The effect of dietary intake of calcium and dry matter on the absorption and excretion of calcium and phosphorus by growing lambs

Abstract

The isotope dilution technique was used to measure endogenous faecal excretion and absorption of dietary Ca and P by growing lambs. Groups of four lambs were given 0-8 or 1-2 kg/day of a pelleted barley and soya-bean diet containing 4-2 g P/kg D.M. and supplemented to contain 1-79, 3-52 or 4-67 g Ca/kg D.M.At the lowest dietary concentration of Ca, absorption of Ca was insufficient to meet the Ca required for deposition in the newly formed bone matrix of the skeleton; retention was 0-39, 1-16 and 1-12 g/day with increasing dietary Ca concentration. The proportion of dietary Ca absorbed decreased (P < 0-001) with increasing Ca intake, ranging from 0-85 to 0-44. Endogenous faecal excretion of Ca was independent of the absorption of dietary Ca, but increased (P < 0-05) from 17-2 at the lower to 21-1 mg/kg live weight/day at the higher D.M. intake. Plasma concentration of Ca was higher for the female (2-51) than the male (2-33 mmol/1) lambs and increased linearly (P < 0-001) with dietary concentration of Ca from 2-31 to 2-52 mmol/1.The retention of P increased with Ca and D.M. intake. On the other hand, the proportion of dietary P absorbed (0-85) was independent of Ca intake and hence of P retention. Increases in P retention were reflected by decreases in urinary P, but not in endogenous faecal excretion. Endogenous faecal excretion of P increased with D.M. or P intake from 17-9 at the lower to 33-5 mg/kg live weight/day at the higher intake.The results are discussed in relation to dietary allowances of Ca and the mechanism of P homoeostasis. TNTROTYTTPTTONmaintenance requirements consist predominantly of unabsorbed secretions of minerals into the digestive Two major difficulties exist in the factorial tract, any factor which alters the rate of secretion approach to the prediction of the dietary require-or the absorbability of the minerals in the secretions ments of ruminants for Ca and P . These are the will alter requirements. Many factors, including the choice of value for the coefficient of absorption of physical nature and quantity of feed (Bailey & dietary Ca and the maintenance requirements of Balch, 1961) are known to increase the volume of P. The difficulty with Ca is that ruminants absorb secretions and Braithwaite (1982,1983) has recently Ca according to needs (Agricultural Research shown experimentally that faecal excretion of endo-Council, 1980). Thus the wide divergence in the genous Ca varies directly with feed intake. Of published values for the proportion of dietary Ca greater concern is whether salivary P, the main absorbed (see Agricultural Research Council, 1980) source of secretion of P, is determined by the can be attributed to the fact that in many of these quantity of P absorbed surplus to requirements or studies Ca intake was rarely limiting and, for a by the actual quantity of P absorbed. The latter given net requirement, the proportion will vary would imply that maintenance requirements are inversely with Ca intake.not constant but vary with the requirements for In the factorial approach t...

Phosphorus kinetics of cattle grazing tropical pastures and implications for the estimation of their phosphorus requirements

“…In our study, endogenous faecal excretion of P ranged from 9-7 to 27-5 mg/kg LW per day and was related to P intake. Our results agree with similar findings for sheep (Braithwaite 1984(Braithwaite , 1985(Braithwaite , 1986Field et al 1985;Ternouth 1989) and add further weight to the argument that the inevitable loss of endogenous P in the faeces is not constant. Since endogenous faecal P is derived almost entirely from unabsorbed salivary P, it follows that excretion rates will be influenced by the amount of salivary P, which is the product of salivary volume and salivary P concentration.…”

EFFECT OF MATURITY OF ALFALFA (Medicago sativa) AT HARVEST ON CALCIUM ABSORPTION IN GOATS

“…Ratios of meq (Na + K -Cl) were similar among diets and exceeded 50 meq 100 g DM-'. Absorption of Ca in goats fed diets containing 48% alfalfahay and divergent ratios offixed cation-to-anion ranged from 17.7 to 44.8% of Ca intake (Fredeen et al 1988b Higher supply relative to requirement corresponds to higher total absorption and lower efficiency of absorption (Ramberg et al 19'76;Braithwaite 1979;Field et al 1985). (Field et al 1985).…”

Efeito do fosfato bicálcico ou monoamônico sobre a digestibilidade ruminal do feno de coast-cross

“…Os rins possuem habilidade de reter fosfatos, sendo menos de 1% perdido pela urina. Porém, quando a concentração sanguínea de P ultrapassa 6 mg P/dL, a capacidade de reabsorção dos túbulos renais é excedida, havendo uma perda significativa do mineral pela urina (FIELD et al,1983;SCOTT et al,1984;FIELD et al, 1985).…”