Influence of mare size on birth explored in Austrian study

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Is the birth process in horses affected by the size of the mare? The answer is yes, according to researchers, who examined a range of hormone and heart-related changes that occur in mares and foals around the time of delivery.

The study, by researchers at the University of Veterinary Medicine Vienna involved the monitoring of 23 heavily pregnant mares and their fetuses.

Small horses were represented by six Shetland ponies, while medium-sized horses were represented by eight Haflinger mares. Nine Warmbloods represented full-sized horses.

Foal weight always approximated 10 percent of mare weight, Christina Nagel and her colleagues reported in the open-access journal Animals. However, the relative placenta weight was highest in the full-size mares.

Relative placental weight did not differ between the Shetlands and Haflingers, they reported.

The concentrations of progestins (hormones that maintain pregnancy) and cortisol (a hormone involved in the onset of foaling but also in an animal’s response to stress) were also highest in the full-size mares.

Progestin concentrations in all the mares decreased towards foaling while cortisol concentrations increased.

The heart rate of mares predictably increased before foaling in the horses, with the most pronounced increase seen in small mares.

Overall, the Shetland mares foaled earlier than larger-size mares.

At foaling, atrio-ventricular (AV) blocks — the physiological omission of heart beats — regularly occurred in the full-size mares but only occasionally in medium-size and small mares, which the study team said probably reflected differences in heart efficiency.

In contrast, these missed beats during the expulsive phase of labor and the first two hours after delivery were only occasionally seen in medium-size Haflinger and small Shetland mares.

This difference, they suggest, is not a size effect but an effect of horse breed.

“Because of their highly efficient cardiovascular and respiratory system, fit athletic horses at rest are under a strong parasympathetic tone, resulting in a reduced heart rate and occurrence of AV blocks,” the researchers said.

Fetal heart rate decreased towards birth, with the most pronounced decrease in full-size horses.

Fetal heart-rate variability showed no consistent changes before birth but increased when the foal was born, this increase being most pronounced in full-size foals.

The authors say late pregnancy represents a substantial burden for the maternal cardiovascular system in horses, and mares towards the end of pregnancy are well adapted to these demands.

In conclusion, the authors said their work showed both similarities and differences in endocrine and cardiac changes around the time of birth in horses of different sizes.

The study team comprised Nagel, Maria Melchert, Christine Aurich and Jörg Aurich.

Nagel, C.; Melchert, M.; Aurich, C.; Aurich, J. Differences in Endocrine and Cardiac Changes in Mares and Her Fetus before, during, and after Parturition in Horses of Different Size. Animals 2020, 10, 1577.

The study, published under a Creative Commons License, can be read here

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