Technical Abstract: Changing climate, growing costs of agricultural inputs, and social and environmental demands are challenges facing agriculture in the 21st century. There is a need for decision support and assessment tools including crop simulation models for estimation of yield and biomass, and management of nutrients and water usage by agricultural crops under existing and future climatic conditions. Among the variables simulated by crop growth models, green leaf area plays a critical role for crop growth and development. Leaf area determines the fraction of incident photosynthetically active radiation (PAR) intercepted by crop canopy and ultimately dry matter production. A majority of simulation models in use today use the thermal time concept of accumulated growing degree days (GDD) with a base temperature. The disadvantage of the GDD concept, however is that it is a linear scale. The metabolic responses of plants to temperature are not necessarily linear, even during the increasing phase, and have an optimum rate after which the rates decline steeply. In this study we used a beta function to represent the response of leaf initiation, addition and expansion rates to temperature in the maize model MAIZSIM. Equations for leaf expansion based on GDD were reformulated and reparameterized to utilize rates based on current temperatures. The parameters for the equations were fit using data from outdoor sunlit growth chambers. The model, MAIZSIM, was then used to simulate maize growth and development at two field sites for three years of data. No further calibration was performed. The model estimated leaf area, leaf addition rate and biomass well. These results suggest that the use of a more realistic representation for temperature response in crop models can reduce the need for parameterization.