Serum bone biomarkers osteocalcin and pyridinoline in mares during pregnancy and lactation, and in foals during early post-natal life.

Breeding mares typically foal yearly. Little is known about the dynamics of maternal bone stores during gestation and lactation, the timing of any maternal bone mobilisation, re-accretion post-foaling, or the dynamics of bone metabolism in foals. We measured serum osteocalcin (OC) and serum pyridinoline (PYD) concentrations in 18 mares monthly from 6months gestation to foaling, and in both mares and foals for 4months after birth. From 6 to 11months of gestation, there was no change in mean monthly OC. Serum PYD increased between 7months gestation and foaling. After foaling, mean serum OC was low up to 14days, rising to peak at 1month. Serum PYD rose concomitantly during this period, but subsequently declined. The mare OC:PYD ratio fell to a nadir within 14days of birth, before rising to a peak at 2months. In foals, OC rose within the first 24h of birth to peak at 3months. PYD fell from birth levels by 1month of age. Maternal bone mobilisation occurs progressively from 8months of gestation until term, before increasing markedly in very early lactation. Net mobilisation switches to accretion by one to two months of foaling, suggesting that this is a period during which mares replenish their own bone stores. Changes in the ratio of OC to PYD indicate adaptation to the prevailing biological milieu. In foals, the increase in the OC:PYD ratio in early life reflects the dominance of bone accretion.