Effects of manipulating intrauterine growth on post natal adrenocortical development and other parameters of maturity in neonatal foals


Beaufort Cottage Stables, High Street, Newmarket, Suffolk CB8 8JS, UK


Reasons for performing study: Intrauterine growth retardation (IUGR) impairs post natal adaptive responses and is associated with increased adrenocortical activity in many species.

Objectives: To determine whether a restricted or enhanced intrauterine environment affects neonatal adaptation and adrenocortical function in horses.

Methods: Embryos from large (577 kg) Thoroughbred (TB) mares were transferred to smaller (343 kg) pony (P) mares and vice versa, to create a restricted (TB-in-P, n = 11) or enhanced (P-in-TB, n = 8) intrauterine environment. Control groups (TB-in-TB, n = 8; P-in-P, n = 7) were also included.

Results: Thirty foals were born live at full term (range 314-348 days) and 4 (3 TB-in-P, 1 P-in-TB) were stillborn between 275 and 335 days. TB-in-P foals were significantly (P<0.05) lighter than TB-in-TB, but heavier than P-in-P foals. TB-in-P foals took longer to first stand and suck and some had fetlock hyperextension and low (<4 g/l) plasma immunoglobulin G concentrations. Other foal groups showed normal behavioural responses. Haematological parameters were normal in all 4 groups of foals. Plasma ACTH levels were high at birth and plasma cortisol concentrations increased after delivery and returned to baseline within 6 h post partum in all but the TB-in-P foals, which had elevated levels until 48 h post partum. Plasma cortisol concentrations increased in all groups following exogenous ACTH administered on Days 1 and 5 post partum.

Conclusions: The TB-in-P foals showed IUGR and impaired post natal adaptive responses with basal hypercortisolaemia.

Potential relevance: Foals born following IUGR may require clinical assistance in the early post natal period, but appear mature with respect to adrenocortical function.