Some observations on phosphorus homoeostasis and requirements of sheep

Abstract

The results of phosphorus kinetic studies from 149 sheep used in previous studies of calcium and P metabolism have been analysed statistically, to provide information on the mechanism of control of P homoeostasis.Whereas the rate of absorption of dietary P was increased in direct relation to P intake, the efficiency of absorption was decreased. Both the rate and efficiency of absorption, however, were increased in response to increased P demands. These results suggest that P may be absorbed by two processes: a passive one, related to P intake, and an active one, related to P demands.Endogenous loss of P in the faeces was directly related to both P intake and P absorption bat inversely related to P demands. Salivary secretion of P was directly related to P absorption and increased at a rate of 1 mg/day/kg body weight for each 1 mg/day/kg body weight increase in absorption. This finding supports the suggestion that the inevitable endogenous faecal loss of P, in animals fed exactly according to their P requirements, is not constant but varies in direct relation to the rate of P absorption, and hence to the P intake needed to supply these requirements.Urinary loss of P was generally low, but high values were found in some animals and were associated with increased rates of P absorption and decreased rates of endogenous faecal loss of P. Possible reasons for these high rates of urinary P are discussed.Increased P demands were met by a combination of an increase in P absorption and a decrease in endogenous faecal loss of P, and P retention was directly related to Ca retention.The relevance of these results to the calculation of P requirements and to the control of P homoeostasis is discussed.149 results are available from animals in various physiological states and given a variety of treatComparable recommendations by the Agri-ments. The purpose of this paper is to provide, by cultural Research Council (1965Council ( , 1980, the Nation-statistical analysis of these 149 results, information al Research Council (1968Council ( , 1975 and the Institut which may be useful in the calculation of P recomNational de la Recherche Agronomique (1978) for mendations and, at the same time, help improve the phosphorus requirements of sheep and cows at our understanding of the control of P homoeodifferent stages of growth, pregnancy and lactation stasis. can vary by as much as two-fold (Field, 1981; MATERIALS AND METHODS Braithwaite, 19836). This variation is due partly to different interpretation by these three bodies of Male and female sheep, ranging from 2 to 5 years the relevant information on P metabolism and of age, in various physiological states and given a partly to a general lack of this information.variety of different treatments were used in these During the past 5 years, an intensive study of studies of Ca and P metabolism. In particular, calcium and P metabolism of sheep has been studies were performed at different stages of pregcarried out at the National Institute for Research nancy and lactation and treatments inclu...

Dietary nitrogen and phosphorus depletion in cattle and their effects on liveweight gain, blood metabolite concentrations and phosphorus kinetics

“…The strong linear relationship between dietary P absorption and P intake, which has been reported by other workers (Braithwaite 1984;Challa et al 1989), indicates that P intake is the major factor influencing P absorption. This is in agreement with Tuen et al (1984) and Bortolussi et al (1992) who have reported that there was no evidence that low dietary N affected absorption of P from the gut.…”

Factors affecting faecal endogenous phosphorus loss in the sheep

“…I N R A (1978), for example, link their estimates of requirement to a level of faecal endogenous P loss that derives from a level of salivary P secretion needed to ensure that rumen microbial requirements for P are met. Braithwaite (1984Braithwaite ( , 1985, on the other hand, has argued that as faecal endogenous P loss increases with increase in P intake even at P intakes below requirement then maintenance P requirements should be based on measurements of faecal endogenous P loss at P intakes needed to meet production requirements; a view accepted by the N R C (1985) in their reassessment of the P requirements for sheep. In the light of these differences, it is of interest to compare current estimates of requirement as determined by T C O R N (1991) with those proposed by these authorities.…”

Mechanisms of phosphate transport in sheep intestine and parotid gland: response to variation in dietary phosphate supply

“…There is evidence of a similar adaptive response in ruminants when they are fed a low-phosphate diet (Care, Barlet & Abdel-Hafeez, 1980). The mechanism controlling this response is not known though it does appear to be independent of changes in parathyroid hormone or 1 ,25-dihydroxycholecalciferol secretion (Braithwaite, 1984;Breves, Ross & Holler, 1985;Maunder, Pillay & Care, 1986). (TCORN, 1990).…”