Previously we developed SOYDEV model that predicts major phenological stages including unifoliate stage (V1). We expanded the phenology simulation to include node appearance (V-Stages) (Fehr and Cavines staging system) from V1 to the appearance of the last node on the main stem (V-End). V-stage is an important parameter that can be used to simulate leaf area index (Sinclair's soybean model) and plant height (CROGRO model). V-stage is simulated by calculating the rate of trifoliate node appearance starting at V1 using a non-linear temperature function and variable LARmax, which is maximum leaf appearance rate at optimum temperature. Near the end of node appearance a chronological function was assigned to reflect decreasing leaf appearance rate and was associated with distinct developmental unit toward R5 in semi-determinate type or toward R7 in indeterminate type. Evaluation of the model for NE3001 (maturity group 3.0, semi-determinate) in 2003 and 2004 and different planting dates in 2004 indicates a satisfactory prediction of V-Stages with RMSE ranging from 0.39 to 1.12 nodes. Among indeterminate cultivars we tested (ranging in maturity group from 3.1 to 3.9), we obtained a very similar LARmax, thus inferring that trifoliate node appearance is relatively constant, while developmental unit toward R7 that initiate reduction of node appearance rate appear to differ between cultivars and not necessarily associated with maturity group.
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