Ovarian Quiescence Post-Foaling
Rates of ovarian quiescence in foaling mares or reports of ovarian failure are low. In foaling mares, lactational anestrus
may be behavioral, related to fescue endophyte toxicosis or physiologic factors. Fescue endophyte toxicosis is treated by
reducing exposure to the endophyte and administering domperidone at 1.1mg/kg PO daily for at least 10 days prepartum and postpartum
to support lactation. Anestrus may follow foal heat in mares that foal early in the breeding season if they are not placed
in a stimulatory artificial photoperiod before foaling. Although physiologic anestrus related to lactation in mares does not
typically occur, and mares return quickly to foal heat estrus, mares occasionally have a failure to cycle as a result of negative
energy balance or prolonged stress such as founder.
Miscellaneous Causes
Maiden mares that fail to cycle may have chromosomal abnormalities. Anabolic steroid administration may be associated with
irregular patterns of ovarian activity. A failure to cycle in barren mares is often idiopathic; however, it is prudent to
rule out a prolonged luteal phase by the administration of prostaglandin or measuring serum progesterone, testosterone, and
inhibin levels if a granulosa theca cell tumor is a possibility. Retained endometrial cups may be demonstrated through the
measurement of eCG. Anti-GnRH vaccination is also associated with a failure to return to estrous cycles in a small proportion
of mares even 2 years after discontinuation of the treatment.37 The anti-GnRH vaccine labeled for use in mares is only available in Australia.Ovarian failure routinely occurs in aged mares; in middle-aged mares this is referred to as premature ovarian failure. Ovarian
failure is associated with a loss of ovarian responsiveness to gonadotropins. The serum gonadotropins levels are very high
in these mares.38
Treatment of Ovarian Quiescence
Treatment for lactation-related ovarian quiescence in mares that are losing weight or in poor body condition typically includes
an increase in the energy density of the diet and early weaning. The combination of PMSG 5000 IU and hCG 5000 IU as a single
treatment has been reported to be effective in barren and foaling mares, with 74% of mares showing estrus and 100% ovulating
within 21 days of treatment.39 It has been reported that basal LH concentrations and GnRH-induced LH release are lower in lactating than in nonlactating
mares, and a lower response to exogenous GnRH has been reported in lactating mares in mid-lactation. Exogenous progesterone
or altrenogest is sometimes used for 10 to 14 days in these mares and presumably influences the hypothalamo-pituitary function.40
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