CALCIUM DECREASES AND PARATHYROID-HORMONE INCREASES IN SERUM OF PERIPARTURIENT MARES

Changes in serum concentrations of Ca and parathyroid hormone (PTH) ma y develop in periparturient mares, may be influenced by dietary Ca, an d may be associated with changes in Ca concentration of mammary secret ion. Milk and blood samples were taken from eight mares on Farm A and eight on Farm B for 10 d before parturition and from four mares on eac h farm for 5 d postpartum. Milk Ca was measured by two commercial test s. Serum samples were analyzed for PTH and total Ca in 16 mares and fo r ionized Ca in six (Farm A). Parturition was induced in eight mares o n Farm A and four on Farm B; no significant difference in serum Ca or PTH was found between mares with induced and spontaneous foaling. Mean serum total Ca decreased from 12.5 mg/dL to a nadir of 11 mg/dL on d 2 postpartum, and mean PTH increased from 46 pg/mL to a peak of 186 pg /mL on d 2 postpartum. Serum total Ca concentrations were lower and se rum PTH concentrations were higher in Farm A mares than in Farm B mare s, probably reflecting dietary Ca (.35% of DM on Farm A and .55% on Fa rm B). The serum PTH peak and Ca nadir occurred on d 2 postpartum, 1 d later than reported previously in dairy cows. Milk Ca concentration i ncreased progressively from 7 d before parturition; this increase prec eded, so was not dependent on, prepartal changes in serum Ca and PTH.