Gestational length of Thoroughbred mares kept in tropical and subtropical climates

Gestation length (GL) in horses varies widely, influenced by multiple variables, including maternal, fetal, and environmental factors. This study aimed to investigate and quantify the relative contributions of climatic (photoperiod and temperature-humidity index – [THI]), maternal (age and parity), fetal (sex) and environmental (year and month of foaling) variables influencing gestation length in Thoroughbred mares. Retrospective data encompassing 704 pregnancies across nine breeding seasons in tropical and subtropical Brazilian climates were analyzed. Stepwise regression analysis identified foaling month and year as the primary determinants of gestation length, with additional contributions from foal sex, mare age, parity, and THI. Subsequent multiple regression analysis ranked foaling month, mare age, and foaling year as the most significant factors, while parity, THI, and foal sex demonstrated smaller but statistically significant impacts. Gestation length in Thoroughbred mares is influenced by a complex interplay of environmental, maternal, and fetal factors, with foaling month, mare age, and foaling year being the most critical. These findings provide valuable insights for optimizing breeding management in tropical and subtropical climates. •Gestation lengths were shorter in mares exposed to elevated temperature-humidity index (THI) during the foaling period.•Male foals exhibited gestation periods that were, on average, two days longer than those of female foals.•Among the evaluated factors, foaling month emerged as the most significant determinant of gestation length, followed by foaling year, foal sex, and mare age.•No significant correlation was found between photoperiod and gestation length.