AbstractThe definitions of the grass growing and grazing seasons as climatic parameters are investigated using a grass growth model. Taking a mean air temperature of 6°C to define the start and end of the growing season at nine UK stations, the model indicates the following: (1) dry matter growth rates at the start are 6 to 9 times greater than those at the end; (2) growth rates at the start of the growing season are higher in those areas where it begins late (i.e. late March/ early April) than in areas where it begins earlier (i.e. late February/ early March). When defining the grazing season in terms of a delay after the start of the growing season, the model indicates that (3) growth rates at the start of the grazing season are higher in upland areas and in areas where the growing season begins late than in lowland areas and in areas where the growing season begins early.A definition of growing season in terms of growth rates shows that 5 and 8°C are more suitable temperatures to define the start and end than 6°C.Grazing season was more adequately described using growth rates than in terms of a delay after the start of the growing seas