A computer model was developed to predict lactation performance responses of ewes to concentrate supplementation. The model considers effects of concentrate supplementation on forage intake, rumen pH and forage digestibility. The user defines body weight, feed composition and concentrate feeding times and amounts. Rumen pools of organic acid from carbohydrate fermentation and buffer from rumination are simulated to determine rumen pH. The pH, in turn, affects fiber degradation rates. Forage intake continues during daylight hours until rumen NDF exceeds 1% body weight (kg), or organic acid concentration exceeds 130 mM. Daily milk production is calculated from the absorption of energy and intake of protein and fat. The model predicts the substitution effect on forage intake of increasing rates of concentrate supplementation, the temporal pattern of rumen pH fluctuation with multiple concentrate feedings per day, and increase in dry matter intake when concentrate meals and amount increases.