THE PHOSPHORUS REQUIREMENTS OF GROWING CATTLE CONSUMING FORAGE DIETS

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 s ets were from individually penned cattle offered barley straw-based di ets nd libitum with dry matter digestibilities of 0.53-0.65, or from c attle grazing tropical pastures with in vitro dry matter digestibiliti es ranging from 0.50 to 0.62. Various concentrations of dietary energy , nitrogen (N), calcium (Ca) and P were imposed during the course of t he experiments with the penned cattle and various rates of application of P fertilizer changed the botanic and nutrient composition of the f orages available to the grazing cattle. The P balances and P kinetics of the cattle were studied using P-32 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 a ge or liveweight. The regression coefficient relating P intake to P ab sorption was 0.77 for unsupplemented grazing cattle and 0.82 for penne d supplemented cattle. When the plasma inorganic P concentrations were < 50 mg/l, 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 re lated to dry matter intake (r = 0.73) in the range 9-17 mg P/kg LW. Th e total P requirements of growing cattle were estimated as g/day and g /kg DM intake from this data. The requirements of cattle consuming for age diers were 40-50% lower than those published by the Agricultural a nd Food Research Council (AFRC 1991), even though the same equation fo r the net requirements for growth was utilized.