PARAMETER-ESTIMATION FOR PREDICTING FLOWERING DATE OF SOYBEAN CULTIVARS

Soybean [Glycine mar (L.) Merr.] growth and yield models depend on good predictions of phenological events such as flowering. Parameters for predicting flowering date of 12 cultivars were estimated for various development-rate models. Date of flowering is predicted by accumulating a daily rate of development, which depends on night length and temperature, until a threshold is reached. Daily development rate is computed by a multiplicative relationship containing two functions: one for describing the variation in development rate with night length under optimal temperature and the other describing variation with temperature under optimal night length. There were 39 to 115 year-location-sowing date combinations for each cultivar, covering latitudes from 18-degrees 03' to 45-degrees 25' N lat. The downhill simplex method was used to estimate phenological parameters for each cultivar by minimizing the error sum of squares between observed and simulated flowering dates. Many formulations of the development-rate model were compared. Linear-plateau functions for both night length and temperature effects provided the best fit and yielded the most consistent results. The root mean square error between observed and simulated dates of flowering ranged from 3.45 to 5.28 d. Correlation coefficients between observed and simulated days from sowing to flowering varied from 0.987 to 0.841, with a decreasing trend from late toward early-maturity cultivars. There was a clear difference among cultivars with respect to night-length sensitivity, but a similar response to temperature.