Abstract
Precise calculations of growing degree days (GDD) are an important component in crop simulation models and managerial decisions. Traditional methods for calculating GDD assume linear developmental responses to temperature and cannot precisely account for the delay in growth or development at temperatures above the optimal temperature (T(opt)). A new nonlinear method for calculating GDD was developed. Variations in the prediction of the dates since sowing to various developmental stages and performance measures for describing the accumulation of dry matter by GDD for two widely planted crops (corn and wheat) were used to evaluate the new method in comparison with the traditional methods. The new method predicted the dates of the developmental stages more precisely (date variations reduced by 1 d), and the errors for the predictions of the accumulation of dry matter for winter wheat and corn were smaller. The method was most promising for spring wheat. The new method was more stable and more precise than traditional methods, especially when T(opt) was lower than the maximum air temperature.