Peripartum Endocrine Orchestration in Mares: How Hormones, Season, and Placenta Shape Post-foaling Fertility

The mare represents a reproductive paradox among domestic species. Despite the physiological stress of late gestation and the immediate demands of lactation, most mares resume ovarian cyclicity rapidly after foaling. More than 80% ovulate within 14 days postpartum and continue to cycle normally thereafter¹. This remarkable capacity is not incidental but reflects a finely tuned endocrine system that integrates pituitary, placental, ovarian, metabolic, and environmental signals. 

Understanding this orchestration is essential for veterinarians involved in broodmare management, particularly when addressing delayed foal heat, early-season infertility, or suboptimal breeding performance.  

Pituitary Adaptation Around Foaling: Beyond Gonadotropins 

The anterior pituitary plays a central role in postpartum fertility, extending far beyond its classic secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH). In the mare, it also produces growth hormone (GH), which stimulates hepatic synthesis of insulin-like growth factor-1 (IGF-1). This somatotrophic axis is critical for linking metabolic status with reproductive readiness¹. 

During early pregnancy, IGF-1 supports fetal growth and is detectable within the allantochorion. As gestation progresses, placental IGF-1 expression declines, reflecting a shift in endocrine priorities from fetal development toward preparation for parturition and postpartum adaptation¹. 

IGF-1 as a Reproductive Signal Rather Than a Metabolic Constraint 

In mares, IGF-1 plays a direct role in ovarian physiology. Intrafollicular IGF-1 concentration increases with follicle size  and acts synergistically with estradiol, activin-A, and inhibin-A to regulate follicular deviation and dominance. Importantly, plasma IGF-1 concentrations rise during the final weeks before foaling, suggesting that the mare enters the postpartum period with a reproductive system already primed for follicular activity. 

This endocrine pattern contrasts sharply with that of dairy cows, where early lactation is characterized by elevated GH but reduced IGF-1 concentrations due to negative energy balance, resulting in suppressed ovarian activity. Although GH can stimulate ovarian steroidogenesis directly or via IGF-1, low circulating IGF-1 limits follicular growth in cattle¹. 

In pigs, postpartum GH and prolactin also increase, but adequate feed intake allows sows to meet lactational energy demands without profound metabolic compromise. As a result, IGF-1 concentrations rise postpartum and correlate positively with follicular fluid IGF-1 levels and oocyte maturation. Feed restriction during lactation reduces IGF-1 and negatively impacts follicle growth and oocyte quality^1,2,3. 

The mare more closely resembles the sow than the dairy cow: when adequately fed, postpartum mares do not experience significant negative energy balance, allowing IGF-1 to support reproductive function rather than being diverted solely toward metabolic adaptation¹. 

Placenta as an Endocrine Contributor Until Foaling 

An important finding of the present study is the continued endocrine role of the placenta late in gestation. Molecular and protein analyses indicate that IGF-1 is synthesized by the allantochorion and amnion until foaling. Although placental IGF-1 mRNA abundance differed between mares with early versus delayed postpartum ovulation, IGF-1 protein expression and plasma concentrations did not differ between groups¹. 

This suggests that placental IGF-1 contributes to a systemic endocrine environment conducive to postpartum fertility, rather than acting as a differentiating factor between individual mares. 

Placental expression of prolactin receptors further indicates that prolactin may regulate placental IGF-1 synthesis. During gestation, prolactin secretion is inhibited by endogenous opioids, but this inhibition ceases after foaling, resulting in a pronounced prolactin surge. While prolactin alone does not directly stimulate follicular growth in mares, its interaction with IGF-1 may enhance the endocrine support for postpartum ovarian activity¹. 

Gonadotropin Release After Foaling: Timing and Functional Significance 

Following parturition, there is a well-defined sequence of gonadotropin release. FSH concentrations rise transiently until approximately five days postpartum, consistent with earlier reports. Plasma activin-A peaks one day before foaling and then declines, likely contributing to this FSH surge through its stimulatory effects on FSH synthesis and release^1,4. 

LH concentrations increase more gradually but continuously during the first two weeks after foaling. This LH rise is more pronounced in mares that ovulate within 15 days postpartum, paralleling enhanced growth of the largest follicle and increasing estradiol production. This positive feedback loop between follicular estradiol and LH secretion is essential for final follicle maturation and ovulation¹. 

Importantly, FSH concentrations early postpartum do not differ between mares with early or delayed ovulation, indicating that LH availability, rather than FSH, is the limiting factor for timely postpartum ovulation¹. 

