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

Abstract

The growth parameters exhibited by seven Thoroughbred (Tb) foals that had experienced a ‘restricted’in uteroexistence following transfer as embryos to the uteri of smaller Pony (P) mares (Tb-in-P) and, conversely, six P foals that experienced a ‘luxurious’in uteroexistence after transfer to larger Tb mares (P-in-Tb), were compared from birth to 3 years of age with those exhibited by six normal Tb-in-Tb and six P-in-P foals conceived by within-breed artificial insemination. Bodyweight, height at the withers, girth, poll-to-nose length, crown–rump length and three foreleg longbone measurements were made at regular intervals. At birth, an approximate 15% reduction or increase in parameters was observed in the Tb-in-P and P-in-Tb respectively, which declined to 5% by 3 years of age. Growth post partum was affected by restricted or enhanced growthin utero. In the first 6 months post partum, growth rate was enhanced in the previously restricted Tb-in-P foals and curbed in the previously enhanced P-in-Tb foals compared with their respective controls. Overall, the similarity of the responses of the offspring to both ‘restriction’ and ‘luxury’in uteroensured that no major changes to conformation resulted from either treatment. Thus, the Thoroughbreds carried by the Pony mares were merely scaled down versions of the Tb-in-Tb controls while the Ponies carried by the Thoroughbred mares were scaled up versions of the P-in-P controls.