The influence of maternal size on pre- and postnatal growth in the horse: III Postnatal growth.

The research investigates the influence of the size of the mother (mare) on the pre and postnatal growth of horse foals. The study finds that Thoroughbred foals carried by Pony mares are smaller but proportional versions of their breed, whereas Pony foals carried by Thoroughbred mares are larger but proportionally similar to their breed.

Research Context and Method

• This study involved comparing the growth parameters of Thoroughbred (Tb) and Pony (P) horse foals that had their in utero environment either ‘restricted’ (Tb foals to smaller Pony mares) or ‘luxurious’ (Pony foals to larger Thoroughbred mares) to those conceived naturally within their breed. The focus was on observing the effects from birth to 3 years of age.
• Growth parameters considered included body weight, height at the withers, girth, poll-to-nose length, crown-rump length, and three longbone measurements of the foreleg. These measurements were taken at regular intervals.

Findings

• The findings showed that at birth, Tb foals carried by P mares were approximately 15% smaller in all observed parameters compared to controls, while P foals carried by Tb mares were roughly 15% larger. By the age of 3 years, this disparity reduced to around 5%.
• Postnatal growth seemed to be impacted by the in-utero conditions. Tb foals which had restricted growth in-utero showed an enhanced growth rate in the first 6 months after birth and vice versa for the P foals.
• In terms of physical conformation, the treatments did not drastically alter the horses’ forms. The Tb carried by the P mares were found to be smaller but proportional versions of the Tb-in-Tb, whereas the P carried by Tb mares were larger but proportional versions of the P-in-P.

Implications and Conclusion

• The research highlights how in-utero conditions can influence postnatal growth rates in horses. However, the eventual proportions remain consistent with that of their breed, with no major conformational changes.
• The results indicate the robustness of the genetic design and growth patterns of these horses—despite changes in in-utero conditions, the horses grow to become scaled versions of their breed.