Utilisation of the phosphorus and magnesium in some calcium and magnesium phosphates by growing sheep

Abstract

Growing wether sheep maintained in metabolism cages were given a basal low phosphorus diet (1.00 g P/day) supplemented (at three different rates) with either tricalcium phosphate or two magnesium phosphates derived from concentrated sea water. The two magnesium phosphates were prepared at different temperatures (either 300° or 500°c). Each diet was compared with feed grade dicalcium phosphate and, where appropriate, with reagent grade magnesium oxide to give equivalent additional intakes of phosphorus (either 0.88, 1.75, or 2.25 g P/day) and magnesium (either 0.69, 1.38, or 1.77 g Mg/day). All four phosphorus sources resulted in similar and significantly improved phosphorus retentions of between 0.53–0.68, 0.88–1.08, and 1.12–1.25 g P/day at the low (0.88 g/day), medium (1.75 g/day), and high (2.25 g/day) rates of phosphorus supplementation respectively, compared with a daily loss of about 0.10 g P/day for the unsupplemented diet. Magnesium retentions were increased from about 0.04 g Mg/day (unsupplemented) to 0.17–0.24 and 0.45–0.67 g Mg/day at the low (0.69 g/day) and high (1.77 g/day) rates of magnesium supplementation and were similar for both magnesium oxide and the two magnesium phosphates. At the medium (1.38 g/day) rate of magnesium supplementation the sheep given magnesium oxide retained significantly more magnesium (0.61 g Mg/day) than those given either of the magnesium phosphates (0.36–0.40 g Mg/day). At all three rates of supplementation, however, significantly greater amounts of the magnesium given as magnesium phosphate heated to 500°c were excreted in the faeces. Blood phosphorus concentrations were significantly increased by all forms of phosphorus addition.