Study: Winter Foals Smaller Than Spring, Summer Foals
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. | Photo: Anne M. Eberhardt/The Horse
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

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