S U M M A R YThe 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.

Calves were fed continuously at 4 h intervals a low P basal diet with or without P supplementation to give three levels of dietary P intake: one was deficient in P (2-5 g/day), the second adequate (6-0 g/day) and the third contained an excess of P (10-0 g/ day) according to Agricultural Research Council (1980) recommendations. Once steadystate conditions were achieved (after 2-3 weeks on the diet) 32 P and 46 Ca kinetic studies were carried out, together with measurements of P flow rates at the reticulo-rumen and P and Ca balances. With increased dietary P intake, the amount of dietary P absorbed increased as initially did the efficiency of P absorption. Both the serum P concentration and the rate of P retention increased in direct relation to increased P intake and increased P absorption. Salivary secretion of P increased with increased P absorption and in direct relation to serum P concentration. Endogenous faecal losses of P were also directly related to P intake and P absorption and results suggest that increased loss with increased P intake is inevitable.Despite a constant and adequate Ca intake, the rate and efficiency of Ca absorption, which was low on the low-P diet, increased significantly on the adequate and high-P diets. Similarly, Ca retention increased substantially on the higher P diets, showing that Ca metabolism can be controlled according to P status.TTMTRf>r>TTPTTniM information on P metabolism and, in particular, the 1J\ U lack of information on endogenous faecal loss of P. Unlike non-ruminant animals where the majorThe 1980 Agricultural Research Council recomcontrol of P homoeostasis is through renal mechanmendations for P requirement for ruminants are isms (Irving, 1964), ruminants usually excrete very considerably lower than previous recommendations little P in their urine (Agricultural Research Council, (Agricultural Research Council, 1965). This is 1980). They do, however, secrete large quantities of mainly because it was assumed that endogenous P through their saliva (Clark et al. 1973; Tomas, faecal loss of P does not vary but remains at a 1973; Bertoni et al. 1976) and much of this salivary constant level characteristic of animals given zero P P is lost in the faeces. It is generally accepted that P intake until all requirements for P have been met. homoeostasis in ruminants is achieved largely by However, this assumption is not accepted by all control of the faecal P loss, though there is workers in the field of P metabolism (Grace, 1980; controversy over the exact mechanisms involved. Perge et al. 1982;Braithwaite, 1984Braithwaite, , 1985 and the Thus, it is not clear whether P homoeostasis is matter remains controversial. It is clear that before brought about by control of P absorption, control of requirements for P can be precisely defined, more salivary P secretion or a combination of the two. information on P metabolism under different nutriThis controversy stems largely from a general lack of tional and physiological conditions must be obtained. The present study was designed t...

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 x 4 latin-square design experiment. Once steady-state conditions were achieved (after 2 weeks of infusion), 32 P 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. INTRODUCTION^' s corn pl' cat i°n> additional P was supplied in the present study by abomasal infusion, rather than by The previous paper (Challa & Braithwaite, 1988) dietary supplementation, thus by-passing the rumen showed that increasing the dietary P intake of and reticulum. Any observed changes in the amount growing calves from deficient to adequate resulted in of P passing the reticulum therefore must be due to an increased rate of P absorption, and a consequen-changes in the salivary P secretion, tial increase in the rate of salivary P secretion and in Since in the present work, the total P supply was the inevitable endogenous faecal loss of P.again varied from deficient, through adequate, to In those studies, increased flow of P at the plentiful for the calculated growth requirements of reticulum was due to a combination of increased the calves, the results add to our knowledge of the dietary P and increased salivary P. In order to avoid mechanisms of homoeostatic control of P.

SUMMARYFriesian bull calves, 2 weeks old, were given an experimental diet of cow's milk supplemented with phosphorus to provide three rates of P intake: adequate, high, and very high. After 1 week's preliminary feeding, studies of absorption, endogenous secretion, urinary excretion, serum concentration, retention and kinetics of Ca and P metabolism were made by a balance and dualradioisotope technique with 32P and 45Ca.The efficiency of P absorption decreased significantly as the P intake increased and the highest absorption efficiency was 96% on the unsupplemented milk diet. Endogenous faecal excretion of P was low on all treatments. Urinary P excretion was substantial even on the milk-only diet and increased further with supplementation of P. The efficiency of absorption of Ca remained constant, and urinary Ca excretion was low in all the groups. Serum P and Ca concentrations did not vary significantly but there was a significant decrease in the retention of P and Ca as the P intake increased.Although urinary excretion appears to be the main process responsible for P homoeostasis in preruminant milk-fed calves, some regulation of the efficiency of absorption of P according to the P intake may also occur.

SummaryCalves were fed continuously at 4 h intervals an adequate-P diet and were infused intravenously with a solution of phosphate supplying either 0, 3, 6 or 9 g P/day in a 4 x 4 latin-square design experiment. Once steady-state conditions were achieved, rates of flow of P at the reticulum, serum P and Ca concentrations, and urinary excretion rates of P and Ca were measured.The concentration of serum P increased significantly with increased intravenous infusion of P as also did the salivary P secretion. Results suggest that it is the serum P concentration that regulates the secretion of P through saliva. Urinary P excretion, which was low in the control group, increased significantly with increased intravenous loading of P. Substantial urinary P excretion occurred, however, only when the serum P concentration exceeded 2·30 mmol/1, which is probably the renal threshold for P. The serum Ca concentration was unaffected by the intravenous infusion of P and urinary Ca excretion remained negligible on all treatments.