Phosphorus homoeostasis in growing calves

Abstract

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.

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

“…Urinary P outputs were negligible in accordance with previous reports (e.g., Vitti et al, 2000). There is some evidence that urinary excretion of P is related to serum P concentration and significant amounts are excreted only when the plasma P concentration exceeds a threshold, which in growing calves was about 2.3 mmol/l (Challa and Braithwaite, 1988b;Challa et al, 1989).…”

Dinâmica do fósforo em ovinos alimentados com dietas contendo diversos níveis deste mineral

“…Verificou-se baixa correlação linear (r = 0,37, P<0,05; Tabela 3), embora positiva, entre o fósforo ingerido e o excretado na urina. Segundo Challa et al (1989), as perdas urinárias de fósforo normalmente não estão relacionadas à ingestão de fósforo, mas estão associadas à eficiência de absorção. Esses autores ressaltam que as glândulas salivares controlam de forma eficiente o fósforo plasmático, fato verificado por Portilho (2003), que observou correlação positiva entre fósforo no plasma e na urina (r = 0,81; P<0,01).…”

Effects of Molybdenum and Sulfur on Minerals in the Digestive Tract of Steers

“…indicating that the net input (duodenal flow minus intake) of p to the stomach region was lower (p<0.05) in steers fed these diets. Saliva is a maior contributor to the flow of P at the duodenum (Kav 1960: Challa et al 1989. fed HM compared to LM diets but with no effect on the apparent digestibility of p.…”