A daily time step model simulating growth and quality of cool season grasses was developed and validated for smooth brome (Bromus inermis Leyss.) under varying environmental and management conditions. Growth predictions are based upon genetic potential, environmental temperature, leaf area, soil moisture and nitrogen fertilization rate. Daily potential growth rate is a composite of two functions; one relating grass species maximum growth rate to minimum, optimum and maximum temperatures for plant growth, and another relating daily minimum and maximum air temperatures to time. Multipliers are developed for available leaf area, moisture and nitrogen, and the minimum of these values modifies the daily potential growth rate to determine daily plant production. Change in sward dry matter digestibility (DMD) is a function of forage material present, daily forage growth and environmental temperature. For dry matter (DM) production, the relationship between model (Y) and observed (X) estimates (kilograms of dry matter per hectare) yielded the regression equation: Y = 218 + 0·94X; SE = 431; R2 = 0·98. The relationship between model (Y) and observed (X) estimates of DMD (%) gave the regression equation: Y = 2·24 + 0·97X; SE = 1·78; R2 = 0·90. The above validation shows that the mathematical logic contained within the plant model accurately predicted smooth brome production and changes in forage quality. Intercept and slope values were similar to 0·0 and 1·0, respectively, and standard error values were similar to observed experiments.