Effect of Chronic Administration of Oxytocin on Corpus Luteum Function in Cycling Mares

Undesirable and variable behavior in mares related to the estrous cycle is a common issue reported in the equine industry, especially for mares in a performance setting. These behaviors can interfere with, as well as decrease, overall performance. The objective of this study was to determine if administration of sixty units of oxytocin once daily for 29 days, regardless of when treatment was initiated during the estrous cycle, would induce prolonged corpus luteum (CL) function in cycling mares. Mares were randomly assigned to two groups: 1) saline-treated control (n=7) and 2) oxytocintreated (n=9). Control mares received 3 cc saline and oxytocin-treated mares received sixty units (3 cc) of oxytocin intramuscularly (IM) for 29 consecutive days. Treatment was initiated in all mares on the same day, independent of the day of the cycle. Jugular blood samples for determination of progesterone concentration were collected three times weekly (M, W, F) for 21 days before treatment was initiated. Beginning on the first day of treatment, blood samples were collected daily for seven days, three times weekly for the remainder of the treatment period, and then three times weekly for 45 days after the last treatment. Mares were considered to have prolonged CL function if serum progesterone remained \u3e1.0 ng/mL for at least 30 days during/after the treatment period. The proportion of mares with prolonged CL function was higher in the oxytocin-treated group compared to the saline-treated group (7/9 vs. 1/7, respectively; P1.0 ng/mL throughout the treatment period and into the post-treatment period. All mares with prolonged CL function maintained elevated progesterone concentrations through at least day 56 of the study. Sixty units of exogenous oxytocin treatment for 29 consecutive days was effective in prolonging CL function as a means of estrus suppression in mares