We have developed a composite dynamic simulation model as the basis for a decision support tool to improve the efficiency of late-spring sidedress nitrogen (N) applications to maize (Zea mays L.) in New York State. Our previous research has shown that spring weather conditions are a critical factor affecting soil N availability and optimum fertilizer rates. The model consists of components of rigorously tested maize/N (Sinclair and Muchow, 1995) and soil N/water (LEACHN; Hutson, 2003) dynamic simulation models. Model output was compared to measured data from multiple year studies at several locations in New York State and southern Ontario. The locations were non-manured sites and represented a range of soil texture classes. Simulated values of critical crop and soil components of the model were compared with field measurements of these components. These included leaf area index, N uptake (kg ha-1) and above ground biomass (kg ha-1) during vegetative growth and grain fill, and stalk, leaf and grain N (kg ha-1) and grain dry matter yield (kg ha-1) at maturity. We compared simulated and measured soil NO3-N (kg ha-1) and water flux below the root zone for specific times during the growing season for a limited number of data sets. Model-generated optimum sidedress N rates were also compared with optimum sidedress N rates for those studies where these data were measured. Model performance will be evaluated based on these comparisons and areas for improvement will be discussed.