Bone metabolic markers in the colostrum of parturient Holstein cows in relation to parity number and milk fever

This research communication aimed to describe the concentrations of major bone metabolic markers in colostrum from dairy cows on the day of calving and to clarify their associations with parity and the incidence of milk fever (MF). Forty-five Holstein cows were categorized into three groups based on parity: P1 (first calving; <i>n</i> = 18), P2 (second calving; <i>n</i> = 11) and P3 (third to sixth calving; <i>n</i> = 16). Within the P3 group, 6 cows developed MF within a few hours after calving (MF cows), whereas 10 showed no signs of MF (non-MF cows). Concentrations of osteoprotegerin (OPG), tartrate-resistant acid phosphatase isoform 5b (TRAP5b), cross-linked N-telopeptide of type I collagen (NTx), bone-specific alkaline phosphatase (BAP or ALP3) and calcium (Ca) were measured. With increasing parity, serum concentrations of Ca, ALP3 and TRAP5b significantly declined (<i>P</i> < 0.001-0.05). Parity also significantly influenced colostral bone markers: colostral BAP was highest in the P2 group (<i>P</i> < 0.01). Among P3 cows, serum Ca concentrations were significantly lower in MF cows than in non-MF cows, although no significant differences were observed in colostral Ca concentrations. Although no statistically significant differences were detected in other colostral bone metabolic markers between MF and non-MF cows, colostral OPG concentrations tended to be higher in MF cows (<i>P</i> = 0.088). Across all 45 cows and within the P3 group, colostral OPG was moderately and positively associated with serum OPG, as well as with both colostral and serum NTx concentrations. In conclusion, colostral OPG concentration in parturient cows with three or more parities may serve as a useful indicator for reflecting the bone metabolic status of dairy cows with and without MF.