Phosphorus and calcium metabolism in growing calves with special emphasis on phosphorus homoeostasis: 1. Studies of the effect of changes in the dietary phosphorus intake on phosphorus and calcium metabolism

Abstract

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...

Phosphorus requirement of finishing feedlot calves

“…The first reason is that maintenance requirements for P were overestimated by NRC (1996). The P maintenance requirements have been fairly well documented due to the ease of feeding cattle maintenance diets low in P and results support NRC recommendations (Ternouth et al, 1996;Challa and Braithwaite, 1988;Challa et al, 1989). A second reason is that gain requirements were overestimated by NRC (1996).…”

III. Quantitative aspects of phosphorus excretionin ruminants

“…In ruminants, saliva secretion controls the homeostasis of the rumen ecosystem by supplying buffers [24] and one of the primary roles of saliva is to transfer phosphorus from the plasma to the digestive tract [25]. This transfer explains why plasma phosphorus concentration rises when the parotid vein is ligated [26] or when chewing is limited [7].…”

Effect of energy and protein supplementation on phosphorus utilization in lactating dairy cows

“…The increase in faecal P output in response to increased P intake may be due to homeostasis and a result of an obligatory increase in unabsorbed endogenous and dietary P (Challa and Braithwaite, 1988a). The lack of significant difference in faecal P output when expressed as the proportion of P intake further confirms the positive linear relationship between the two variables.…”