Evaluation of a Proprietary Slow-Release Oxytocin Formulation on Corpus Luteum Function in Mares

Prolonging function of the corpus luteum (CL) is a method of suppressing estrus that relies on continued secretion of endogenous progesterone to keep mares out of heat naturally. The use of oxytocin treatment to prolong CL function is gaining increasing use, and the most common treatment protocol involves administration of 60 units of oxytocin intramuscularly (IM) once daily on days 7-14 after ovulation (eight daily treatments). Although that protocol induces prolonged CL function in ≥70% of treated mares, the need for daily administration is a drawback to its use. Therefore, the objective of this study was to evaluate the efficacy of a proprietary slow-release oxytocin formulation (SR-OT) for prolonging CL function that requires only two treatments. Mares were examined via transrectal palpation and ultrasonography to determine the day of ovulation (day 0) and then randomly assigned to a nontreated control group and an SR-OT treatment group (n = 8 mares/group). Mares in the treated group received 1.0 mL of SR-OT containing 2,400 IU oxytocin IM once on day 7 and again on day 10 after ovulation. Jugular blood samples were collected on day 0 and then every Monday, Wednesday, and Friday for 50 days for determination of the serum progesterone concentration. Mares were classified as having prolonged CL function if their progesterone concentration remained >1.0 ng/mL continuously for at least 30 days. Corpus luteum function was prolonged in 0/8 (0%) control mares and 6/8 (75%) of the SR-OT-treated mares (P < .01). The demonstrated efficacy of this two-injection, SR-OT protocol represents a 75% reduction in the number of oxytocin treatments compared with daily administration of oxytocin from day 7-14, making it a more practical treatment protocol.