Using Estrous Behavior to Time Initiation of Oxytocin Administration to Prolong Luteal Function in Mares

The objective of this study was to use estrous behavior alone to determine the appropriate time for beginning an oxytocin treatment protocol for estrus suppression. We hypothesized that administration of oxytocin beginning 8 days after the onset of estrus will prolong the luteal phase in mares. Twenty-three light breed mares (aged 4–20 years) were exposed to a stallion and observed for signs of sexual receptivity. Mares not displaying signs received 250 μg of cloprostenol intramuscularly (IM) and were teased again 3–4 days later. On the day that estrous behavior was observed (Day 0), mares were randomly divided into two groups: oxytocin (n = 11): oxytocin (60 IU, IM) was administered once daily from Day 8–17; control (n = 12): did not receive treatment. Blood was collected from all mares every 4 days throughout Day 17, and every 7 days thereafter until Day 45. Serum progesterone concentrations >1.0 ng/mL were indicative of a functioning corpus luteum. Interestrus interval was defined as the period between Day 0 and the day when progesterone next reached