Dairy cattle spend most of their lives producing milk. This, as well as selection of dairy cattle for high milk yields creates a significant metabolic demand for all nutrients including calcium which must be met by changes in bone homeostasis. These changes may be associated with significant welfare implications if not dealt with appropriately. However, the effects of selection for increased milk yield and nutrient partitioning on bone loss and skeletal health in dairy cows have not been fully studied. The goal of the present study was to monitor dynamics of bone turnover and metabolic state at different stages of lactation in a herd of high yielding dairy cows.
Blood samples were collected from 30 Holstein-Friesian cows during late pregnancy, as well as early, mid and late lactation stages. Samples were assayed for the bone resorption marker CTX-I, the bone formation marker osteocalcin and metabolic markers including β-hydroxybutyrate (BHB) and glucose. Daily milk volume was recorded by a Lely robotic milking system.
A repeated measure ANOVA with a Greenhouse-Geisser correction determined that mean CTX-I (p<0.01) and glucose (p<0.05) concentration differed significantly between stages, whilst osteocalcin and BHB did not. Post-hoc analysis revealed that for CTX-I there was a significant decrease from early to late (63%, p<0.001) and mid to late lactation (42%, p<0.05) stages. Glucose concentration increased significantly from early to late lactation (15%, p<0.001). Lactation stage also explained milk yields (p<0.0001) with amounts decreasing from early to mid-lactation (38%, p<0.0001) and mid to late lactation (39%, p<0.0001) stages.
The data suggest that dairy cows adapt to the calcium demands of lactation through changing levels of bone resorption while maintaining constant bone formation.