The mechanisms by which phosphorus homoeostasis is achieved in growing calves in response to changes in P supply ranging from deficient to excess were investigated in three studies in which additional P was given in the diet, infused into the abomasum, or infused directly into the blood.
Whereas the rate of P absorption was directly related to P supplied, the efficiency of absorption of P differed according to the supply. Thus, the absorption efficiency was low from the P-deficient basal diets, increased with P supplementation until the supply was sufficient to meet requirements and then decreased at higher rates of P supply, possibly as a result of homoeostatic control.
The serum P concentration, which was directly related to the rate of P absorption, appeared to determine the rates of P retention, salivary P secretion and urinary P excretion. Urinary P excretion was negligible at P intakes below those needed to supply requirements, and any additional P absorbed was partitioned between retention and salivary P secretion in the ratio 0–88:1, these two processes continuing to have equal priority for absorbed P until retention reached a maximum, approximately equal to the postulated growth requirement. Only when the growth requirement for P had been met, did urinary P excretion occur, increasing progressively as the serum P concentration exceeded the renal threshold.
The endogenous faecal loss of P increased in direct relation to salivary P secretion and hence to the rate of P supply, the increase occurring even in P-deficient animals. This finding supports the claim that the inevitable endogenous faecal loss of P is not maintained at the constant rate expected at zero P intake until all requirements are met, as assumed by the Agricultural Research Council (1980) in their calculations of P requirements, but varies in direct relation to the P intake needed to supply a particular requirement. Indeed, these results suggest that the ARC (1980) recommendations for P, particularly for growing animals, are too low.
Finally, these studies show that bone resorption, salivary P secretion, P absorption and urinary P excretion all play a part in P homoeostasis in the growing ruminating calf, and that the relative importance of each of these processes depends upon the P status of the calf.