Calves, fed continuously a P-deficient basal diet, were infused abomasally with a phosphate solution supplying either 0, 3, 6 or 9 g P/day in a 4 × 4 latin-square design experiment. Once steady-state conditions were achieved (after 2 weeks of infusion), 32P kinetic studies were carried out. At the same time, P and Ca balances and rates of flow of P at the reticulum were measured.

The rate of dietary P absorbed increased significantly with increasing P supply as did the serum P concentration. The rate of salivary P secretion was directly related to the serum P concentration and hence to P absorption, but the efficiency of absorption of salivary P remained relatively constant, ranging from 70 to 74% depending on treatment.

Despite the high requirements for P, the rate of endogenous faecal P loss increased directly with increased P supply and absorption, suggesting that some increased loss of P with increased P supply was inevitable. Urinary P secretion was negligible when the P supply was low but increased substantially as the serum P concentration increased from 2 to 4 mmol/1, possibly because the renal threshold was exceeded.

The rates of retention of P and Ca were low on the low-P treatment but increased significantly with increased P supply until this reached about 70 mg/day per kg live weight, beyond which they remained fairly constant, presumably because requirements had been met. Since the growth rate did not reach a maximum until the P supply exceeded 65 mg/day per kg live weight, it is suggested that the Agricultural Research Council (1980) recommendations of P requirements (34 mg/day per kg live weight) for these growing calves must be inadequate.

These results suggest that loss of P in saliva and urine may be determined by the serum P concentration whereas the efficiency of absorption of P from the intestine may be regulated according to P requirements.