Modeling the Occurrence of Reproductive Stages after Flowering for Four Soybean Cultivars

Soybean growth and yield models require good predictions of vegetative and reproductive development stages, as a function of specific environmental variables. Parameters for predicting R5 (beginning of seed growth) and R7 (physiological maturity) dates of four soybean cultivars were estimated for a development rate model. R5 is predicted by accumulating a daily rate of development, which depends on night length and temperature, starting at Rl (flowering) until a threshold is reached. Daily development rate is computed by a multiplicative relationship containing two linear‐plateau 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. The downhill simplex method was used to estimate phenological parameters for each cultivar, minimizing the error sum of squares between observed and simulated dates of R5 occurrence. The same type of model and methodology was used to estimate parameters for R7 prediction, but beginning at R5 until a threshold is reached. The results indicate that as plants develop from V1 (first true leaf) to R7 they become more responsive to photoperiod and less sensitive to temperature. Differences among cultivars with respect to optimal night length tend to diminish as the plants approach physiological maturity; with respect to temperature, the reverse happens. The optimal temperature for reproductive development varied between 25 and 29 °C, without great differences among cultivars of differing maturity, but with a slight increasing trend from V1 toward R7.