Growing Degree Days (GDD)

Growing Degree Days (GDD): heat units that drive crop development

GDD tracks how much heat has built up above a crop's base temperature (Tbase), which is the point below which growth basically stops. You compute it with GDD = ((Tmax + Tmin) / 2) − Tbase, and any negative result is clamped to zero. Common Tbase values run around 10 °C for corn/maize and soybeans, 5 °C for wheat, and 7 °C for tomato. Take Tmax = 28 °C, Tmin = 16 °C, Tbase = 10 °C: that works out to GDD = 22 − 10 = 12 °C·day. Add those daily values up over a season and you can predict phenological stages. Corn usually needs about 2,700 GDD from sowing to harvest, while soybeans land somewhere between 2,400 and 3,000 GDD depending on the cultivar.

Applications

Precision agriculture leans on GDD for picking sowing windows, planning harvests, and forecasting pest and disease pressure since many insect life cycles accumulate GDD too. It also guides variety selection by region. In Brazil it feeds into platforms like EMBRAPA Sigma and the Agroclimatic Zoning (ZARC) that determine credit and insurance eligibility.

FAQ

What if Tmin is below Tbase? The simple method just caps the mean at Tbase and returns 0. If you grow fruit, the modified and sine-wave methods deal with cool nights more accurately.

Is GDD the same across cultivars? No. An early-maturing corn hybrid hits harvest with fewer GDD than a late-maturing one. Check the seed packet or an EMBRAPA bulletin for the target that matches your cultivar.

Does GDD account for water stress? No, it watches temperature and nothing else. Drought, frost, or waterlogging can hold development back even when the crop has banked plenty of heat units.