E-Poster Presentation ESA-SRB-ANZBMS 2021

In dairy cattle, selection for high milk yields creates significant metabolic demand for all nutrients including calcium which is met by changes in bone homeostasis. The goal of the present study was to monitor the dynamics of bone turnover at different stages of lactation in a herd of high yielding dairy cows. Blood and milk samples were collected from 42 Holstein Friesian cows during late pregnancy (LP), early lactation (EL), mid lactation (ML), late lactation (LL), extended lactation (ExL) as well as non-pregnant, non-lactating young (heifers) and older (empty) cows. Samples were assayed for the bone resorption marker, CTX-I, and bone formation marker, osteocalcin. Rib biopsies were obtained from 33 cows during EL, ML, LL and LP. Biopsy samples were evaluated using micro-CT and dynamic histomorphometry. Generalised mixed model was used for statistical analysis in R. CTX-I concentration was decreased in LL, ML and ExL compared to baseline category EL. Some evidence was obtained that CTX-I was higher in heifers than during EL (Table 1). Small effects on serum osteocalcin levels were observed in cows at stages of lactation. Micro-CT evaluations showed total volume was higher in ML (β: 0.17, CI: 0.07-0.26) and LP (β: 0.28, CI: 0.13-0.41) as compared to EL (β: 0) indicating a gradual increase in bone mass as milk production declines. Interestingly, the average total porosity was higher in ML 3.3% (CI: 2.8-3.8) and LP 3.5% (CI: 2.8-4.5) than in EL 2.8% (CI: 2.4-3.3) and LL 2.4% (CI: 1.9-3.0) cows, probably due to the porous nature of new bone. The data suggest that dairy cows adapt to the calcium demands of lactation by changing levels of bone resorption while maintaining constant bone formation. These findings can be used to further study how dairy management practices and breeding programs affect bone health in dairy cattle.