Calcium decreases and parathyroid hormone increases in serum of periparturient mares.

The research paper explores the fluctuations in serum calcium and parathyroid hormone concentrations in mares around parturition (birthing), their potential relationship with dietary calcium, and their possible effects on milk calcium concentrations.

Study Design and Methodology

• The researchers collected milk and blood samples from two groups of eight mares, each located on a different farm (Farm A and B), for a period of ten days leading up to parturition.
• Postpartum samples were taken from four mares on each farm for 5 days.
• The calcium content in the milk samples was determined using two commercial tests.
• The blood samples were analysed to measure parathyroid hormone and total calcium concentrations in all 16 mares. Additionally, ionized calcium levels were analysed in six of the mares on Farm A.
• Each farm induced parturition in a different number of mares (eight on Farm A, four on Farm B). This enabled the researchers to compare the serum calcium and parathyroid hormone levels between those with induced and spontaneous foaling.

Findings

• The study found that mean serum total calcium decreased from 12.5 mg/dL before parturition to 11 mg/dL on the second day postpartum, indicating a drop in calcium levels around the birth period.
• In contrast, the mean parathyroid hormone increased from 46 pg/mL to a peak of 186 pg/mL on day two postpartum, suggesting an increase in this hormone around the birthing period.
• The study also revealed that the levels of serum total calcium were lower and the concentrations of serum parathyroid hormone were higher in mares from Farm A as compared to mares on Farm B. This might be influenced by differences in dietary calcium content (.35% of dry matter on Farm A and .55% on Farm B).
• The peak in serum parathyroid hormone levels and nadir in calcium levels were observed on day 2 postpartum, a day later than what has been reported in previous studies on dairy cows.
• The study also found that milk calcium concentrations increased progressively from seven days before parturition, suggesting that this increase was not dependent on changes in serum calcium and parathyroid hormone occurring close to parturition.

Conclusion

The research establishes that changes in dietary calcium, parathyroid hormone, and serum calcium levels occur in periparturient mares. This offers better understanding of the calcium and hormone dynamics in mares around parturition, which might have implications for dietary considerations and clinical practices around mare birthing.