Purdue University Department of Agronomy

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Nov 2012
URL: http://www.kingcorn.org/news/timeless/HeatUnits.html

Heat Unit Concepts Related to Corn Development

R.L. (Bob) Nielsen
Agronomy Dept., Purdue Univ.
West Lafayette, IN 47907-2054
Email address: rnielsen at purdue.edu

rowth and development of corn are strongly dependent on temperature. Corn develops faster when temperatures are warmer and more slowly when temperatures are cooler. For example, a string of warmer than normal days in late spring will encourage faster leaf development than normal. Another example is that a cooler than normal grain filling period will delay the calendar date of grain maturity.

The phrases "string of warmer than normal days" and "cooler than normal grain filling period" can be converted mathematically into measures of thermal time by calculating the daily accumulations of heat using temperature data. Commonly used terms for thermal time are Growing Degree Days (GDDs), Growing Degree Units (GDUs), or heat units (HUs).

Different methods exist for calculating heat units depending on a) the crop or biological organism of interest and b) the whim or personal preference of the researcher. The calculation method most commonly used throughout the U.S. for determining heat unit accumulation relative to corn phenology is the formula first suggested by the National Oceanic and Atmospheric Administration in 1969 and labeled as the "Modified Growing Degree Day" formula in 1971.

This method calculates daily accumulation of GDDs as the average daily temperature (degrees F) minus 50. The "modification" refers to the limits imposed on the daily maximum and minimum temperatures allowed in the calculation. Daily maximums greater than 86 degrees F are set equal to 86 in the calculation of the daily average temperature. Similarly, daily minimums less than 50 degrees F are set equal to 50 in the calculation.

Example 1:
If the daily maximum temperature was 80 degrees F and the minimum was 55 degrees F, the GDD accumulation for the day would be ((80 + 55) / 2) - 50 or 17.5 GDDs.

Example 2 (Illustrating the limit on daily maximums):
If the daily maximum temperature was 90 degrees F and the minimum was 72 degrees F, the GDD accumulation for the day would be ((86 + 72) / 2) - 50 or 29 GDDs.

Example 3 (Illustrating the limit on daily minimums):
If the daily maximum temperature was 68 degrees F and the minimum was 41 degrees F, the GDD accumulation for the day would be ((68 + 50) / 2) - 50 or 9 GDDs.

In late April to early May, normal daily GDD accumulations for central Indiana are about 10 GDDs. By late July, the normal daily accumulation rises to about 23 GDDs. For a typical corn growing season in central Indiana, say from late April to late September, the total seasonal accumulation of GDDs is about 2800 GDDs.

Historical normal GDD accumulations on a weekly basis for Indiana's nine Crop Reporting Districts are listed in the accompanying table and are also available in Purdue's Corn & Soybean Field Guide, Extension publication ID-179. These historical GDD values can be used to estimate weekly GDD accumulations if you do not have access to current temperature data.

Obviously, the ability to calculate daily heat unit accumulations is dependent on your having access to daily maximum and minimum temperatures. If you do not own your own max / min recording thermometer, you can guesstimate the daily highs and lows by manually recording the temperatures shortly after sunrise (approximate daily low) and late in the afternoon (approximate daily high).

Another option is to try your luck with the online Indiana State Climate Office data archive. At this Web site, you can search for temperature data from the automated Purdue weather stations located at a number of outlying Purdue Ag. Centers, cooperative weather stations maintained by the National Weather Service, and airport weather stations also maintained by the National Weather Service. Also, the weekly Purdue Pest & Crop Newsletter lists accumulated GDDs for selected sites across the state based on various start dates.

Related References

Corn & Soybean Field Guide (ID-179). 2012. Purdue Univ. Extension. To order, call Purdue Extension toll-free at (888) 398-4636.

Nielsen, R.L. (Bob). 2008. Use Thermal Time to Predict Leaf Stage Development in Corn. Corny News Network, Purdue Univ. [On-Line]. Available at http://www.kingcorn.org/news/timeless/VStagePrediction.html. [URL accessed Nov 2012].

Nielsen, R.L. (Bob). 2010. Determining Corn Leaf Stages. Corny News Network, Purdue Univ. [On-Line]. Available at http://www.kingcorn.org/news/timeless/VStageMethods.html. [URL accessed Nov 2012].

Purdue Pest & Crop Newsletter. 2012. Purdue Univ. Pest Mgmt. Program. [On-Line]. Available at http://extension.entm.purdue.edu/pestcrop/index.html. [URL accessed Nov 2012].