Relationships between phosphorus intake, plasma phosphorus and faecal and urinary phosphorus excretion in young sheep

Abstract

The effects of 6 levels of phosphorus (P) intake on plasma P concentrations and P excretion were studied in young sheep individually fed a basal diet of wheaten chaff, lucerne chaff, cane sugar and wheat gluten meal that provided 0.68 g P/day. Monosodium phosphate was used to supplement the remaining 5 diets with 1, 2, 3, 4 or 5 g P/day, to give intakes of 20-150 mg P/day per kg liveweight (LW). Faecal P excretion, apparent P absorption (P intake-faecal P) and total P excretion all showed marked linear responses to P intake (P<0.001). Urinary P excretion was low at intakes below 80 mg P/day per kg LW, but was higher and more variable above this intake. Urinary P excretion and apparent P absorption were closely related (P<0.001) in a manner well described by 2-phase linear regression with phases intercepting at an apparent absorption of 43.6 mg P/day per kg LW. Below this point, only about 13% of apparently absorbed P appeared in urine, but above it, urinary excretion of absorbed P appeared complete. The effect of P treatment on plasma P concentrations was not marked (P=0.057) and observed concentrations were low (<45 mg P/L) compared with published studies. Nevertheless, there was a significant (P<0.001) exponential relationship between urinary P excretion and plasma P, though this explained less than 60% of the variance in the former. Urinary P excretion was thus better predicted from P intake and faecal P, either by using them to calculate apparent P absorption, or by using them as separate terms in a multiple regression. It is suggested that, following field validation, these relationships could be useful in modelling P cycling in sheep grazing systems.