Discussion

Suppression of oestrous behaviour is a common practice in performance mares that is used to obviate real and/or perceived deleterious effects of oestrous behaviour on training and related performance activities (McCue 2003). General methods that have been used to suppress oestrous behaviour in mares include: 1) administration of exogenous progesterone/progestins; 2) extending the duration of corpora luteal (CL) function; 3) suppressing ovarian follicular activity; and 4) ovariectomy (Vanderwall and Nie 2011). Of these, administration of exogenous progesterone/progestins and extending the duration of CL function are most commonly employed.
Daily intramuscular administration of 100 mg progesterone in oil (0.2 mg/kg bwt) effectively suppresses signs of oestrus in mares; however, the need for daily administration and the potential for soreness at the site of injection are limitations to its use. Altrenogest (ReguMate, Intervet) is a synthetic progestin approved for use in horses for suppressing oestrus, and is generally considered the 'gold-standard' for suppressing oestrus. Daily oral administration of altrenogest at a dose of 0.044 mg/kg bwt (1 ml per 110 lbs of bodyweight) is highly effective for suppressing oestrus in mares. The need for daily administration of altrenogest is a disadvantage to its use, as is concern for human safety, since it is readily absorbed through the skin. Although there are several other synthetic progestins such as medroxyprogesterone acetate, hydroxy-progesterone caproate, hydroxyprogesterone hexanoate, melengestrol acetate and norgestomet available in various formulations (e.g. injectable, implant, oral, etc.) none have been found to be efficacious for suppressing oestrus in mares.
In contrast to using exogenous progesterone/progestins to block oestrus, CL function can be extended allowing continued secretion of endogenous progesterone to block oestrus. One method of extending CL function is intrauterine insertion of a sterile glass ball (i.e. marble). Nie et al. (2003) reported that placement of a 25 or 35 mm sterile glass ball into the uterine body immediately following ovulation resulted in prolonged CL function and absence of oestrous behaviour for approximately 90 days in
7 of 18 (39%) mares that retained the glass ball after insertion (6 of 12 mares expelled the glass marble). Although placement of a glass ball appeared to be an efficacious means of blocking oestrous behaviour 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 (i.e. continued to cycle normally despite the presence of the glass ball in the uterine lumen), 3 of the 7 glass ball treated mares with extended CL function had one or 2 oestrous cycles of normal duration after placement of the glass ball before CL function was prolonged. Therefore, on a 'per-cycle' basis the incidence of prolonged CL function was only 11% (7/62 cycles) in the glass ball treated mares compared to 8% (4/50 cycles) in nontreated control mares, which was not significantly different between groups.

It is important to note that if a glass ball is not eventually removed, some mares may retain the ball for prolonged periods of time (i.e. years), such that its presence in the uterine lumen may be unbeknown to individuals working with the animal. This has led to situations in which attempts have been made to breed a mare when an intrauterine glass ball has been present, and although the presence of the glass ball will likely cause iatrogenic infertility and interfere with the establishment of pregnancy, there are anecdotal reports of mares becoming pregnant despite of the presence of the glass ball in the uterine lumen with subsequent compromise and loss of the pregnancy due to the presence of the glass ball. In addition, there are anecdotal reports of glass balls that have spontaneously fragmented in the uterine lumen leaving behind glass shards that could permanently impair fertility; therefore, there is the potential for severe unintended consequences if a glass ball is not eventually removed from the uterine lumen, so their use should be considered carefully.
In contrast to using an intrauterine glass ball to prolong CL function, administration of exogenous oxytocin during dioestrus is another method of blocking luteolysis and extending CL function. Endogenous oxytocin is involved in regulating PGF2alpha secretion from the endometrium during spontaneous luteolysis in the mare, and although administration of oxytocin to mares around the time of luteolysis (i.e. Days 11 - 15 post ovulation) stimulates an acute onset of PGF2alpha secretion, when oxytocin is administered prior to the expected time of luteolysis (i.e. before Day 10 post ovulation) it does not induce PGF2alpha secretion and often disrupts luteolysis causing prolonged CL function. We have published the results of a 'proof of principle' study (Vanderwall et al. 2007), in which our objective was to determine if intramuscular (i.m.) administration of 60 units of oxytocin twice daily on Days 7 - 14 post ovulation would block luteolysis and cause prolonged CL function in mares. On Day 7, mares were randomly assigned to one of two groups (n = 6/group): 1) saline- treated control and 2) oxytocin-treated. Beginning on Day 7, control mares received 3 cc sterile saline i.m. b.i.d. and oxytocin- treated mares received 60 units oxytocin i.m. b.i.d. through Day 14. Mares were considered to have prolonged CL function if progesterone levels remained >1.0 ng/ml continuously through Day 30. The proportion of mares with prolonged CL function was significantly greater (P = 0.001) in the oxytocin-treated group compared to the saline-treated control group (6/6 vs. 0/6, respectively). In a follow-up study (Vanderwall et al. 2012), we compared the same dose of oxytocin (60 units) given twice daily vs. once daily on Days 7 - 14 post ovulation on the duration of CL function. In that study, CL function was maintained for 50 days post ovulation in 5/7 mares (71%) treated twice daily, 5/8 mares (63%) treated once daily and in 1/7 (14%) untreated control mares. There was no difference (P>0.05) in the proportion of mares with extended CL function between once daily and twice daily administration of oxytocin, whereas collectively oxytocin treatment increased (P<0.05) the proportion of mares with extended CL function; therefore, once daily administration of oxytocin on Days 7 - 14 after ovulation can be used to prolong CL function clinically in mares and it appears to be a plausible method of suppressing oestrus.