Growing Degree Units (GDUs), Explained
Corn develops on heat, not calendar days. GDUs are how you count that heat — here's the 86/50 formula, how hybrids are rated, and why soybeans don't play by the same rules.
What GDUs are
Growing Degree Units (GDUs, also called growing degree days) measure the heat a crop has accumulated. Corn development runs on temperature — a cool May and a hot May are weeks apart in crop terms even if the calendar says otherwise — and GDUs are how you count it.
The 86/50 formula
The standard method for corn caps the day's high at 86°F and floors the low at 50°F (corn develops little outside that range), then:
Daily GDU = (capped high + floored low) ÷ 2 − 50
A day with a high of 84 and a low of 62 contributes (84 + 62)/2 − 50 = 23 GDUs. A 95° scorcher with a 75° night counts as (86 + 75)/2 − 50 = 30.5 — the cap matters, and skipping it quietly inflates a whole season.
Accumulation starts at planting and runs daily. A central-Illinois season typically delivers on the order of 2,600–2,900 GDUs by black layer.
How hybrids use it
Seed companies publish, per hybrid, GDUs to silking and GDUs to physiological maturity (black layer) — a "2,650 GDU" hybrid needs that much accumulated heat to finish. Comparing your field's running total against the hybrid's published figures is how growth-stage estimates, pollination-window warnings, and harvest-readiness projections are made honestly.
Two cautions from the trenches: use the seed company's published figure, not a number back-derived from relative maturity (RM is a harvest-moisture rating, not a thermal one — same-RM hybrids can differ by 150 GDUs); and check whether a company counts from planting or emergence — the difference is real (~120 GDUs).
Why soybeans are different
Soybean development is driven substantially by photoperiod — day length — not heat alone. That's why beans are grouped by maturity group rather than GDU ratings, and why extension agronomists stage soybeans with planting-date and calendar-day tables instead of heat units. If a tool claims a soybean growth stage from GDUs, be suspicious.
Clevis tracks GDUs per field from your actual planting dates using the 86/50 method against your hybrids' published ratings — it's the backbone of the in-season staging in our corn management guide.
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