Determining accurate total body phosphorus status and requirements is important in ruminants around the world. Environmental concerns are associated with pollution of surface water and rapid algae growth caused by phosphorus in developed countries, 1,2 and extensive areas of phosphorus deficiency in developing countries affect livestock produc- tion. 3,4 Various indicators of body phosphorus status, such as the concentrations of bone phophorus, 5–7 and inorganic phosphorus (Pi) concentrations in plasma (PPi), serum, 8,9 feces, 9 whole blood (WBPi), 7,10,11 saliva, 10,12 rumen fluid, 7,10 hair, and some body organs10 fall short of the goal of accurately assessing whole body phosphorus status. Eryth- rocyte uptake of phosphorus-32 also has been studied. 13 A major part of the Pi concentration in whole blood consists of intra-erythrocytic phosphorus. The goals of this study were to: 1) compare 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. Collectively, the inter- relationships among PPi, RBCPi, and WBPi were also studied.

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