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‘Winter foals’ smaller than herd mates born later
Foals born in February and March were smaller than those born at a later time.
Study shows effect of seasonal change on pregnancy

Foals born in the winter are smaller than herd mates born later in the year, a new study has concluded.

Writing in the journal Theriogenology, researchers show that seasonal changes during winter have a strong influence on pregnancy and foetal development.

Foals born in February and March were smaller than those born at a later time and these differences persist to at least 12 weeks after birth, researchers explain.

In the study, researchers at Vetmeduni Vienna assessed 27 broodmares and their foals. Mares and foals were allocated to three groups by the date of foaling. Foaling occurred between February and early March in Group One, from early March until early April in Group Two, and from mid-April to May in Group Three.

From birth to the age of 12 weeks, researchers repeatedly checked the foals’ weight and took a variety of parameters to assess size. The weight and the size of the placenta were determined at foaling.

“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,” explained first author Elisabeth Beythien. “The size parameters clearly demonstrate that foals born in February were smaller than those born later in the year.”

The team did not observe any difference among foal groups for birth weight. But 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,” Elisabeth continues. “The placenta is thus not the only factor that determines foetal growth.”

Winter foals are a rare occurrence. Most mares show regular oestrous cycles for a limited period in spring and summer. With a gestation period of 11 months, most foals are born at a time when temperature and nutrient supply would favour their survival in the wild.

Modern technology, however, has made it possible to genetically fix reproductive cycles to allow for earlier foalings. This can be advanced by artificial light programmes, medical treatments and optimising housing and nutrition under stud farm conditions. For some breeds, this has strong economic implications.

“Although winter foals need at least two weeks to make up their size deficit, they can still be several months ahead of their later born conspecifics,” adds study coordinator Christine Aurich. “This time window affects performance at competitions when all young horse born in the same year compete in the same class.”

The authors say that effects in nutrition among horse groups in the study could be excluded. The mares were fed similar diet throughout the study period.

“This confirms genetically based seasonal changes in maternal metabolism as a cause of foetal development and subsequent size of neonatal foals,” Elisabeth concludes. 

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