The rate of growth and development of corn plants is highly correlated with temperature. Scientists have determined that little or no growth and development occurs in corn below 50 degrees F (10 degrees C). It has also been determined that temperatures above 86 degrees (30 degrees C) can be stressful to corn plants. Using these two values as upper and lower thresholds, air temperature measurements are used to monitor and predict phenological development in corn. Commonly used terms for these calculations are Growing Degree Days, Growing Degree Units, and Heat Units. For an overview of Corn Growing Degree Days, see an excellent article offered by the North Dakota Agricultural Weather Network (NDAWN).
A climate normal is defined as the average of a variable over a continuous three-decade period. At the end of each decade, a new 30-year average is calculated. The statistic is a measure of the central tendency of the variable. Minnesota's highly variable climate leads to large year-to-year swings above and below this benchmark value.
The maps below depict normal seasonal (May-September) corn growing degree days (GDD) for two periods: the most recent normal period (1981-2010), and the preceding normal period (1971-2000). Click on the maps to view larger renditions. The newest of the normal maps indicates relatively little change in total seasonal corn GDDs from the previous normal period. The average change statewide is an increase of less than one percent. For some counties in southwest, south central, east central, northwest, and northeast Minnesota, an increase of roughly two percent is noted. For a handful of locations, a small decrease in total GDDs is seen.
Those who follow climate trends in Minnesota may be surprised to learn of the modest increases observed in growing degree day totals for this most recent normals period. Intuitively, one would expect well-documented rising temperatures to lead to increased seasonal growing degree day totals. A few thoughts on this apparent inconsistency:
- Much of the increase in annual average temperatures in Minnesota is due to significant rises in winter temperatures, especially winter minimum temperatures. To visualize seasonal temperature trends, see this presentation tool.
- As described in the NDAWN explanation of the corn growing degree day calculation methodology (referenced above), minimum temperatures below 50 degrees F are artificially lifted in the equation to equal 50 degrees F before computing the average temperature for the day. It is thought that much of Minnesota's growing season warming trend is attributable to increases in overnight temperatures. Therefore, the artificial mathematical elevation of minimum temperature values in the growing degree day arithmetic may mask time trends in the growing degree day metric. Also, any upward trends in the number of daily maximum temperatures above 86 degrees F will be unaccounted for in the calculation technique.
It should also be noted that there is a discernible tendency towards an early onset of spring in Minnesota. This phenomenon, along with research linking planting dates to production, leads producers towards progressively earlier statewide average planting dates. Earlier planting often leads to earlier emergence. The earlier-emerging plants are taking advantage of heat units previously unavailable to seed still in the ground.