Seasonal nutrient intake and phosphorus kinetics of grazing steers in northern Australia

Abstract

The growth of Bos indicus cross-steers grazing native grass and native grass-legume (Stylosanthes scabra cv. Seca) pastures was monitored for 392 days in five unreplicated experimental paddocks, each grazed by three steers. Two phosphorus (P) fertilizer rates: F1, 4-5 kgP/ha every 2 years and F2, 9.0 kgP/ha annually, together with an unfertilized control, FO, were studied. P supplement was supplied to cattle in two additional paddocks (FOS and F1S) at the rate of 5 gP/ha/day-l. In the dry, wet and late wet seasons, pasture yield and pasture component yield were measured. At the same time, the nutrient intake and P kinetics of resident steers were measured using chromic oxide capsules and intravenous injections of 32P . Additional non-resident steers, fistulated at the oesophagus, grazed the pastures intermittently to provide estimates of dry matter digestibility, fibre, nitrogen (N) and P content of the selected diet. Phosphorus fertilizer increased legume and native grass growth which resulted in more dry matter containing higher concentrations of P and N than unfertilized pastures. Both P fertilizer and supplement increased dry matter intake and liveweight gain of cattle. During the experimental period of 13 months, steers which grazed pastures that received the (F2) fertilizer treatment gained on average 0.43 kg/day compared with 0.19 kg/day for those which grazed unfertilized FO pastures. Supplementation increased wet season liveweight gain from 0.45 to 0.59 kg/day and 0.46 to 0.73 kg/day for FO and F1 treatments respectively. Only cattle which grazed pastures that received the F1S 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 recommended by recognized authorities for the steer growth rates we recorded. Both the absorption and faecal excretion of dietary P were closely related to P intake. Endogenous faecal P values varied with P intake in the range 9-22 mg/kg liveweight. Endogenous faecal P was also related to dry matter intake and plasma inorganic P. The coefficient of P absorption was lowest (0.65) for steers grazing unfertilized native pasture (FO), but increased with P intake to 0.85 for steers grazing the F1S treatment. We conclude that the P requirements of growing cattle grazing native grass and native grass-legume pastures in northern Australia are about half those recommended by most authorities.