Growing Degree Unit Calculator
Corn & Soybean Growing Degree Day Calculator
How GDUs Work:
What Are Corn GDUs?
Corn Growing Degree Units (GDUs), also known as Growing Degree Days (GDDs), are a measure of heat accumulation used to predict the growth and development stages of corn (maize). GDUs quantify the amount of warmth a corn plant experiences over time, which directly influences its progression from planting to maturity. Unlike calendar days, which don’t account for temperature variations, GDUs provide a more accurate way to track how corn responds to its environment.
How Do Corn GDUs Work?
Corn GDUs are calculated using daily temperature data, specifically the average of the maximum and minimum temperatures, adjusted with base and ceiling thresholds that reflect corn’s biological needs. The formula is:
-
Base Temperature: Typically set at 50°F (10°C), because corn growth slows significantly below this point.
-
Ceiling Temperature: Often capped at 86°F (30°C), as corn growth doesn’t increase much beyond this due to heat stress.
-
Adjustments: If the max temp exceeds 86°F, it’s adjusted to 86°F; if the min temp falls below 50°F, it’s set to 50°F.
Example Calculation:
-
Day 1: Max Temp = 78°F, Min Temp = 52°F
-
Average = (78 + 52) / 2 = 65°F
-
GDU = 65 – 50 = 15 GDUs
Each day’s GDUs are added to a running total from planting onward. Corn hybrids are rated by the total GDUs they need to reach maturity (e.g., 2,500 GDUs for a 110-day hybrid). Key growth stages—like emergence, silking, and grain fill—also correspond to specific GDU thresholds.
Corn is a heat-loving crop, and its development is driven by temperature rather than just time. GDUs measure the thermal energy available for processes like:
-
Germination and Emergence: Corn needs about 100-120 GDUs to sprout and emerge from the soil.
-
Vegetative Growth: Leaf development (e.g., V6 stage, 6 leaves) might occur around 500-600 GDUs.
-
Reproductive Stages: Silking (when pollen-receiving silks appear) typically requires 1,200-1,400 GDUs, depending on the hybrid.
-
Maturity: Full grain maturity (black layer formation) aligns with the hybrid’s total GDU requirement.
Because temperatures vary by region and season, GDUs are standardized for these stages across different climates.
Why Are GDUs Important?
GDUs are a critical tool for farmers, agronomists, and researchers for several reasons:
-
Hybrid Selection:
-
Corn hybrids are classified by their GDU requirements (e.g., early-season hybrids at 2,200 GDUs, full-season at 2,800 GDUs). Farmers use GDUs to choose hybrids that match their local growing season length and climate, ensuring the crop matures before frost.
-
-
Planting and Management Timing:
-
GDUs help determine the best planting window. For example, planting too late in a cool region might not allow enough GDUs for maturity. They also guide decisions like applying fertilizers or scouting for pests at specific growth stages.
-
-
Yield Optimization:
-
By tracking GDUs, farmers can predict when corn will reach critical stages like pollination, maximizing yield potential. For instance, drought or heat stress during silking (a GDU-defined window) can drastically reduce yields if not managed.
-
-
Climate Adaptation:
-
As weather patterns shift due to climate change, GDUs help farmers adapt. Warmer seasons might accumulate GDUs faster, allowing longer-season hybrids, while cooler or shorter seasons favor quicker-maturing ones.
-
-
Research and Forecasting:
-
Agronomists use GDUs to model corn development, compare hybrid performance, and forecast harvest dates. This data also feeds into tools like weather apps or crop insurance models.
-
Practical Example
Imagine a farmer in Iowa planting a 2,600-GDU hybrid on May 1, 2025. Using historical weather data, they calculate daily GDUs:
-
May: 300 GDUs
-
June: 450 GDUs
-
July: 600 GDUs
-
August: 550 GDUs
-
September: 400 GDUs
By late September, the total reaches 2,300 GDUs—close but not quite mature. If a frost hits early, the crop might not fully ripen. To plan ahead for this potential, the farmer can utilize Clarity by BAM Weather to monitor season forecasts to optimize the best planting time.
Why It Matters
Corn GDUs transform raw temperature data into actionable insights. They bridge the gap between weather trends and predictable plant growth, empowering farmers to make informed decisions. Utilizing Clarity by BAM Weather, farmers can monitor GDU forecasts on a daily basis and track season accumulations of GDUs for each individual field from planting to harvest.
Monitor GDUs with Clarity by BAM Weather
Track total GDUs from plant date to harvest.
Monitor total GDUs for the entire season with Clarity’s accumulated GDU tool. Simply input your start date (and end date once applicable) and your accumulated GDUs will update daily! Accurate down to the field level – so you can monitor totals for each of your fields!
Monitor the expected GDU value for each day.
Using Clarity’s high and low temperature forecast and the Corn GDU method, we provide a daily estimate of GDUs in our daily forecast tab.
See a full 10-day total GDU forecast.
Get an accurate estimate of expected GDUs over the next 10 days. Simply click any spot on the interactive Clarity radar – or click the dot representing any of your locations to view the 10-day forecast.
Examine trends in GDUs vs. normal.
Will conditions ahead promote more GDUs or less compared to normal for the time of year. Use this tool to help determine whether you can expect growing towards maturity to be faster or slower than normal.