Australian researchers based at Coolmore Stud in New South Wales, Australia, reported in a new study that placental weight and parity (number of foals a mare has had) are positively associated with foal weight. In contrast, neither gestation length nor age of mare were implicated in affecting birth weight.
"It is widely perceived by the Thoroughbred industry that a large foal equates to a large yearling which will therefore sell for more money due to their assumed superior racing ability," explained Chris Elliott, BVSc (hons), from Main Ridge Veterinary Clinic in Victoria, Australia.
Previous research in this field has identified relationships between placental weight, age, parity, and foal birth weight in horses in the United States and the United Kingdom; however, this data has not been explored in Australian horses and the relationship between gestation length and resulting foal and placental weight have not been widely evaluated.
During the 2006 breeding season, data from 348 foals was collected at the time of parturition. These included foal weight, placental weight (i.e., the entire placenta including amniotic membranes, umbilical cord, and allantochorion), gestation length, and mare age and parity at time of foaling.
"Key findings in this study were that neither the length of gestation or the mare's age or parity seem to play an important role in foal weight," said Elliott. "We did find, however, that a positive association between placental weight and foal weight existed, but only to a placental weight of 6.5 kg. Above, this threshold, the association between these two parameters was no longer significant."
This means that either the placental weights up to 6.5 kg limit foal birth weight or that placental weight increases concurrently with foal weight.
Elliott points out that this finding is unique from the rest of the current accepted research which states that placental weight and foal weight have a direct, continuous linear relationship with no threshold point.
The positive relationship between parity and foal weight is thought to be due, at least in part, to increased placental weight.
According to Elliott, the take-home message is that regardless of gestation length or age, the more foals a mare has, the more likely it is that each subsequent foal will be larger than the previous and more likely to grow into a large, successful Thoroughbred.
The study, "Factors affecting foal birth weight in Thoroughbred horses," is scheduled to be published in an upcoming edition on the journal Theriogenology.
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.