Calcium balance, bone density, and performance of late-lactation dairy cows with supraphysiological concentration of blood vitamin D

Abstract

Vitamin D can affect calcium (Ca) metabolism, bone turnover and, lactation performance of dairy cows. To assess bone metabolism, it is important to standardize the technique of radiographic optical densitometry analysis. The objective was to standardize the radiographic optical densitometry technique in dairy cattle and to correlate the values with age, weight and, serum markers; to evaluate lactation performance, body Ca retention and, bone density of dairy cows in late lactation. Thirty Holstein cows (230 ± 137 days in milk, DIM) submitted to physiological (Phy) or supraphysiological (Supra) concentrations of vitamin D (calcidiol, 25(OH)D) in blood for 12 weeks were evaluated. With advancing age, bone mineral density (BMD) increased. Older animals had higher weight and lower blood concentration of phosphorus and carboxyterminal telopeptide of collagen type I. With advancing DIM, there was an increase in weight, total calcium, and ionized calcium. BMD was higher for the accessory carpus and correlated positively with total calcium and ionized calcium from the accessory carpus, metacarpal, lateral, medial, and medullary cortex regions. Body weight of the cows showed a positive correlation with BMD on radiographs of the lateral, medial and medullary metacarpal cortex. Bone alkaline phosphatase correlated negatively with lateral metacarpal BMD. The mean plasma concentration of 25(OH)D in the Supra group was twice as high as in the Phy group. Total Ca and ionized Ca concentrations were increased in the Supra group on days 28 and 56. Cows on Supra excreted more Ca in urine at week 12 than cows on Phy. Bone deposition markers osteocalcin and bone alkaline phosphatase were reduced in the Supra group on day 84. There was a trend towards reduced bone mineral density in the accessory carpal and metacarpal medullary region with Supra, suggestive of osteoporosis. The high level of vitamin D was associated with increased milk production and did not affect dry matter intake (DMI). The energy corrected milk, feed efficiency, body condition score, body weight gain, and apparent digestibility of the total tract of dry matter and organic matter did not differ between treatments. The Supra group was associated with a reduction in milk somatic cell count (SCC). It is concluded that the BMD determined by optical densitometry in radiographic images is a methodology that can be reproduced. Dairy cows exposed to a supraphysiological concentration of 25(OH)D for 12 weeks, even with similar DMI, produced more milk with lower CCS, had higher concentrations of Ca in the blood and excretion in the urine, reduced markers of bone mineral deposition and bone mineral density.