Corn maturation is a direct response to the accumulation of heat throughout the growing season. This required energy is measured as thermal time. The standardized agricultural metric used to track this thermal accumulation is the Growing Degree Day (GDD), sometimes called a Growing Degree Unit (GDU). GDD allows growers and researchers to accurately predict when a corn hybrid will reach specific developmental milestones. By quantifying the available heat, GDDs provide a consistent and reliable measure of a plant’s progression from seed to full maturity.
Understanding Growing Degree Days
GDDs are a superior metric to calendar days because they account for yearly variations in weather that influence plant metabolism. A specific corn hybrid requires a constant number of GDDs to reach a growth stage, though the calendar days needed to accumulate those units vary considerably. Corn growth only occurs within a defined temperature window, and GDD quantifies the effective heat contributing to development. GDD provides a daily numerical value representing the heat units available for growth by measuring the difference between the daily average temperature and a specific base temperature. The cumulative total of these daily units throughout the season dictates the plant’s progression toward maturity.
The Calculation Formula and Corn’s Temperature Limits
For corn, GDD calculation uses the 86/50 system, which incorporates the plant’s physiological temperature limits. The base temperature is 50°F (10°C), below which corn growth effectively ceases. The ceiling temperature is 86°F (30°C), above which the rate of growth does not significantly increase.
The standard daily GDD calculation is: GDD = ((Daily High Temp + Daily Low Temp) / 2) – 50. To account for the temperature limits, the daily high temperature is capped at 86°F if it is higher, and the daily low temperature is set to 50°F if it falls below that value. This ensures that extremely hot or cool temperatures are not inaccurately factored into the growth accumulation. For example, a day with a high of 94°F and a low of 60°F would use 86°F for the high. The calculation is ((86 + 60) / 2) – 50, resulting in 23 GDD for that day.
GDD Targets for Corn Maturity
GDD accumulation is tracked from planting until the plant reaches physiological maturity, a stage identified by the “black layer” at the base of the kernel. This black layer signifies that maximum dry weight has been achieved and kernel development has ceased. The total GDD required for a hybrid to reach this final stage is the primary number used to rate its maturity.
The specific total GDD needed varies depending on the hybrid’s genetics, but typical ranges exist for common maturity classes. An earlier-maturing hybrid (e.g., 95-day relative maturity) may require 2,350 to 2,400 GDDs from planting to black layer. A later-maturing hybrid (e.g., 110-day relative maturity) typically requires a higher accumulation, often around 2,670 to 2,750 GDDs.
Several key intermediate milestones also have predictable GDD targets. Corn emergence generally requires 100 to 150 GDDs. The silking stage, which marks the beginning of the reproductive period, typically occurs after approximately 1,500 GDDs. Tracking these intermediate targets allows for timely management decisions, such as fungicide applications.
Hybrid Selection and Environmental Variables
Selecting the correct hybrid is the primary factor influencing the GDD total required for maturity, as genetics determine the inherent heat-unit requirement. Shorter-season varieties complete their life cycle with fewer GDDs, making them suitable for northern climates. Full-season hybrids require higher total GDDs and are suited for regions with longer, warmer summers.
External factors can influence the rate of GDD accumulation and modify the total requirement. For instance, “growing degree compression” can occur when planting is delayed, potentially causing the hybrid to reach maturity with a slightly lower GDD accumulation than expected.
Non-temperature environmental stresses, such as drought or nutrient deficiencies, also disrupt the predictable relationship between GDD accumulation and development. When a corn plant is under significant water stress, its growth rate slows down. GDD provides the best estimate of development under non-stressed conditions.