Results and discussion Results and Discussion Figure 3. Observed (symbols) and simulated (lines) V-Stages of soybean cultivars (MG 3.0 to 3.9) grown at Lincoln, Nebraska in Colors of the symbols and lines indicate V-Stages of plants from difference emergence dates: 10-May (red), 23-May (blue), 7-June (green), and 24-June (yellow). Table 1. Cultivar information (associated with model parameters), final node number and root mean square error (RMSE) of V-Stages simulation for each of the 14 soybean cultivars and emergence dates. MG= Relative maturity group Type: ID = Indeterminate, SD = Semi-determinate DG = V-End indicator group for indeterminate cultivars V n,10-May = Final node number for 10-May emerged plants Figure 4. Mean and standard error of V-End Indicator (a), maximum node appearance rate (R LA,MAX ) (b), R1-R5 duration (c), and final number of nodes (V n ) (c) for each of the emergence dates. On average, vegetative stage prediction can be achieved with a precision of less than 1 node using the proposed model as long as the effects of emergence dates and maturity on maximum node appearance rate and that of emergence date on V-End indicator are taken into account. The effect of emergence date on V-Stages simulation parameters may be linked to the inter- relationship between phenology and growth and warrants further investigation. Shorter R1 to R5 duration, smaller maximum node appearance rate, and earlier V-End indicator cause the late planted (2-June and 17-June) soybeans to end with approximately 4 nodes less than the earlier planted (28-April and 16-May ) soybeans. Variables definitions V-Stages simulation begins at unifoliate stage (V1), which is simulated using Eq. 1. and emergence (Emg) as the starting point. Afterwards, rate of main stem node appearance is driven by daily average temperature (Eq. 2 and Fig. 2a) and V-Stages for a given day is calculated as a cumulative node appearance rate (Eq. 3) and modifiable by a dimensionless (0-1) chronological function (CF) at the later phase of V-Stages. During the initial phase of V-Stages CF equals 1. Determination of end of V-Stages requires input from the reproductive development sequence, particularly the assignment of a V-End indicator (D). The V-End indicator is dependent on the V-End coefficient (r D ), which is a dimensionless developmental progress from R1 toward R5. R1 to R5 developmental rate is driven by temperature and photoperiod functions (Eq. 4). Once V-End indicator is reached, a chronological function associated with reduction of node appearance rate is calculated using Eq. 5. Different sets of parameters of the chronological function are used for indeterminate and semi-determinate cultivars to describe the differences in node appearance rate of the two types of soybean near the end of vegetative development (Fig. 2b). Figure 2. Temperature function (a) and chronological function (b) used in simulation of soybean node appearance (vegetative stage). Field Experiments Data for model development and evaluation was obtained from an irrigated field experiment at the University of Nebraska, Lincoln, Nebraska, in Fourteen soybean cultivars (maturity group 3.0 to 3.9) were grown on a conventionally tilled Kennebec silt loam (Cumulic Hapludolls) following corn at four different planting dates (28-April, 16-May, 2-June, and 17-June). The row spacing, seeding rate, and planting depth was 76 cm, 370,658 seeds ha -1 and 2.54 to 3.84 cm, respectively. The seed yield obtained from the experiment was from 3.2 Mg ha -1 (averaged 17-June planting) to 4.6 Mg ha -1 (averaged 28-April planting). Vegetative and reproductive stages (Fehr and Caviness, 1977) were recorded biweekly by using a numerical average of 10-random plants for each of the cultivar x planting date treatments. Abstract Development and evaluation of an approach to predict soybean V- stages (node appearance) is being presented. V-Stages are simulated using two main functions: 1) a non-linear temperature function associated with rate of main stem node appearance, and 2) a chronological function related with decreasing node appearance near the end of V-Stages. Model evaluation based on 13 indeterminate maturity group III cultivars and a semi-determinate cultivar at Lincoln, NE in 2004 indicates a satisfactory prediction of V- Stages with overall RMSE of 0.7 nodes. Maximum node appearance rate did not differ between cultivars with MG 3.0 to 3.6, but a higher rate was found in MG 3.9. Maximum node appearance rate and signal determining end of V-Stages tended to be lower at later planting dates. These phenomena along with shorter duration between R1 and R5 are responsible for the observed lower number of final nodes among the late planted plants. Rationale Accurate prediction of vegetative development (node appearance) in soybean is useful for scheduling field management and simulating leaf area index (Sinclair soybean model) and plant height (CROPGRO model). Soybean node appearance responds to temperature under controlled environment in a non-linear fashion (Hesketh et al., 1973). A type of non-linear temperature function, the beta function, can describe developmental response to temperature in several crops such as cassava, maize, rice, and sorghum (Yan and Hunt, 1999; Yin et al., 1995) and was used to predict leaf appearance rate in winter wheat (Streck et al., 2003). The beta function version used in Wang and Engel ( 1998) and Streck et al. (2003) is of particular interest because of simplicity of its parameters. Soybean vegetative development under field conditions and how final node number is determined, however, is more complicated because there is an apparent interaction with reproductive stage as seed development and growth compete for available assimilates. Objective To develop soybean vegetative stage simulation by using a beta function to describe the response of node appearance rate to temperature and by relating the developmental progress with reproductive stage. Material and Methods Model Description Figure 1. Sequence description and equations used in the model for simulation of soybean vegetative development. Soybean Phenology: Simulating V-Stages (Node Appearance) Using Non-Linear Temperature and Chronological Functions Related to Reproductive Stage T. Setiyono 1), A. Dobermann 1), A. Weiss 2), J. Specht 1), A. Bastidas 1) 1) Department of Agronomy and Horticulture, University of Nebraska-Lincoln 2) School of Natural Resources, University of Nebraska-Lincoln