Bone metabolism in dairy cows in three lactations during dry period and early lactation

The paper presents a study of bone metabolism in Holstein cows during transition period and early lactation in 2(nd), 3(rd) and 4(th) lactation. Samples of blood were taken on three occasions: 14 days before calving ("-14"), on day 10 and on day 30 after calving ("+10" and "+30", respectively). Concentrations of calcium, phosphorus, parathyroid hormone (PTH), C terminal telopeptide of type I collagen (CTx) and the activity of bone-specific alkaline phosphatase (BAP) were determined in the blood serum. A significant increase in the concentrations of CTx on "+10" in comparison with "-14" in the second (p < 0.001), third and fourth lactation (p < 0.05), and on "+30" in comparison with "-14" in the second, third and fourth lactation (p < 0.001, p < 0.01 and p < 0.05) was observed. A significant influence of the lactation number was found for BAP in total (p < 0-01) and on "-14" (p < 0.05). Significant changes in concentration of Ca and P were observed in the fourth lactation (p < 0.05); the concentration of Ca was significantly lower on "+10" in comparison with "-14", and the concentration of P was significantly higher on "+30" in comparison with "+10" (p < 0.05). Obtained results indicate a fast mobilisation of calcium stores from bones during early lactation in dairy cows. The results also indicate a decreased activity of osteoblasts in animals with a larger number of lactations, and, consequently, a decreased bone synthesis and a difficult maintainance of permanent Ca and P concentrations at the beginning of lactation.