13.4 Growing Degree Days (GDD) & Thermal Unit Calculations

13.4 Growing Degree Days (GDD) & Thermal Unit Calculations

Crop development is overwhelmingly driven by temperature rather than calendar days. A cool spring can significantly delay growth, while a hot summer can accelerate it. To accurately predict crop development stages, estimate maturity dates, and manage pests, agronomists rely on heat unit calculations, most commonly known as Growing Degree Days (GDD) or thermal units. This section details the fundamental concepts and calculations behind GDD, with a specific focus on corn.

The Concept of Growing Degree Days

Plants require a certain amount of accumulated heat to progress from one developmental stage to the next. The GDD system quantifies this heat accumulation by calculating the daily average temperature and comparing it to a baseline temperature below which the plant does not grow. By summing the GDD values for each day over the course of a season, we can track the physiological progress of the crop.

Using GDD instead of calendar days allows for consistent predictions across different years and geographic locations. For example, a specific corn hybrid may always require 1,200 GDD to reach silking, regardless of whether it takes 55 calendar days in a warm year or 70 days in a cool year.

The General GDD Formula

The fundamental formula for calculating the Growing Degree Days for a single day is:

GDD = [(Daily Maximum Temperature + Daily Minimum Temperature) / 2] - Base Temperature

The variables are defined as:

• Daily Maximum Temperature (Tmax): The highest temperature recorded during a 24-hour period.
• Daily Minimum Temperature (Tmin): The lowest temperature recorded during the same period.
• Base Temperature (Tbase): The minimum temperature required for the specific crop to grow. If the daily average temperature is below the Tbase, the GDD for that day is calculated as zero (negative GDD values are not accumulated).

Different crops have different base temperatures:

• Corn: 50°F (10°C)
• Soybeans: 50°F (10°C)
• Wheat/Small Grains: 32°F (0°C) or 40°F (4.4°C) depending on the specific model.
• Cotton: 60°F (15.5°C)

The Modified 86/50 Formula for Corn

Corn has specific temperature thresholds for optimal growth. It does not grow when the temperature drops below 50°F, and its growth rate plateaus and can even become stressed when temperatures exceed 86°F (30°C). To account for this physiological reality, agronomists use a modified GDD formula for corn, often referred to as the "86/50 cutoff method."

In this method, the standard formula is used, but specific limits (caps) are placed on the Tmax and Tmin values before the calculation is performed:

1. Minimum Cap (Tmin Rule): If the actual Daily Minimum Temperature drops below 50°F, it is adjusted to 50°F for the purpose of the calculation. This accounts for the fact that the plant doesn't "lose" ground during cold nights; it simply stops growing.
2. Maximum Cap (Tmax Rule): If the actual Daily Maximum Temperature exceeds 86°F, it is adjusted down to 86°F for the calculation. This reflects that extreme heat does not proportionally accelerate growth and may, in fact, cause heat stress that slows physiological development.

Example GDD Calculations for Corn

Let's calculate the GDD for corn (Tbase = 50°F, Max Cap = 86°F) under a few different weather scenarios.

Scenario 1: An Ideal Summer Day

• Actual High: 82°F
• Actual Low: 60°F
• Application of caps: Neither cap is triggered. Tmax = 82, Tmin = 60.
• Calculation: [(82 + 60) / 2] - 50 = [142 / 2] - 50 = 71 - 50 = 21 GDD

Scenario 2: A Hot Summer Day

• Actual High: 95°F
• Actual Low: 70°F
• Application of caps: The high exceeds 86°F, so Tmax is capped at 86. Tmin = 70.
• Calculation: [(86 + 70) / 2] - 50 = [156 / 2] - 50 = 78 - 50 = 28 GDD
  • (Note: Without the cap, the calculation would yield 32.5 GDD, which overestimates the plant's actual physiological progress on a stressfully hot day.)

Scenario 3: A Cool Spring Day

• Actual High: 65°F
• Actual Low: 42°F
• Application of caps: The low is below 50°F, so Tmin is adjusted to 50. Tmax = 65.
• Calculation: [(65 + 50) / 2] - 50 = [115 / 2] - 50 = 57.5 - 50 = 7.5 GDD

Scenario 4: A Very Cold Spring Day

• Actual High: 48°F
• Actual Low: 35°F
• Application of caps: Both values are below 50°F. Tmax is adjusted to 50, and Tmin is adjusted to 50.
• Calculation: [(50 + 50) / 2] - 50 = [100 / 2] - 50 = 50 - 50 = 0 GDD
  • (The crop accumulated no heat units and made no developmental progress.)

Practical Applications of GDD

Agronomists and farmers use GDD tracking for several critical management decisions:

1. Hybrid Selection: Seed companies rate corn hybrids by the total GDD required to reach black layer (physiological maturity). A farmer in North Dakota must select a hybrid with a lower GDD requirement (e.g., 2,200 GDD) than a farmer in Iowa (e.g., 2,700 GDD) to ensure the crop matures before the first killing frost.
2. Predicting Growth Stages: GDDs are used to predict when critical stages like V6, VT (tasseling), or R1 (silking) will occur. For instance, emergence typically requires about 90 to 120 GDDs after planting. Silking might require 1,400 GDDs.
3. Pest Management: Many insects, such as corn rootworm or European corn borer, also develop based on heat units. By tracking GDDs, agronomists can predict insect emergence and time insecticide applications for maximum efficacy.
4. Harvest Scheduling: Tracking GDDs after silking helps predict when the crop will reach maturity and dry down, allowing for better harvest planning.

While GDD is a powerful tool, it is an estimation model. Other factors, such as day length, soil moisture, and severe nutrient deficiencies, can also influence the rate of crop development. However, thermal tracking remains the most reliable baseline for agronomic planning.