Season of Birth Can Affect Foal Size

Researchers have found that winter foals (those born in February to early March) tend to be smaller at birth than those born during the natural foaling season (between April and September).
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A horse’s metabolism can slow down when the weather gets cold. It’s one way the horse’s body can realign resources to stay warm. With modern breeding practices often manipulating mares’ natural reproductive cycles to deliver winter foals (born in February to early March), a group of researchers wondered if that metabolic reduction could affect fetal development.

The researchers, led by Christine Aurich, DVM, PhD, Dipl. ECAR, of the University of Veterinary Medicine Vienna, and Elisabeth Beythien, Tierarzt (a veterinarian from a German university) of the Graf Lehndorff-Institute for Equine Science, studied 27 broodmares and their foals. All the mares had similar diets and a similar number of previous foalings. Gestation length and the sex ratio of foals did not differ.

The mare–foal pairs were grouped by foaling date: Group 1 foals were born between February and early March; Group 2 between early March and early April; and Group 3 from mid-April to May.

The researchers measured the size and weight of each mare’s placenta at birth. The measurements were smaller in the winter-foaling mares than mares delivering later. This indicates a reduction in nutrient transfer to the fetus, which could influence fetal growth.

A number of size parameters were also recorded for each foal weekly from birth to 12 weeks of age. Researchers found that most parameters—including height at withers, distance from fetlock to carpal joint and length from poll to nose—were lower in the earliest foaling group compared to the other groups. Foal weight did not differ among the groups.

“The size difference of the early-born foals was present over the whole observation period, i.e., until 12 weeks of age,” says Dr. Beythien. After that time, the mares and foals were sent out to pasture and could not be easily measured.

Despite the initial size disparity, Dr. Beythien notes that early-born foals will be taller and larger than later-born foals during the first few months of life simply due to the difference in age. “If you imagine a foal presentation scheduled in June, foals born in March will be taller than foals born in May because they are approximately eight weeks older at this time,” she explains.

For this reason, breeders often feel early-born foals have an advantage in young-horse competitions at 3 to 4 years of age. Ultimately, though, breeding mares too early in the year could be detrimental to the foals’ mature size, says Dr. Beythien. Based on other studies, she adds, the lower energy supplied to the winter foals during gestation may also contribute to health problems during adult life. Therefore, the team concluded, breeders may be better off aiming for foals to be born during the natural foaling season (typically between April and September) when they’ll “experience a healthier environment during gestation,” says Dr. Beythien.

This article was originally published in the November 2017 issue of Practical Horseman. 

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