Effect of mid-diestrus oxytocin treatment on early pregnancy in the mare

Abstract

Progesterone supplementation in early pregnancy is commonplace in equine reproduction. Recently, there has been some controversy questioning the beneficial effects of routine progesterone administration and rising concerns of medication exposure as an occupational hazard for humans. Although premature luteal regression and resulting low plasma progesterone concentrations are rare in the mare, select cases benefit from luteal support. Duration of luteal function can be prolonged following oxytocin administration during mid-diestrus in the non-pregnant mare (Vanderwall.et.al. Journal of Equine Veterinary Science. 2012; 32:536-541). The aim of this field study was to investigate the effects of oxytocin administration on early pregnancy in the mare. Thoroughbred mares (n = 39; age 5 to 19 years) were enlisted and bred over 55 estrous cycles. Prior to natural breeding, uterine cultures were obtained, and mares were assigned to 1 of 2 groups: Oxytocin treatment (OT); oxytocin 60 units, intramuscularly, once daily, days 7-14 post-ovulation; or Control (CT); no treatment. Mare reproductive cycles were monitored by transrectal palpation and ultrasonography. Ovulation was documented and defined as day 0. Pregnancy diagnosis was performed by transrectal ultrasonography day 14 post-ovulation. If pregnancy was confirmed, a blood sample was collected and assayed for plasma progesterone concentration. Mares that did not conceive were administered prostaglandin F2αanalogue (cloprostenol; 250mcg once intramuscularly) at day 16 post-ovulation. These mares were rebred and treatment was determined by the stage in the study; therefore, some mares underwent the same treatment more than once. Subsequent pregnancy examinations were performed on days 28 and 45 of pregnancy. Data were analyzed using two-tailed Fisher's Exact tests for small counts, with significance defined at p < 0.05. First and per-cycle pregnancy rates for OT and CT were 90.9 and 91.3% and 68.2 and 55.1%, respectively. Median progesterone concentrations for OT were 6.9 and 7.0 ng/ml for CT. There were no differences between OT and CT groups for first (p = 0.165) or per cycle (p = 0.289) pregnancy rates, days 28 and 45 pregnancy rates (p = 1.0), or progesterone concentrations (p = 1.0). In this population of mares, oxytocin administration during mid-diestrus in early pregnancy did not induce a negative effect on pregnancy rate nor a positive effect on progesterone concentration. Whilst the study design did not assess the effect of OT on the inter-ovulatory interval (IOI), the authors’ hypothesize that the IOI would be increased, due to prolonged luteal function, in the non-pregnant OT mare if prostaglandin F2α analogue was not administered. Further investigation in this area may reveal potential beneficial effects of oxytocin administration in early pregnancy.