Foals born in winter are smaller than those born in the warmer months, according to researchers, who believe a seasonal reduction in energy metabolism in the mare affects foetus size at a crucial stage.
The study team found that while the birth weight of foals was not affected by season, there were other important differences.
Foals born closer to the start of the year – that is, closer to the depths of the northern hemisphere winter – were shorter at the withers, with shorter legs than foals born later around the traditional spring window.
Seasons have long been known to determine behaviour, metabolism and reproductive activity in many animal species, including horses.
Even in domesticated horses, metabolic activity is reduced in winter. However, its effect on pregnant mares and their foals has not been investigated until now.
Researchers from the University of Veterinary Medicine, Vienna, also known as Vetmeduni Vienna, have shown that seasonal changes during winter strongly influence pregnancy and foetal development.
Foals born early in the northern hemisphere year – the winter months – are smaller than herd mates born at a later time, in the warmer months. These differences persist to at least 12 weeks after birth, the researchers reported in the journal Theriogenology.
Horses can reduce their metabolic activity during the cold season to reduce heat loss. The last weeks of pregnancy correspond to a time of rapid foetal 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 foetus,” says principal investigator Christine Aurich.
To test their hypothesis, the scientists studied 27 broodmares and their foals at the Graf Lehndorff Institute, a joint research unit of the University of Veterinary Medicine, Vienna and the Brandenburg State Stud at Neustadt, 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.
From all foals, weight and a variety of parameters to assess their size were determined repeatedly from birth to the age of 12 weeks. In addition, weight and size of the placenta were determined at foaling.
First author Elisabeth Beythien explains: “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. 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 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 foetus via the placenta, however, placental function appears to be sufficient also during winter,” Beythien says.
“The placenta is thus not the only factor that determines foetal growth.”
The number of foals a mare has produced is known to affect foal size, but in this study the seasonal effects were independent of this.
In the wild, foals are rarely born in winter. Most mares show regular oestrous cycles only for a limited time in spring and summer.
With a pregnancy of 11 months, most foals are born at a time when temperature and nutrient supply would favour their survival in the wild. Modern breeding technologies, however, allow earlier foalings.
The genetically fixed reproductive cycle of mares can be advanced by artificial light programmes, medical treatments, or by optimizing housing and nutrition under stud farm conditions.
In certain breeds this has strong economic implications.
Aurich continues: “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.”
Effects of differences in nutrition among horse groups in the study could be excluded. All mares were fed similarly throughout the study period.
“This,” says Beythien, “confirms genetically based seasonal changes in maternal metabolism as a cause of foetal development and subsequent size of neonatal foals.”
Beythien E, Aurich C, Wulf M, Aurich Jö, Effects of season on placental, foetal and neonatal development in horses, Theriogenology (2017), doi: 10.1016/j.theriogenology.2017.04.027.
The abstract of the study can be read here.