Phosphorus and calcium metabolism in growing calves with special emphasis on phosphorus homoeostasis. 4. Studies on milk-fed calves given different amounts of dietary phosphorus but a constant intake of calcium

Abstract

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.

III. Quantitative aspects of phosphorus excretionin ruminants

“…Relationship between the ratio of faecal endogenous phosphorus to total faecal phosphorus (PFEC ENDO / PFEC TOT ) and urinary phosphorus (PURI, g·kg [18] as illustrated in model 16 and Figure 3. We found urinary phosphorus flow non significant when dietary supply was low but significant above a step of requirements [7,19], and correlated (r 2 = 0.53) with ingested phosphorus (model 17, Fig. 4).…”

“…This ratio of endogenous faecal phosphorus to total faecal phosphorus is highly variable from 0.12 to 0.95 in our database. The lowest value (0.12) was obtained in a growing calf fed a milk-based diet [7] and in adult sheep fed a diet rich in wheat bran [21]. The highest value (0.95) was obtained with high forage content diets as in the study of [22] for the determination of phosphorus availability in alfalfa hay.…”

“…In growing calves urinary phosphorus could represent as much as 0.88 of the total phosphorus loss [7].…”

“…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]. Saliva production was investigated by [27].…”

Determination of macromineral requirements for preweaned dairy calves in tropical conditions

“…The RC of Ca obtained in this study was 73.18% (Table 3); this is higher than that of BR-CORTE (2016), which was reported as 56.8% for Zebu beef cattle. Challa and Braithwaite (1989) reported an RC for Ca of 83% for 138-kg calves, and Yuangklang et al (2004) reported an absorption coefficient for Ca of 95% for 47kg calves. NRC (2001) and AFRC (1991) recommended values of 70 and 68%, respectively, as absorption coefficients for Ca.…”