There are times when it is necessary or expedient to induce foaling in a mare. However, the key is to coordinate induction with timing of when the fetus is ready to leave the womb. This is important for preventing risk of dystocia or adverse effects on foal health. Historically, corticosteroids like dexamethasone have been used as has PGF2α or its analogues for induction.
Of importance to an uncomplicated birth of a viable foal is the readiness of the mare and foal for parturition. Physiologic gestation narrows to a window of 320-360 days, which is variable based on genetics, size and age of the mare, day length, and gender of the foal. Still, some foals that are 360 days in utero may remain dysmature and unready for birth. Changes in mammary gland development, pH and biochemistry of mammary secretions help predict imminence of foaling. Specific criteria are necessary for safe induction of parturition: Gestation ≥ 330 days; pre-colostrum pH ≤ 6.5; calcium carbonate ≥ 250 ppm; and clinical signs preparatory to birth—relaxing vulva, waxing of the teats, for example.
An Austrian study examined the use of low doses of oxytocin as an effective and safe means of parturition induction. [Nagel C, Aurich C. Induction of parturition in horses – from physiological pathways to clinical applications. Domestic Animal Endocrinology Jan 2022, vol. 78; doi.org/10.1016/j.domaniend.2021.106670]
The authors of the study explain that there is a “pronounced endogenous release of oxytocin only during the short expulsive phase of labor.” This is associated with higher concentrations of PGF2α (prostaglandin F 2α) metabolites, and strong uterine contractions for rapid foal delivery. While high doses of oxytocin did not affect foal viability, they have other adverse effects of premature placental separation (40%) and dystocia (25%). Both of these events threaten foal survival particularly if immediate veterinary intervention isn’t available. The authors also point out that high doses of oxytocin may stimulate parturition of foals that are not fully developed to term.
However, a low dose of oxytocin releases PGF2α as well as progesterone, estrogen, and prolactin in mares close to term. Repeated low doses (2.5 IU) of oxytocin at 20-minute intervals are not advised based on birth of compromised foals in half the cases. Yet, a single dose of 2.5 IU oxytocin given to mares with sufficient mammary gland calcium carbonate secretion results in uncomplicated parturition. If the mare fails to respond to this single treatment, then it is concluded that she isn’t ready to foal, and the treatment can be re-administered 24 hours (or more) later.
At least 94% of mares that fit the strict criteria—calcium carbonate ≥ 250 ppm, gestation ≥ 330 days, decreased pre-colostrum pH, and clinical signs of impending foaling such as a full udder—and given 2.5-3.5 IU oxytocin did not require a second injection of oxytocin for foaling induction. Foals are usually born within 2 hours of treatment. In the studies, while the expulsive phase of foaling is not affected by the oxytocin protocol, the foals experience some slight hypoxia and delayed time to stand and suckle compared to foals born spontaneously. Veterinary intervention for malpresentation occurred in 5%, similar to spontaneous parturition. Premature placental separation and time to expulsion of placental membranes was found to be similar to spontaneous foaling mares.
The study concludes: “Low doses of oxytocin given once daily apparently stimulate the release of PGF2α and stimulate uterine contractions only in mares at term and well prepared for foaling.” The authors are referring to this technique as “augmentation” rather than “induction” of parturition. Enabling foaling during daytime hours improves access to supervision and medical intervention when necessary as well as monitoring of the foal and colostral intake in the initial hours following birth.