A potential new biomarker to monitor the phosphorus balance in dry dairy cows

Abstract

The present study aimed at comparing different candidate blood biomarkers in their suitability to determine activation of regulatory mechanisms responding to phosphorus (P) deprivation in dry dairy cows on deficient dietary P supply. Fifteen multiparous dairy cows in late pregnancy were assigned to either a diet with low (1.6 g/kg DM) or adequate P content (3.5 g/kg DM) during the last 4 weeks of gestation. Blood samples were obtained immediately prior to initiating the experimental feeding period (T-bl), after 2 weeks of experimental feeding (T-1) and 4 days before calving, which was equivalent to at least 3 weeks of experimental feeding (T-2). Studied parameters were the plasma concentrations of phosphate (Pi), calcium (Ca), CrossLaps (CTX), a biomarker for bone mobilization, 1,25-(OH)2-Vitamin D (1,25-(OH)2D) and the activity of alkaline phosphatase (AlP). Treatment and time effects were studied with repeated measures ANOVA, and the different parameters were correlated to the dietary P intake in the days before sampling. Treatment effects were only identified for the individual increase in the plasma concentration of CTX related to baseline values (ΔCTX) and to a lesser extent for the Ca:Pi ratio in plasma. ΔCTX was the only parameter correlated to P intake at T-1 and with the strongest association with P intake at T-2. The results of this study provide first indications for the suitability of ΔCTX to determine the level of dietary P supply at which counter regulatory mechanism to negative P balance are triggered during the dry period. This parameter that is easily applicable in a field setting can assist the validation of current recommendations for the dietary P supply in dry cows.