Seed dormancy could be a factor related to natural reseeding of hairy vetch (Vicia villosa ssp. villosa Roth.), a winter annual species cultivated for seed, pasture, hay, green manure and cover crop. The presence of combinational dormancy (physical dormancy   physiological dormancy, PY   PD) in hairy vetch was explored by a model using laboratory and field measures. At the stage of natural dispersal, dry seeds of hairy vetch were stored under laboratory conditions at 5, 10, 20 and 30°C (±2°C) or buried at 5 cm depth in an experimental field. Germination at 5, 8, 10, 15, 20 and 25°C was assessed at regular intervals up to 295 days after harvest. Following the hypothesis of the existence of a combinational dormancy mechanism, model development was based on the estimation of: (i) the fraction of non-PY seed as a function of after-ripening thermal-time accumulation, and (ii) seed population thermal parameters associated with a given level of PD. The developed model adequately described the after-ripening thermal-time requirements for PY   PD release of V. villosa. Based on model predictions, under a semi-arid thermal regime, >45% of vetch seeds shed during the summer season would be able to germinate during early autumn. Thus, the seed-bank size threshold at the end of the first growing season should be >65 seeds m^–2 in order to reach a minimum stand of 30 plants m^–2 necessary for a productive pasture.