Abstract A model was constructed to simulate the canopy development of a vegetative grass sward, continuous grazed to a constant mean height. Additional submodels were constructed or adapted to simulate grazing and the physiological response of the plants to defoliation and the environment. This enabled growth, its partitioning between offtake and senescence, green sward mass and leaf area index to be simulated over a range of mean sward heights and under a range of environments. The changes in above ground growth, offtake and senescence rates with increasing mean sward height predicted by the model were similar to those previously observed in the field. The model predicts little change in the rate of offtake over a range of mean sward heights from 4 to 15 cm, independent of the environment. The relationships between both green sward mass and leaf area index and mean sward height are also predicted to be independent of the environment. Simulated values of green sward mass and leaf area index at a given mean sward height were slightly higher than those previously observed in the field. The model suggests that mean sward height should provide a reliable indicator of the balance between senescence and offtake and a good predictor of the green sward mass and leaf area index of moderate to intensively managed grassland.