Effects of soils, fertilizers and stocking rates on pastures and beef production on the Wallum of south-eastern Queensland. 3. Relation of liveweight changes to chemical composition of blood and pasture

Abstract

During the concluding phase of a six-year grazing trial a study was made of the steers and the pasture material from paddocks annually fertilized with 125 kg ha-1and 250 kg ha-1of superphosphate. Plucked pasture samples from the low and high superphosphate treatments contained 0.11 and 0.15 per cent phosphorus, 0.83 and 0.91 per cent nitrogen and 0.12 and 0.12 per cent sulphur, respectively. The N: S ratio of these samples were 6.8 and 7.6 and the in vitro digestibilities were 41.6 and 44.9 per cent. There were marked seasonal patterns in the concentrations of all components with minimum values in the winter months when liveweight gain was lowest. Greater superphosphate application increased mean liveweight gain (0.29 to 0.41 kg head-1day-1), mean plasma concentrations of both urea-nitrogen (6.3 vs 5.0 mg 100 ml-1) and of inorganic phosphorus (6.7 vs 6.3 mg 100 ml-1). The mean blood sulphate-sulphur concentration of 28 ¦g ml-I was not influenced by treatment. Comparisons of the levels of nutrients in pasture and blood with published requirements indicated that neither sulphur nor phosphorus were deficient in either treatment. However, the between treatment differences in both nitrogen and phosphorus levels could not be ignored as possible reasons for the difference in animal production between the two levels of superphosphate application. Improved animal performance associated with increased application of superphosphate was probably influenced more by the induced changes in the organic components rather than the mineral levels of the pasture material.