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Foal Size Related to Characteristics of Mare and PlacentaBy Kentucky Equine Research Staff · April 2, 2015

All breeders hope to produce healthy, well-grown foals that continue to grow and thrive. It is known that larger mares tend to deliver larger foals and that a mare’s first foal may be somewhat smaller than her later foals. A study conducted in Brazil looked at a number of maternal, paternal, and placental characteristics that might be related to foal size, newborn vitality, and status at 30 days of age in 18 Mangalarga Paulista and 20 Brazilian Sport Horses.

For each foal researchers recorded the sire’s height, weight, and thoracic perimeter, as well as dam’s height, weight, post-partum thoracic perimeter, age, length of gestation, and parity (whether the foal was the mare’s first, second, and so on). Umbilical cord length and placental weight were also recorded. Data for foals included height, weight, and thoracic perimeter at birth and at 7 and 30 days of age; Apgar score (a measure of overall neurologic and physiologic status) at 3 and 60 minutes after birth; time to stand and nurse; and time before passing meconium.

Analysis of data showed that bigger, older, and multiparous mares had larger foals than smaller, younger mares foaling for the first time. The foals from larger mares were taller and heavier and had greater thoracic perimeters at birth and also at 7 and 30 days of age. Older and multiparous mares also produced heavier placentas, which is possibly related to the better ability of these mares to allow the uterus to expand during pregnancy. Fetal nutrition is provided by the placenta, and a small or poorly developed placenta may be less able to deliver nutrients and support fetal growth.

There were no significant relationships between sire characteristics and neonatal foal characteristics for the horses in this study.

In foals, higher Apgar scores were correlated with standing and nursing soon after birth. Heavier placentas were also correlated with foals that stood and nursed quickly. Foals born after unusually long gestation periods had lower Apgar scores and took longer to stand and nurse.  

In this study, maternal thoracic perimeter was one of the most important factors in foal weight and size. If all other factors are equal, choosing a large-barreled mare that has had several foals might be a better option than using a smaller mare for breeding if a large foal is considered desirable.