The aim of the study was to determine the average length of gestation and its variation in a population of Silesian mares bred during a period of 55 years (1945-2000) in national studs and private farms in southern Poland. Considered was the sex of foals, season, month and year of birth,environmental conditions (national or private breeding centres), breeding history of a mare (number of foals born)and influence of the preceding reproductive period.Gestation period in Silesian mares lasted on average 338.66 days ±13.57 and ranged from 299 (the shortest) to 386 (the longest) days. Gestations lasting from 311 to 356 days were considered normal for the breed. Gestations resulting in a birth of a colt lasted one day longer (339.23±13.44 days compared to 338.19±13.60 days for fillies). Gestations from the autumn reproductive period (October–December), when photoperiods become shorter, were the shortest, whereas gestations from the late spring season (April-June), when photoperiods become longer, were the longest(329.82±10.25 and 341.14±13.87 days, respectively). Significant differences were also found in the length of gestations that ended in particular months: the shortest gestations ended in October (327.45 days ±12.24 days), while the longest in May (342.19±14.19 days). The analysis of the years of foaling,grouped in ten-year periods, showed that the shortest gestations occurred before 1960 (335.41±13.87 days), whereas the longest between 1961 and 1970 (339.64±15.15 days). A slow decrease was observed in the length of gestation in subsequent ten-year periods. Mean gestation period in mares from national studs lasted 5 days longer than that in mares from private breeding centres (334.79±11.21 and 339.60±13.92 days, respectively). The number of foalings did not affect the length of gestation significantly; a small difference was found in the case of multiparous mares (more than eight foals born), whose gestation lasted one day longer compared to remaining mares (338.71±13.52 and 337.46±12.32 days, respectively). Gestations in mares whose preceding reproductive period ended with a miscarriage were the shortest, while gestations in mares that were barren in the previous season were the longest (336.36±13.11 and 340.05±13.71 days, respectively).
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