Endocrine profiles of periparturient mares and their foals

The aim of this study was to characterize concentrations of leptin, IGF-I, and thyroid stimulating hormone (TSH) in the blood serum of mares pre-and postpartum, in the milk serum of mares postpartum, and in the blood serum of their foals. Nine pregnant Quarter Horse mares and their offspring were used in this study. Once weekly between 1000 and 1200 h for 2 wk before their predicted parturition date, mares were weighed, assigned a BCS, and blood was sampled via jugular venipuncture. Within 2 h of parturition and before the foals nursed (d 0), blood samples were obtained from the mares and foals, and a milk sample was collected from the mares. Blood from the foals and blood and milk from the mares were collected again at 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 12, 19, 26, 33, and 61 d postpartum. Mares and foals also were weighed and assigned a BCS on d 0, 5, 12, 19, 26, 33, and 61. Additionally, on d 5, 33, and 61, ultrasound images of fat depth and area of the LM immediately cranial to and parallel with the last rib on the left side of the foals were measured to characterize changes in fat depth and LM area over time. There were no changes in mare blood concentrations TSH (P = 0.15), nor were there any changes in foal blood concentrations of leptin (P = 0.54) or TSH (P = 0.10) during the trial period. Mare blood concentrations of IGF-I tended to change over time (P = 0.07), whereas leptin changed over time (P < 0.001), initially decreasing and then remaining relatively stable after d 5. Foal blood concentrations of IGF-I increased initially, peaked at d 19, and stabilized thereafter (P < 0.001). Milk concentrations of leptin and TSH were greatest on d 0 and decreased over time (P < 0.007), reaching nadir concentrations at d 61. Milk concentrations of IGF-I also changed over time (P = 0.02), being greatest on d 0 and undetectable by d 12. There was no difference in BCS (P = 0.94) in mares over time, but there was a difference between pre- and postpartum BW (P < 0.001) due to foaling. However, no differences were detected in pre- (P = 0.70) or postpartum BW (P = 0.76) of mares over time. Mean ultrasonic fat depth and LM area increased (P < 0.04) as the foals aged, as did BCS and BW (P < 0.001). Recognizing changes in metabolic hormones surrounding the time of parturition in the mare and foal provides a basis for further determination of the role, if any, these hormones play in the milk, as well as in the neonate.