In the Netherlands, rotational grazing, with grazing periods of 2 to 5 days, is the most common grazing system at present. In contrast with other countries of North-western Europe, the continuous grazing system is used here only to a limited extent. However, the results of numerous comparative trials at high nitrogen fertilization levels and high stocking rates, carried out in the 1970's, suggest that there is no significant difference in animal production between the two grazing systems.Experiments were carried out to determine the physiological and environmental limits to herbage production under continuous and rotational grazing. This was done by measuring the seasonal patterns and seasonal totals of sward CO 2 assimilation and animal production. The experiments were carried out on heavy clay soils at two nitrogen fertilization levels (125 and about 450 kg N ha -1yr -1). With 450 kg N ha -1yr -1cumulative gross assimilation over the grazing season was 9% higher with rotational than with continuous grazing, but there was no difference in animal production. The higher efficiency of utilization of gross assimilation products under continuous grazing was due to lower topping losses and lower costs of above-ground maintenance respiration. Under both grazing systems, gross CO 2 assimilation per unit leaf area was not depressed at all at 125 kg N ha -1yr -1. but there was a marked reduction of the rate of leaf area development in the second half of the grazing season. The absence of any effect in the first half of the grazing season was due probably to a residual effect of previously applied nitrogen. This effect can be considerable on heavy clay soils.The herbage intake under rotational grazing is often estimated using Linehan's formula, which takes into account the herbage production during grazing. This formula was evaluated by means of dynamic simulation, with measured assimilation-light response curves as the main input. It emerged that in some practical situations of rotational grazing, the herbage production ...