Seasonality as a Major Modulator of Postpartum Fertility 

One of the most clinically relevant observations is the strong influence of season on postpartum ovarian function. Mares that failed to ovulate within 15 days postpartum foaled, on average, one month earlier in the year than mares with early ovulation. These mares experience conflicting endocrine signals: parturition promotes ovarian activity, while winter photoperiod suppresses LH secretion¹. 

Delayed foal heat in early-season foaling mares has been reported previously and reflects photoperiod-driven suppression of hypothalamic–pituitary activity. Although most horse mares can overcome seasonal suppression, only a small proportion of pony mares can do so¹. Haflinger mares appear more pony-like in this regard. 

From a management perspective, artificial light programs initiated approximately 60 days before foaling, or the use of blue LED light masks, represent effective tools to mitigate seasonal suppression and advance postpartum ovulation^1,5. 

Metabolic Signals Supporting Lactation and Reproduction 

Concomitant with increased IGF-1 concentrations postpartum, plasma leptin concentrations decrease. Reduced leptin may stimulate feed intake, enabling mares to meet the energy demands of early lactation without entering negative energy balance. This metabolic adaptation further distinguishes mares from dairy cows and supports uninterrupted reproductive function. 

Concentrations of IGF-1 decrease in mares subjected to restrictive feeding, emphasizing the importance of adequate nutrition. However, under appropriate feeding conditions, differences in time to first ovulation appear to be driven primarily by season rather than metabolic status or IGF-1 availability¹. 

Limited Endocrine Role of IGF-2 

Only a minor increase in plasma IGF-2 concentration occurs at foaling, with no differences between early- and late-ovulating mares. Although receptors for IGF-1 and IGF-2 are strongly expressed in the placenta, evidence suggests that IGF-2 primarily functions as a binding protein rather than an active signaling molecule for growth promotion. Consequently, IGF-2 appears to play a limited role in regulating postpartum ovarian activity in mares^1,6. 

Conclusion 

Postpartum fertility in the mare is the result of an intricate endocrine integration involving pituitary hormones, placental IGF-1 synthesis, metabolic signals, and environmental photoperiod. While follicular development is already underway before foaling, successful ovulation depends on adequate LH release after parturition, a process strongly influenced by season. Prolactin-stimulated IGF-1, adequate nutrition, and appropriate light exposure together create the hormonal environment that allows mares to resume cyclicity rapidly despite lactation. 

For clinicians and breeding managers, optimizing postpartum reproductive performance requires not only monitoring ovarian structures but also managing season, nutrition, and endocrine context as a unified system. 

References 

1. Melchert M, Aurich J, Ertl R, Reichart U, Walter I, Gautier C, Kaps M, Aurich C. Involvement of somatotrophic hormones in the postpartum regulation of ovarian activity in mares. Domestic Animal Endocrinology. 2024 Jul 1;88:106852. https://www.sciencedirect.com/science/article/pii/S0739724024000158 

2. Costermans NG, Teerds KJ, Middelkoop A, Roelen BA, Schoevers EJ, van Tol HT, Laurenssen B, Koopmanschap RE, Zhao Y, Blokland M, van Tricht F. Consequences of negative energy balance on follicular development and oocyte quality in primiparous sows. Biology of Reproduction. 2020 Feb 14;102(2):388-98. https://academic.oup.com/biolreprod/article-pdf/102/2/388/32892899/ioz175.pdf 

3. Han T, Björkman S, Soede NM, Oliviero C, Peltoniemi OT. IGF-1 concentration patterns and their relationship with follicle development after weaning in young sows fed different pre-mating diets. Animal. 2020 Jul;14(7):1493-501. https://www.sciencedirect.com/science/article/pii/S1751731120000063 

4. Dhakal P, Tsunoda N, Nambo Y, Taniyama H, Nagaoka K, Watanabe G, Taya K. Circulating activin A during equine gestation and immunolocalization of its receptors system in utero-placental tissues and fetal gonads. Journal of Equine Science. 2021;32(2):39-48. https://www.jstage.jst.go.jp/article/jes/32/2/32_1931/_pdf 

5. Lutzer A, Nagel C, Murphy BA, Aurich J, Wulf M, Gautier C, Aurich C. Effects of blue monochromatic light directed at one eye of pregnant horse mares on gestation, parturition and foal maturity. Domestic animal endocrinology. 2022 Jan 1;78:106675. https://www.sciencedirect.com/science/article/pii/S0739724021000722 
6. Arfuso F, Giannetto C, Bazzano M, Assenza A, Piccione G. Physiological correlation between hypothalamic–pituitary–adrenal axis, leptin, UCP1 and lipid panel in mares during late pregnancy and early postpartum period. Animals. 2021 Jul 9;11(7):2051. https://www.mdpi.com/2076-2615/11/7/2051