This study analysed the effects of shortening the breeding period from 85-90 to 63-66 days on herd fertility. The data set included 1,063 calving events comprising two breeds (Parda de Montaña, PA, and Pirenaica, PI), two calving seasons (autumn and spring) and 8 years (1999 to 2006). Overall actual pregnancy rate over a breeding period of 85-90 days was moderate in both seasons, but slightly higher in autumn than in spring (68% vs. 52%; P<0.10). Nevertheless, pregnancy rate within the first 63-66 days was similar in both seasons (56% vs. 46%; P>0.10). The day 65 after initiation of the calving period 86% and 92% of total calving had occurred in autumn and spring, respectively, regardless the season and year (P>0.10). Short breeding periods may allow for concentrated calving seasons, resulting in greater calf homogeneity at weaning and a more efficient labour organisation.