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  • Author or Editor: Desirée M. Rasmussen x
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Objective—To determine whether IM administration of exogenous oxytocin twice daily on days 7 to 14 after ovulation blocks luteolysis and causes prolonged function of corpora lutea (CL) in mares.

Design—Prospective study.

Animals—12 mares.

Procedures—Beginning on the day of ovulation (day 0), jugular blood samples were collected every other day until day 40 for determination of progesterone concentration. On day 7, mares (n = 6/group) were treated with saline (0.9% NaCl) solution (control group) or oxytocin. Beginning on day 7, control mares received 3 mL of sterile saline solution every 12 hours, IM, and oxytocin-treated mares received 60 units of oxytocin every 12 hours, IM, through day 14. Mares were considered to have prolonged CL function if progesterone concentration remained > 1.0 ng/mL continuously through day 30.

Results—The proportion of mares with prolonged CL function was significantly higher in the oxytocin-treated group (6/6), compared with the control group (0/6). All control mares underwent luteolysis by day 16, at which time their progesterone concentrations were < 1.0 ng/mL. In contrast, all 6 oxytocin-treated mares maintained progesterone concentrations > 1.0 ng/mL continuously through day 30.

Conclusions and Clinical Relevance—IM administration of 60 units of oxytocin twice daily on days 7 to 14 after ovulation was an efficacious method of inhibiting luteolysis and extending CL function in mares. Disrupting luteolysis by administering exogenous oxytocin during diestrus appears to be a plausible and practical method of long-term suppression of estrus in mares.

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in Journal of the American Veterinary Medical Association