DIETARY NITROGEN AND PHOSPHORUS DEPLETION IN CATTLE AND THEIR EFFECTSON LIVEWEIGHT GAIN, BLOOD METABOLITE CONCENTRATIONS AND PHOSPHORUS KINETICS

In a 15-week animal-house experiment, 24 steers were offered one of si x diets based on molasses and an libitum barley straw. Three levels of dietary nitrogen (N) and three levels of dietary phosphorus (P), in f actorial combination, were formulated by the addition of urea, formald ehyde-treated wheat gluten and monosodium orthophosphate. Food intake, liveweight gain, plasma metabolites and P kinetics were measured unde r dietary regimens similar to those experienced by cattle grazing Aust ralia's northern semi-arid rangelands. The adverse effect of the low d ietary N on both liveweight change and feed intake was greater and mor e immediate than that of the dietary P deficiency. The reduction in fe ed intake due to the P deficiency approached that caused by the N defi ciency after 10 weeks. Under conditions of adequate dietary N, there w as a trend for the effects of P deficiency on liveweight gain to be ex acerbated. Dietary N and P deficiency reduced the concentrations of pl asma urea-N and inorganic P respectively. Dietary N deficiency had no effect on cortical rib bone thickness but P deficiency markedly decrea sed bone thickness. Faecal endogenous loss of P and P absorption effic iency ranged from 9 to 21 mg/kg LW and 0.63 to 0.82 respectively for P intakes from 6 to 41 mg/kg LW. Faecal endogenous losses were closely related to dry matter Intake and plasma inorganic P together. Dietary N deficiency affected the efficiency of absorption of P. The results o f this experiment indicate that cattle consuming diets containing low levels of N and P require supplementary N and P in combination to avoi d severe depletion since an increase in N intake alone exacerbated the P deficiency. The results are also discussed in relation to the publi shed findings regarding P metabolism and the implications for the calc ulation of P requirements.