ASSOCIATION OF SEX OF FETUS, SIRE, MONTH OF CONCEPTION, OR YEAR OF FOALING WITH DURATION OF GESTATION IN STANDARD-BRED MARES

Objective-To determine whether sex of fetus, sire, month of conception , or year of foaling was associated with duration of gestation in mare s. Design-Epidemiologic retrospective cohort study. Animals-500 foalin gs for 296 Standardbred mares. Procedure-Data for reproductive events from 1986 to 1992 were analyzed. Analyses were conducted to determine whether duration of gestation was associated with sex of fetus, sire, month of conception, or year of foaling. Results-Mean duration of gest ation was 343.3 days and was significantly greater for colt fetuses (3 44.4 days) than for filly fetuses (342.2 days). Sire was associated wi th duration of gestation; gestation after mating with certain sires wa s consistently less than 340 days in duration, whereas duration after mating with other sires was consistently more than 350 days. Duration of gestation was associated with month of conception, decreasing by ab out 2.5 d/mo for mares conceiving later during the breeding season. Cl inical Implications-Duration of gestation is affected by mating to spe cific sires and by month of conception. Stallions associated with exce ptionally prolonged gestations might be used to breed mares early in t he breeding season, whereas stallions associated with shorter gestatio ns might be desirable for mares bred later in the breeding season. Pre parations for impending parturition should be more effective and effic ient with improved information on expected date of parturition.