Control of gastrointestinal strongylosis in heifers is often based on systematic treatments at standard periods, without any adaptation to the specific parasitic risk for the animals in each herd. This parasitic risk depends on the immunological status against gastrointestinal nematodes (GIN), and on the pasture infectivity level (PIL) (GIN larval challenge). A model based on these 2 key factors has been developed to evaluate in field conditions parasitic risk periods. The development of immunity is modeled using data regarding heifers’ grazing and treatment history: the time of effective contact (TEC, in months) with GIN larvae is calculated, heifers being considered as non-immune when TEC<8 months. The increasing PIL during the grazing season results from the succession of parasite cycles. In the model, this increase is evaluated calculating the numbers parasitic cycles realized since turn out, the duration of one cycle being the addition of the prepatent period and the development time from egg to infective larvae. As this development time depends on temperature, it is calculated using a previously model developed by Smith et al. based on daily average temperature. By taking into account the grazing management (one or several pastures, time spent on each pasture), a parasitic risk is estimated by the model when non-immune heifers are grazing a pasture where the PIL generated by the number of parasitic cycles is considered to be high enough to represent a danger (e.g. to cause weight gain losses or clinical signs). As long as such heifers remain on such a pasture, the risk period continues, and different therapeutic or agronomic control strategies can then be tested and recommended. This model is a useful tool on the field to optimize the use of anthelmintics in the control of GIN through targeted treatment.