The growth of phosphorus kinetics of steers grazing a subtropical pasture

Abstract

The growth and nutrient balances of steers grazing predominantly buffel (Cenchrus ciliaris) grass pastures fertilized with phosphorus (P) at annual rates of 0, 2.5, 5 and 15 kg P ha-1 were studied in three different seasons. There were two additional treatments in which steers grazing the 0 and 2.5 kg P ha-1 treatments were supplied daily with phosphorus supplement at 5 g head-1 throughout the study. Liveweight gain was increased by P fertilization but not P supplementation. Over a period of 15 months, the steers grazing the 15 kg P ha-1 pastures gained 80 kg more than those on the unfertilized pastures. This was associated with a small increase in dry matter intake and larger increases in P and nitrogen (N) intakes. Generally, the apparent absorptions of N, P and calcium by the steers were positive. In each season, the P intake, dietary faecal P and endogenous faecal P were all higher in cattle grazing fertilized pastures. After making allowance for an underestimation of pasture intake, our estimates of P intake were considerably less than published requirements to sustain the liveweight gains recorded. Endogenous faecal P values (13-19 mg kg-1 liveweight) were lower and the efficiency of absorption (mean 76%) was significantly higher so that the P requirements of cattle grazing tropical pastures appear to be much lower than recently published values.