The relationship between blood indicators of phosphorus status in cattle

Background: Determining accurate body phosphorus status and requirements is important in ruminants because of environmental concerns of surface water pollution by overzealous phosphorus consumption in developed countries, and extensive regions of phosphorus deficiency in developing ones. Current indicators, such as concentrations of bone, plasma, fecal, and rumen inorganic phosphorus (Pi), fall short of this goal. In addition, plasma Pi (PPi) concentrations may be falsely increased by hemolysis during storage of blood samples. Objectives: The goals of this study were to: 1) compare whole blood Pi (WBPi), red blood cell Pi (RBCPi), and PPi concentrations in different groups of dairy calves and cows; 2) evaluate relationships among the concentrations of these indicators of phosphorus status; and 3) investigate the possibility of predicting RBCPi and PPi concentrations according to WBPi concentration. Methods: Heparinized whole blood samples were obtained from 53 suckling calves, 29 weaned calves, 14 heifers, 20 dry pregnant cows, and 19 high-producing lactating dairy cows. Pi concentration was measured in plasma and in lysates of whole blood and packed RBCs using an automated phosphomolybdate method. PCV was determined using the microhematocrit method. Results were analyzed using 1-way ANOVA and regression analysis. Results: Heifers and dry cows had significantly higher RBCPi concentrations, and weaned calves and dry cows had significantly higher WBPi concentrations, compared with other groups. In all groups, WBPi was significantly associated with other variables, including PCV. Regression models for each group were calculated to predict RBCPi and PPi concentrations according to WBPi concentration. Conclusion: Results of different blood phosphorus indicators vary according to age and stage of lactation. The models that predict PPi and RBCPi according to WBPi may have some value in estimating total body phosphorus status in cattle.