Study: Winter Foals Smaller Than Spring, Summer Foals

Study: Winter Foals Smaller Than Spring, Summer Foals

Foals born in the winter were smaller than those born later in the year, but there was no difference in body weight among the groups.

Photo: iStock

Season determines behavior, metabolism, and reproductive activity in many animal species, including horses. Even in domesticated horses, metabolic activity is reduced in winter.

Although researchers have known about these seasonal effects for several years, the effects on pregnant mares and their foals hadn’t been investigated. But researchers from Vetmeduni Vienna, in Austria, have now demonstrated that seasonal changes during the winter have a strong influence on pregnancy and fetal development. They learned that foals born early in the year are smaller than herdmates born at a later time, and these differences persist to at least 12 weeks after birth.

The last weeks of pregnancy correspond to a time of rapid fetal growth—this is a key phase in foal development.

“When a foal is born in winter, it is, thus, likely that the seasonal reduction in energy metabolism affects the fetus,“ explained principal investigator Christine Aurich, DVM, PhD, Dipl. ECAR.

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, in Neustadt, Germany. They grouped mares and foals according to foaling date: 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. The team collected a variety of data on each foal regularly to assess their size from birth to an age of 12 weeks. In addition, they measured the weight and size of the placenta at foaling.

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

There was no difference among foal groups for birth weight, although both weight and size of the placenta were smaller in winter-foaling mares than in mares foaling later in the year.

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

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 gestation of 11 months, most foals are born at a time when temperature and nutrient supply would favor their survival in the wild.

Modern breeding technologies, however, allow earlier foalings. Mares’ reproductive cycles can be advanced by artificial light programs, medical treatments, and optimizing housing and nutrition conditions at stud farms.

In certain breeds, smaller foals could have 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,” Aurich said. “This time window affects performance at competitions when all young horse born in the same year compete in the same class.“

All mares were fed similarly throughout the study period, the team added, so there were no effects of variations in nutrition among the groups.

“This confirms genetically based seasonal changes in maternal metabolism as a cause of fetal development and subsequent size of neonatal foals,“ Beythien said.

The study, “Effects of season on placental, foetal and neonatal development in horses,“ was published in Theriogenology

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