The effects of low levels of dietary phosphorus upon the dry matter intake and metabolism of lambs

Abstract

The effects of feeding roughage-based diets deficient in phosphorus (P) were investigated in two 20- week experiments in young sheep. In Experiment 1, six-month-old lambs were offered diets containing low concentrations of either calcium (LCa HP) or phosphorus (HCa LP) or both minerals (LCa LP) ad libitum for 20 weeks. In Experiment 2,4-month-old lambs were offered the HCa HP, HCa LP and LCa LP diets ad libitum with a further group pair-fed the HCa HP diet at the same dry matter rate as the lambs offered the LCa LP diet. The semi-synthetic diet, based on barley straw, contained adequate quantities of energy and nitrogen. The major effect of the deficiencies was to reduce the dry matter intakes of the lambs. The lambs offered the LCa HP, HCa LP and LCa LP diets had 15,41 and 40% lower dry matter intakes than the HCa HP lambs in Experiment 1, whilst lambs offered the HCa LP and LCa LP diets consumed 34 and 3 1% less in Experiment 2. The dry matter intakes of the lambs were closely related to their plasma inorganic P concentrations in both experiments. In both experiments, the lambs offered the low P diets had lower dry matter digestibility coefficients than those offered the high P diets. This was considered to be due to changes in caeco-colic rather than rumeno-reticular digestion. There was no difference in liveweight gain of lambs pair-fed on the LCa LP and HCa HP diets. The diets, particularly the low P diets, caused significant changes in plasma Ca and P concentrations and demineralization of the bones as measured by rib biopsy and neutron activation analysis. Lambs offered the low P diets had lower faecal endogenous P losses, but the coefficients of absorption of P were similar. Lambs offered the low P diets had smaller slowly exchangeable-pool sizes and lower rates of recycling and irreversible losses of P.