Phosphorus nutrition of beef cattle. 4. The use of faecal and blood phosphorus for the estimation of phosphorus intake

Abstract

In two experiments measurements were made of the phosphorus content of faeces and blood of penned Angus steers which received diets containing different amounts of phosphorus. In experiment 1, mature carpet grass (Axonopus affinis) hay was supplemented with different amounts of sodium dihydrogen orthophosphate and in experiment 2, different mixtures of carpet grass hay and lucerne hay were given. Phosphorus intake (X) was related to total daily faecal phosphorus excretion (YT) by the equation: YT= 2.442 + 0.289 ¦ 0.234 (r2 = 0.96; P < 0.001) It is therefore possible to estimate phosphorus intake (X) from total daily faecal phosphorus excretion from the equation : X = 3.460 YT- 8.450 Because the constant 8.450 will vary with the size of the cattle, both variables should be expressed in units of metabolic body weight SO that the prediction equation then is : Xc = 3.401 Yc - 0.1 73 When the regression equation calculated for cattle from these experiments was compared with that calculated for sheep the differences in the slopes of the two regressions suggested that sheep may be more efficient than cattle in retaining phosphorus when intakes of this nutrient are low. Regressions relating phosphorus intake to phosphorus content of faecal dry matter and organic matter differed for the two experiments (P < 0.05) but residual variability was significantly reduced when dry matter digestibility was included as a second independent variable. Plasma inorganic phosphorus concentration was significantly related to phosphorus intake (P < 0.05) but the relationship varied (P < 0.05) depending on the time of day at which samples were collected.