How this instrument works
Growing Degree Units (GDU), also called modified Growing Degree Days, measure how much heat accumulated in a day that a corn plant can actually use for growth. Corn's growth rate doesn't keep climbing past about 86°F, and it effectively stops below about 50°F, so a plain average of the day's high and low would overstate heat accumulation on very hot days and understate it on cool mornings. The modified method fixes that by capping the day's high at 86°F and flooring the day's low at 50°F before averaging.
The formula runs in three steps: cap Tmax at 86°F if it's higher (a 95°F day still only counts as 86°F for this purpose), floor Tmin at 50°F if it's lower (a 40°F morning still only counts as 50°F), average the capped high and floored low, then subtract the 50°F base temperature below which corn doesn't meaningfully grow. This cap-and-floor rule is the specific detail that distinguishes corn GDU from a plain GDD formula, and it was proposed by Barger in 1969 as the standard heat-unit method later adopted for corn development tracking.
Farmers and agronomists accumulate daily GDU totals across a season to track corn's progress toward key stages — emergence typically needs roughly 90-120 accumulated GDU, and each hybrid has a GDU rating (often called relative maturity or CHU/GDU rating) indicating how many total units it needs to reach physiological maturity. Tracking accumulated GDU against a hybrid's rating helps growers predict harvest timing and compare how a season's heat is running against a normal year.
- Enter the day's high temperature into Daily high temperature (°F).
- Enter the day's low temperature into Daily low temperature (°F).
- Read Capped high (86°F ceiling) and Floored low (50°F floor) to see how each raw reading was adjusted before averaging.
- Read Growing Degree Units (modified GDD) for that day's total — add each day's GDU across the season to track accumulated heat units.
- Enter both readings in Fahrenheit; a very hot high above 86°F or a cool low below 50°F will show its capped or floored value automatically, not the raw number you entered.
Worked example — a 90°F high, 45°F low
Enter 90 into Daily high temperature (°F) and 45 into Daily low temperature (°F) — a hot day with a cool morning. Capped high (86°F ceiling) reads 86.00 (the 90°F high is capped, since it exceeds 86°F), Floored low (50°F floor) reads 50.00 (the 45°F low is floored, since it's below 50°F), and Growing Degree Units (modified GDD) reads 18.00: ((86+50)/2)-50 = 68-50 = 18.0 GDU.
That result shows exactly why the cap-and-floor rule exists: without it, the raw average of 90 and 45 is 67.5, giving 17.5 GDU instead of 18.0 — a small difference here, but on more extreme days the gap between the raw and modified calculation grows much larger, which is why the modified method is the one actually used for corn development tracking.
Questions
Why cap the high temperature at 86°F instead of using the actual reading?
Because corn's growth rate stops increasing once temperatures climb past about 86°F — a 95°F day doesn't grow corn any faster than an 86°F day, and a plain average would incorrectly credit the extra heat as useful growing degrees. Capping Tmax at 86°F before averaging keeps the GDU total from overstating how much growth actually happened on very hot days.
Why floor the low temperature at 50°F?
Because corn growth effectively stops below about 50°F, so a cold morning low of, say, 35°F contributes no more to growth than a 50°F morning would — but a plain average would still drag the day's number down by the full amount. Flooring Tmin at 50°F prevents an unusually cold night from making the day's GDU total artificially, and inaccurately, low.
What's the difference between GDU and plain GDD?
Plain GDD (Growing Degree Days) just averages the day's high and low and subtracts a base temperature, with no upper or lower limit on the inputs. GDU, sometimes called modified GDD, adds the 86°F cap and 50°F floor specifically calibrated to corn's growth response, which is why it's the version corn producers and agronomists use rather than a generic GDD formula built for a different crop.
How many GDU does corn need to emerge, and to reach maturity?
Corn typically needs roughly 90-120 accumulated GDU to emerge from planting, while reaching physiological maturity takes anywhere from about 2,200 to over 3,000 accumulated GDU depending on the hybrid's relative maturity rating. Seed companies publish a GDU (or CHU) rating for each hybrid, letting growers estimate maturity date by tracking accumulated GDU against that rating through the season.
Can two very different days produce the same GDU total?
Yes — because both the high and low get capped or floored before averaging, a scorching 95°F/40°F day and a milder 86°F/50°F day can land on an identical GDU total once both extremes are clamped to the same 86°F and 50°F limits. This is a direct consequence of the cap-and-floor rule, not a coincidence of the arithmetic.