum & maximum temperature, then subtracting 32 from the result.   32 is subtracted from the average daily temperature because cereal plant development stops at 32 degrees.   If the result of this calculation is zero or a negative number, the GDD is recorded as zero.  

For example, on October 31st of this year, a maximum temperature of 50 and a minimum of 35 were recorded in Arlington.   The GDD calculation for this day is: 50 + 34 = 84; 84 divided by 2 = 42; 42 – 32 = 10.   From this calculation it is shown that the Arlington area accumulated 10 growing degree days of heat on October 31st , 2003.   An additional 10 GDD of heat accumulation contributed to cereal plant development in Arlington on that day.   Because of its higher elevation and cooler temperatures, Condon accumulated zero GDD heat units on October 31st .

Wheat growth stages are fairly predictable when the accumulated GDD is known.    The growing degree day total can be counted from the estimated date of seeded grain germination.   The publication “Early Growth and Development of Wheat in Northeast Oregon”, by Cook, Johlke & Karow identifies wheat growth stage with the associated GDD.  

Assuming adequate soil moisture, germination occurs at 145 (degrees F) GDD, followed by emergence at 327 GDD, with the first leaf fully developed at 509 GDD, the second leaf at 691 GDD, the third leaf at 873 GDD, the fourth leaf and first tiller at 1054 GDD, and so forth.   As daily growing degree days are added to the total, the accumulated GDD can be used to estimate plant development.

Comparison of estimate plant development with what is seen out in the field tells us whether or not a plant is under stress (due to drought, etc).   A stressed plant will have retarded development when compared to the GDD predicted growth stage.   This difference can provide us with an estimate of whether or not the wheat field is going to yield at it's full potential.

Stress can also have an impact on the end-use functional quality of wheat, depending upon what growth stage the stress occurs in.   For this reason, calculating GDD is an important tool in forecasting potential wheat quality prior to harvest.   It gives us one additional tool in working to meet our customer's grain quantity and quality needs.

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