SEASONAL NUTRIENT INTAKE AND PHOSPHORUS KINETICS OF GRAZING STEERS INNORTHERN AUSTRALIA

The growth of Bos indicus cross-steers grazing native grass and native grass-legume (Stylosanthes scabra cv. Seca) pastures was monitored fo r 392 days in five unreplicated experimental paddocks, each grazed by three steers. Two phosphorus (P) fertilizer rates: F1, 4.5 kgP/ha ever y 2 years and F2, 9.0 kgP/ha annually, together with an unfertilized c ontrol, F0, were studied. P supplement was supplied to cattle in two a dditional paddocks (F0S and F1S) at the rate of 5 gP/ha/day-1. In the dry, wet and late wet seasons, pasture yield and pasture component yie ld were measured. At the same time, the nutrient intake and P kinetics of resident steers were measured using chromic oxide capsules and int ravenous injections of P-32. Additional non-resident steers, fistulate d at the oesophagus, grazed the pastures intermittently to provide est imates of dry matter digestibility, fibre, nitrogen (N) and P content of the selected diet. Phosphorus fertilizer increased legume and nativ e grass growth which resulted in more dry matter containing higher con centrations of P and N than unfertilized pastures. Both P fertilizer a nd supplement increased dry matter intake and liveweight gain of cattl e. During the experimental period of 13 months, steers which grazed pa stures that received the (F2) fertilizer treatment gained on average 0 .43 kg/day compared with 0.19 kg/day for those which grazed unfertiliz ed F0 pastures. Supplementation increased wet season liveweight gain f rom 0.45 to 0.59 kg/day and 0.46 to 0.73 kg/day for F0 and F1 treatmen ts respectively. Only cattle which grazed pastures that received the F 1S and F2 treatments gained weight in the dry season. Nitrogen was the primary nutrient limiting cattle growth from unfertilized pastures in the dry season, but when pastures were fertilized (4.5 kgP/ha every 2 years) and legumes established, P was the primary limiting nutrient. Intakes of P ranged from 7 to 35 mg/kg LW and were lower than those re commended by recognized authorities for the steer growth rates we reco rded. Both the absorption and faecal excretion of dietary P were close ly related to P intake. Endogenous faecal P values varied with P intak e in the range 9-22 mg/kg liveweight. Endogenous faecal P was also rel ated to dry matter intake and plasma inorganic P. The coefficient of P absorption was lowest (0.65) for steers grazing unfertilized native p asture (FO), but increased with P intake to 0.85 for steers grazing th e F1S treatment. We conclude that the P requirements of growing cattle grazing native grass and native grass-legume pastures in northern Aus tralia are about half those recommended by most authorities.