Enhanced or Reduced Fetal Growth Induced by Embryo Transfer Into Smaller or Larger Breeds Alters Postnatal Growth and Metabolism in Weaned Horses

Embryo transfer between breeds of different sizes impacts fetal growth in horses. We have shown that it elicits various postnatal adaptations in terms of growth and glucose metabolism until weaning. Postweaning effects remain to be described. Pony (P), saddlebred (S), and draft (D) horses were used. Control Pony-in-Pony (P-P; n = 21) and Saddlebred-in-Saddlebred (S-S; n = 28) pregnancies were obtained by artificial insemination. Enhanced and restricted pregnancies were obtained by transferring P or S embryos into D mares (Pony-in-Draft [P-D], n = 6 and Saddlebred-in-Draft [S-D], n = 8) and S embryos into P mares (Saddlebred-in-Pony [S-P], n = 6), respectively. Control and experimental foals were raised by their dams and recipient mothers, respectively and weaned on day 180. Weight gain, growth hormones, and glucose metabolism were investigated in foals between days 180 and 540. Pony-in-Draft (P-D) remained heavier than P-P on days 180, 360, and 540, with lower glucose and higher non-esterified fatty-acids on days 180, 360, and 540 and higher T₃ on day 180. Insulin sensitivity was similar between pony groups on days 200 and 540. S-P were lighter than S-D on day 180 but caught up by day 540. S-P had higher glucose than S-D on days 180, 360, and 540, as well as lower non-esterified fatty-acids and higher T₃ on day 180. Insulin sensitivity was higher in S-P than in S-D on day 200. No difference was observed between saddlebred groups thereafter. In conclusion, in horses, fetal growth is determinant for postnatal metabolism, especially for energy availability.