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 6 months gestation to foaling, and in both mares and foals for 4 months after birth. From 6 to 11 months of gestation, there was no change in mean monthly OC. Serum PYD increased between 7 months gestation and foaling. After foaling, mean serum OC was low up to 14 days, rising to peak at 1 month. Serum PYD rose concomitantly during this period, but subsequently declined. The mare OC:PYD ratio fell to a nadir within 14 days of birth, before rising to a peak at 2 months. In foals, OC rose within the first 24 h of birth to peak at 3 months. PYD fell from birth levels by 1 month of age. Maternal bone mobilisation occurs progressively from 8 months 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.