How to Use Oxytocin Treatment to Prolong Corpus Luteum Function for Suppressing Estrus in Mares

• The most common method of prolonging function of the corpus luteum (CL) for estrus suppression in mares has been intrauterine insertion of a glass ball.
• An alternative method of prolonging function of the CL is oxytocin treatment.
• Two protocols for using oxytocin treatment to prolong function of the CL for estrus suppression in mares are described.

Prolonging function of the corpus luteum (CL) is a method of suppressing estrus that uses continued secretion of endogenous progesterone to keep mares out of heat naturally. The most common method of prolonging CL function has been intrauterine insertion of a glass ball (i.e., marble). However, several recent reports have described deleterious complications associated with their use, including the presence of multiple glass balls, fragmentation of the glass ball(s), and/or pyometra. Therefore, the use of other methods for prolonging CL function is warranted. Alternatives to using an intrauterine glass ball for prolonging CL function include (1) oxytocin treatment, (2) inducing a late-diestrus ovulation, (3) intrauterine infusion of plant oils, and (4) manual reduction of the conceptus after day 16 of gestation. Of these, oxytocin treatment is the most practical and efficacious method of prolonging CL function. As described here, one oxytocin protocol involves administering 60 units of oxytocin intramuscularly (IM) once daily on days 7 to 14 after ovulation, which induces prolonged CL function in 60% to 70% of treated mares. Alternatively, by extending the duration of oxytocin treatment to 29 days, administration of 60 units of oxytocin IM can be initiated randomly at any point in the estrous cycle with no loss in efficacy (i.e., over 70% response).