The results of six experiments on growing cattle weighing 140–480 kg, and with liveweight gains of —0·46 to 1·11 kg/day, were reanalysed to provide estimates of their phosphorus (P) requirements. The 158 data sets were from individually penned cattle offered barley straw-based diets ad libitum with dry matter digestibilities of 0·53–O·65, or from cattle grazing tropical pastures with in vitro dry matter digestibilities ranging from 0·50 to 0·62. Various concentrations of dietary energy, nitrogen (N), calcium (Ca) and P were imposed during the course of the experiments with the penned cattle and various rates of application of P fertilizer changed the botanic and nutrient composition of the forages available to the grazing cattle. The P balances and P kinetics of the cattle were studied using 32P as a tracer.
Over the range of P intakes normally observed in cattle consuming forage diets (10–60 mg/kg LW), the coefficient of P absorption was high and not affected by age or liveweight. The regression coefficient relating P intake to P absorption was 0·77 for unsupplemented grazing cattle and 0·82 for penned supplemented cattle. When the plasma inorganic P concentrations were < 50 mg/1, urinary P excretion of the penned cattle was low, as were the endogenous faecal P losses of both the penned and grazing cattle. These losses were concluded to represent obligatory losses and were related to dry matter intake (r = 0·73) in the range 9–17 mg P/kg LW.
The total P requirements of growing cattle were estimated as g/day and g/kg DM intake from this data. The requirements of cattle consuming forage diets were 40–50% lower than those published by the Agricultural and Food Research Council (AFRC 1991), even though the same equation for the net requirements for growth was utilized.