Estradiol-Mediated Induction of Parturition and Associated Neonatal Outcomes in Ewes

The initiation of parturition in sheep is influenced by multiple factors, with estrogens recognized as key contributors. However, the specific effects of estrogens in regulating the timing of delivery, fetal organ maturation, and neonatal adaptation to extrauterine life remain unclear. This study aimed to clarify the role of estradiol in the timing of parturition and neonatal outcomes in periparturient Rambouillet ewes by evaluating the relationship between maternal systemic estradiol and progesterone levels, the timing of delivery, and lamb birth weight, vigor, cortisol levels, mortality, and postnatal weight gain until weaning. Pregnant Rambouillet ewes were randomly assigned to one of two groups between gestational d 139 and 142: the E group (n = 13), which received six Silastic implants containing 50 mg of estradiol each (300 mg total per ewe), or the C group (n = 12), which received six empty Silastic implants. The implants were inserted subcutaneously in the axillary space and removed 2 d postpartum. Ewes were allowed to deliver naturally. Maternal blood samples were collected from the jugular vein at five timepoints: -1 d (pre-treatment), +8 h and +24 h (post-treatment), day of birth (Birth), and +2 d postpartum (PP), for analysis of E2 and P4 concentrations. Lamb blood samples were collected from the jugular vein on the day of birth and at +2 d (PP) to determine cortisol concentrations, and lamb weight gain was monitored biweekly until weaning. The results demonstrated that estradiol treatment shortened the time to parturition (P < 0.008) without altering maternal progesterone concentrations. Importantly, estradiol treatment had no significant effects on lamb birth weight, vigor, cortisol concentrations, or weight gain until weaning. This research demonstrates that E2 induces parturition in sheep, without a withdrawal in systemic P4, supporting the possibility of functional progesterone withdrawal, and highlights the potential use of E2 silastic implants for reproductive management, including the synchronization of parturition in livestock.