Administration of exogenous progesterone or synthetic progestins, such as altrenogest, to mares is commonly used to block estrus behavior when that behavior is deemed undesirable or interferes with intended activities.1 Intramuscular injection of 0.2 mg of progesterone2/kg (0.09 mg/lb) in oil or oral administration of 0.044 mg of altrenogest3/kg (0.02 mg/lb) on a daily basis will block estrus behavior; however, the need for daily administration is a drawback of these formulations. In an effort to eliminate the need for daily administration, there have been anecdotal reports of the use in horses of growth-promoting implants containing progesterone and estradiol that are labeled for use in cattle; however, when cattle implants were tested in horses under controlled conditions, they failed to induce blood progesterone concentrations > 0.5 ng/mL and they did not suppress estrus behavior.4 More recently, a compounded long-acting injectable formulation of progesterone has become available that will maintain blood concentrations of progesterone > 1.0 ng/mL for approximately 10 days,5 which is a sufficient concentration of progesterone to block estrus behavior2,6; however, because of the need for re-administration every 10 days, it cannot be considered an optimal method of long-term suppression of estrus.
One alternative to the use of exogenous progesterone for estrus suppression is intrauterine insertion of a glass ball to extend CL function. Nie et al7 reported that placement of a 25or 35-mm sterile glass ball into the uterine body immediately following ovulation resulted in prolonged CL function in 7 of 18 mares that retained the glass ball after insertion. In mares that developed prolonged CL function following placement of the glass ball, CL function was maintained for approximately 90 days, during which time progesterone concentrations remained > 1.0 ng/mL and estrus behavior was not evident. In nontreated control mares, spontaneous prolongation of CL function occurred in 4 of 32 (13%) mares. Although placement of a glass ball appeared to be an efficacious means of blocking estrus behavior for an extended period of time, it should be noted that in addition to the 11 mares that retained the glass ball and never developed extended CL function (ie, continued to cycle normally), 3 of the 7 glass ball–treated mares with extended CL function had 1 or 2 estrous cycles of normal duration after placement of the glass ball before CL function was prolonged. Therefore, on a per-cycle basis, the frequency of prolonged CL function was 11% (7/62 cycles) in the glass ball–treated mares, compared with 8% (4/50 cycles) in the nontreated control mares, which was not significantly different between groups. Because of its variable efficacy among mares and the need to physically remove the glass ball when the resumption of cyclical reproductive activity is desired, placement of an intrauterine glass ball does not appear to be an optimal method of suppressing estrus behavior in mares.
In contrast to using an intrauterine glass ball to extend CL function, administration of exogenous oxytocin during diestrus is a plausible alternative method of blocking luteolysis to prolong CL function. Endogenous oxytocin secretion is involved in regulating PGF2α secretion from the endometrium during spontaneous luteolysis in mares,8,9 and although administration of exogenous oxytocin to mares around the time of luteolysis (ie, days 11 to 15 after ovulation) stimulates an acute onset of PGF2α secretion,10-12 when oxytocin is administered in the midluteal phase prior to the expected time of luteolysis (ie, before day 10 after ovulation), it does not induce PGF2α secretion and often disrupts luteolysis, causing prolonged CL function.11 Experimentally, continuous infusion of oxytocin by use of a subcutaneous osmotic mini pump from days 8 to 20 after ovulation blocked luteolysis in 4 of 5 mares, whereas luteolysis occurred at the expected time in all 4 control mares that received saline (0.9% NaCl) solution infusion.13 Although it successfully induced prolonged CL function, continuous infusion of oxytocin to disrupt luteolysis would not be a practical method of long-term suppression of estrus behavior. The purpose of the study reported here was to determine whether IM administration of exogenous oxytocin twice daily on days 7 to 14 after ovulation would block luteolysis and cause prolonged CL function in mares.
Oxytocin, Bimeda-MTC Animal Health Inc, Cambridge, ON, Canada.
DSL-3400, Diagnostic Systems Laboratories Inc, Webster, Tex.
SAS, version 9.1, SAS Institute Inc, Cary, NC.
Nie GJ. Estrous suppression. In: Samper JC, Pycock JF, McKinnon AO, eds. Current therapy in equine reproduction. St Louis: Saunders Elsevier, 2007;26–31.
Squires EL, Stevens WB & McGlothlin DE, et al. Effect of an oral progestin on the estrous cycle and fertility of mares. J Anim Sci 1979;49:729–735.
McCue PM, Lemons SS & Squires EL, et al. Efficacy of synovex-S implants in suppression of estrus in the mare. J Equine Vet Sci 1997;17:327–329.
Vanderwall DK, Marquardt JL, Woods GL. Use of a compounded long-acting progesterone formulation for equine pregnancy maintenance. J Equine Vet Sci 2007;27:62–66.
Hawkins DL, Neely DP, Stabenfeldt GH. Plasma progesterone concentrations derived from the administration of exogenous progesterone to ovariectomized mares. J Reprod Fertil Suppl 1979;27:211–216.
Nie GJ, Johnson KE & Braden TD, et al. Use of an intra-uterine glass ball protocol to extend luteal function in mares. J Equine Vet Sci 2003;23:266–273.
Vanderwall DK, Silvia WJ, Fitzgerald BP. Concentrations of oxytocin in the intercavernous sinus of mares during luteolysis: temporal relationship with concentrations of 13,14-dihydro-15-keto-prostaglandin F2A. J Reprod Fertil 1998;112:337–346.
Shand N, Irvine CHG & Turner JE, et al. A detailed study of hormonal profiles in mares at luteolysis. J Reprod Fertil Suppl 2000;56:271–279.
Betteridge KJ, Renard A, Goff AK. Uterine prostaglandin release relative to embryo collection, transfer procedures and maintenance of the corpus luteum. Equine Vet J 1985;(suppl 3):25–33.
Goff AK, Pontbriand D, Sirois J. Oxytocin stimulation of plasma 15-keto-13,14-dihydro prostaglandin F-2A during the oestrous cycle and early pregnancy in the mare. J Reprod Fertil Suppl 1987;35:253–260.
Starbuck GR, Stout TAE & Lamming GE, et al. Endometrial oxytocin receptor and uterine prostaglandin secretion in mares during the oestrous cycle and early pregnancy. J Reprod Fertil 1998;113:173–179.
Stout TAE, Lamming GE, Allen WR. Oxytocin administration prolongs luteal function in cyclic mares. J Reprod Fertil 1999;116:315–320.
Pineda MH, Ginther OJ, McShan WH. Regression of corpus luteum in mares treated with an antiserum against an equine pituitary fraction. Am J Vet Res 1972;33:1767–1773.
Pineda MH, Garcia MC, Ginther OJ. Effect of antiserum against an equine pituitary fraction on corpus luteum and follicles in mares during diestrus. Am J Vet Res 1973;34:181–183.
Sharp DC, Thatcher MJ & Salute ME, et al. Relationship between endometrial oxytocin receptors and oxytocin-induced prostaglandin F2A release during the oestrous cycle and early pregnancy in pony mares. J Reprod Fertil 1997;109:137–144.
Boerboom D, Brown KA & Vaillancourt D, et al. Expression of key prostaglandin synthases in equine endometrium during late diestrus and early pregnancy. Biol Reprod 2004;70:391–399.