Phosphorus kinetics of cattle grazing tropical pastures and implications for the estimation of their phosphorus requirements

Twelve yearling Droughtmaster heifers were grazed on a tropical grass/legume pasture in north-east Australia for 1 year in four paddocks, each of 4 ha. One paddock was fertilized annually with 10 kg P/ha (treatment HPF). The other three paddocks had low soil P and low pasture P status. In one of the low soil P paddocks, P was added to the drinking water (treatment HPS) but no supplement was provided in the other two paddocks (treatments LP1 and LP2). In February, May and August 1989, the P kinetics of the heifers were monitored following oral administration of ruminal chromic oxide capsules and i/v injection of P. At the same times, oesophageal-fistulated steers were grazed in the paddocks to obtain extrusa samples to estimate the P content and digestibility of selected forage. The liveweight gain per heifer was higher in the HP treatments (196 kg) than in the LP treatments (152 kg). The intake, absorption and excretion of P were closely related but the faecal excretion of P relative to P intake was less in heifers with than without P supplement. The P intakes of the LP heifers were about half the quantities recommended in the nutrient feeding standards for the observed food intakes and weight gains. The coefficients of P absorption in the LP heifers in May and August were high (0.8) and did not decrease with age or liveweight. The coefficients of P absorption of heifers given supplementary P were as high as those for LP heifers, but those of heifers on the fertilized pasture were significantly lower in May and August. The endogenous faecal excretion of P was related to P intake but more closely to dry matter intake and plasma inorganic P together. The reasons for the difference between observed P intakes of LP heifers and recommended requirements are discussed in relation to the assessment of P requirements of grazing cattle. The results from this experiment suggest that recommended P requirements are higher than necessary to sustain the measured liveweight gains of cattle on grass/legume pastures in the tropics