Endocrine Regulation of Peripartum Ovarian Activity in the Mare: Clinical Insights Into Rapid Postpartum Cyclicity

Horse mares exhibit a unique reproductive characteristic among domestic animals, the ability to resume ovarian cyclicity remarkably quickly after parturition. Despite the substantial metabolic and physiological demands of late pregnancy and early lactation, more than 80% of mares ovulate within 14 days after foaling and continue to cycle thereafter. Understanding the endocrine mechanisms that enable this rapid reproductive rebound is essential for veterinarians involved in breeding management, fertility optimization, and stud farm practice. 

Early Reactivation of Ovarian Function: A Species-Specific Advantage 

Unlike many domestic species, mares do not experience prolonged postpartum anestrus. Central to this rapid recovery is the anterior pituitary gland, which not only secretes follicle-stimulating hormone (FSH) and luteinizing hormone (LH) but also synthesizes growth hormone (GH). Growth hormone, in turn, stimulates hepatic production of insulin-like growth factor-1 (IGF-1), a key mediator of ovarian and placental function. 

IGF-1 plays a well-documented role in early pregnancy, supporting fetal growth and being detectable in the allantochorion¹. However, its expression in placental tissues declines as pregnancy progresses¹. Clinically, this shift reflects a transition from fetal dependency to preparation for postpartum reproductive and metabolic demands. 

IGF-1 and Follicular Dynamics in the Mare 

In the mare, intrafollicular IGF-1 concentration is positively correlated with follicle size¹. Together with estradiol, activin-A, and inhibin-A, IGF-1 contributes to follicular deviation and selection of the dominant follicle¹. Notably, plasma IGF-1 concentrations rise during the final weeks before foaling [12], coinciding with a pronounced peripartum increase in prolactin. While prolactin alone appears insufficient to directly stimulate follicular growth, its interaction with IGF-1 is thought to enhance follicular development in the immediate postpartum period¹. 

From a clinical standpoint, this explains why mares can exhibit significant follicular activity even while nursing a foal, a phenomenon that contrasts sharply with lactational anestrus seen in other species¹. 

Comparative Endocrinology: Lessons from Ruminants and Swine 

In ruminants, early lactation is characterized by increased GH release but decreased plasma IGF-1 concentrations, a hormonal pattern that favors energy mobilization at the expense of reproductive activity. Although GH can stimulate ovarian steroidogenesis directly or via IGF-1, suppressed IGF-1 levels contribute to delayed postpartum fertility in cattle¹. 

Pigs present a contrasting model. Postpartum increases in GH and prolactin are accompanied by adequate feed intake, preventing severe negative energy balance¹. As a result, IGF-1 concentrations rise after farrowing and gradually decline during lactation¹. Importantly, IGF-1 levels in plasma correlate with follicular fluid concentrations and oocyte maturation. Feed restriction during lactation reduces IGF-1 levels, leading to impaired follicle growth and oocyte quality^2,3. 

These interspecies comparisons highlight a key clinical takeaway: energy balance and IGF-1 availability are critical modulators of postpartum fertility, but mares are uniquely capable of maintaining both during lactation. 

Follicular Growth Begins Before Foaling: New Clinical Evidence 

The present study tested the hypothesis that follicular growth in mares is re-initiated before foaling and that mares with early versus delayed postpartum ovulation differ in somatotrophic hormone dynamics. Blood hormone concentrations, placental hormone expression, and ovarian follicular growth were evaluated in peripartum mares¹. 

Results demonstrated that ovarian tertiary follicle growth resumes well before foaling. During the last two weeks of gestation, the number of antral follicles exceeding 10 mm increased, and the diameter of the largest follicle detectable by ultrasonography doubled. After foaling, follicular growth continued, facilitating rapid resumption of ovulatory cycles¹. 

Clinically, this finding is highly relevant: postpartum ovulation is not an abrupt event triggered solely by parturition but the culmination of follicular development initiated during late pregnancy, even while mares remain under the influence of fetoplacental progestogens¹. 

Interestingly, mares that ovulated within 15 days postpartum showed more pronounced follicular growth already before foaling compared to mares with delayed ovulation. Although all ovulations detected occurred on the right ovary, this was likely coincidental given the limited sample size. 

Gonadotropin Patterns After Foaling 

In alignment with earlier studies, FSH concentrations increased transiently until five days postpartum, while mean LH concentrations rose steadily during the first two weeks after foaling¹. Plasma activin-A peaked one day before foaling and declined thereafter, likely contributing to the postpartum rise in FSH^1,4. 

Mares with early postpartum ovulation exhibited a more pronounced increase in LH concentrations, consistent with enhanced estradiol production from larger dominant follicles and the resulting positive feedback on LH release. In contrast, mares with delayed ovulation had foaled earlier in the year, suggesting a strong seasonal influence. 

Seasonality: A Practical Constraint in Early-Foaling Mares 

Photoperiod remains a critical determinant of reproductive activity in the mare. Mares foaling early in the year experience conflicting endocrine signals: the stimulatory effect of parturition versus the suppressive influence of the anovulatory winter season due to reduced LH release [. Delayed onset of foal heat in such mares has been reported previously¹. 

While most horse mares can overcome seasonal suppression, only a small percentage of pony mares do so . Haflinger mares appear to resemble ponies in this respect. From a management perspective, artificial light programs initiated approximately 60 days before foaling, or the use of blue LED light masks can be effective tools to advance postpartum ovulation^1,5. 

Placental IGF-1, Prolactin, and Metabolic Adaptation 

Confirming earlier findings, plasma IGF-1 and prolactin concentrations increased before foaling and peaked on day 1 postpartum. Molecular analyses revealed that IGF-1 is synthesized, at least in part, by the allantochorion and amnion until foaling. Although placental IGF-1 mRNA abundance differed between early- and late-ovulating mares, protein expression and plasma IGF-1 concentrations did not^1,5. 

Prolactin release, previously inhibited during gestation by endogenous opioids, increases sharply after foaling. While prolactin alone does not directly stimulate follicular growth, placental prolactin receptor expression suggests that prolactin may enhance placental IGF-1 synthesis, indirectly supporting postpartum follicular development¹. 

Adequate nutrition further supports this endocrine environment. Unlike dairy cows, postpartum mares do not experience marked negative energy balance when properly fed. Reduced leptin concentrations postpartum may stimulate feed intake, ensuring sufficient energy supply during early lactation. Consequently, differences in time to first ovulation are more likely attributable to seasonality rather than energy balance or IGF-1 availability^1,6. 

Limited Role of IGF-2 

Only a minor increase in plasma IGF-2 concentration was observed at foaling, with no group differences in plasma levels or placental expression. Although IGF-2 receptors are present in the placenta, evidence suggests that IGF-2 functions primarily as a binding protein rather than a signaling molecule for growth promotion¹. 

Conclusion 

Ovarian follicular growth in mares is initiated during late pregnancy, well before foaling, enabling rapid resumption of cyclic ovarian activity postpartum. While follicular development to the preovulatory stage depends on increased FSH and subsequent LH release after foaling, seasonal suppression of LH can delay ovulation in early-foaling mares. Placental and systemic IGF-1, potentially enhanced by prolactin, appears to support this transition. 

For clinicians and breeding managers, these findings emphasize the importance of seasonal management, nutritional adequacy, and strategic light manipulation to optimize postpartum fertility in mares. 

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