Seasonal and diurnal rhythms determine the life cycle of many animal species, including livestock. In equids, this is true not only for wild species such as the Przewalski, but season-dependent metabolic changes also exist in domesticated horses.

Horses can reduce their metabolic activity during the cold season and, thus, reduce heat loss. The effects of such seasonal changes on pregnancy and fetal development, however, have not been investigated so far.

Researchers from the University of Veterinary Medicine-Vienna (Vetmeduni Vienna) in Austria have demonstrated that foals born in winter are smaller than their herd mates born later in the year.

The last weeks of pregnancy correspond to a time of rapid fetal growth. This phase is a key moment for development of the foal. "When a foal is born in winter, it is thus likely that the seasonal reduction in energy metabolism affects the fetus," principal investigator Christine Aurich explained.

To test their hypothesis, the scientists studied 27 broodmares and their foals at the Graf Lehndorff Institute, a joint research unit of Vetmeduni Vienna and the Brandenburg State Stud at Neustadt (Dosse), Germany. Mares and foals were allocated to three groups by the date of foaling. Foalings occurred between February and early March in group 1, from early March until early April in group 2 and from mid-April to May in group 3.

Weight and a variety of parameters were determined repeatedly for all foals to assess their size from birth to an age of 12 weeks. In addition, the weight and size of the placenta were determined at foaling.

Clear seasonal effects on foal size

"Among the foal groups, we compared circumference of the thorax, height at withers, the distance from the fetlock to the carpal joint and to the elbow as well as length of the head from poll to nose. The size parameters clearly demonstrate that foals born in February were smaller than those born later in the year," first author Elisabeth Beythien said. "The winter foals did not completely compensate their size deficit within the first 12 weeks of life."

No difference among foal groups existed for birth weight, although both the weight and size of the placenta were smaller in winter-foaling mares than in mares foaling later in the year, the researchers said.

"The smaller placenta indicates a reduced nutrient transfer to the fetus via the placenta. However, placental function appears to be sufficient also during winter. The placenta is, thus, not the only factor that determines fetal growth," Beythien suggested. "Parity, i.e., the number of foalings a mare has had, is known to affect foal size, but in our study, the seasonal effects were also independent from parity."

Winter foals rare in nature

In wild or feral horses, foals are rarely born in winter. Most mares show regular estrous cycles only for a limited time period in spring and summer. With a pregnancy of 11 months, most foals are born at a time when the temperature and nutrient supply would favor their survival in the wild.

Modern breeding technologies allow earlier foalings, however. The genetically fixed reproductive cycle of horse mares can be advanced by artificial light programs and medical treatments as well as just by optimizing housing and nutrition under stud farm conditions, the researchers noted. In certain breeds, this has strong economic implications.

"Although winter foals need at least 12 weeks to make up their size deficit, they can still be several months ahead of their later-born conspecifics. This time window affects performance at competitions when all young horses born in the same year compete in the same class," Aurich said.

Effects of differences in nutrition among horse groups in the study could be excluded because all mares were fed similarly throughout the study period. "This confirms genetically based seasonal changes in maternal metabolism as a cause of fetal development and subsequent size of neonatal foals," Beythien concluded.

The article "Effects of Season on Placental, Foetal and Neonatal Development in Horses," by Beythien, Christine Aurich, Manuela Wulf and Jörg Aurich, was published in Theriogenology and is available at http://www.sciencedirect.com/science/article/pii/S0093691X17301875?via%3Dihub.