Superphosphate and stocking rate effects on a native pasture oversown with Stylosanthes humilis in central coastal Queensland. 5. Bone phosphorus levels in grazing cattle

Abstract

The influence of fertilizer on bone phosphorus levels in grazing cattle was examined on native spear grass (Heteropogon contortus) pastures in central coastal Queensland oversown with Townsville stylo (Stylosanthes humilis). The first experiment compared superphosphate at nil, 125 or 250 kg ha-1 annually (F0, F1, F2, respectively), at three stocking rates. In the eighth and final year of this experiment, bone samples from Brahman-cross cattle obtained by rib biopsy were analyzed for their phosphorus content. Analyses of pasture samples indicated that Fo provided a diet deficient in phosphorus, but that F1 and F2 were adequate. Animals on F0 grew slowly, and after one year bone from the twelfth rib contained 121.5 mg P cm-3 In contrast, bone samples from cattle on F1 and F2 contained over 150 mg P cm-3 . At the end of the year there was no difference in bone phosphorus levels between animals on F1 and F2, although a significant advantage of F2 over F0 became apparent earlier in the year than that for F1. It is suggested that levels around 120 mg P cm-3 in bone from the twelfth rib indicate a deficiency of phosphorus, and that levels over 150 mg P cm-3 indicate adequacy. The diet selected from F0 was apparently deficient in protein for six months of the year. However, the quality of herbage selected from the fertilized pasture was much higher throughout the year, probably mainly through the consequent availability of much greater amounts of Townsville stylo. A second experiment, involving potassium fertilizer plus superphosphate, was run for three years following the development of potassium deficiency in the legume. During the first two years only small changes in bone phosphorus were observed, but in the third year, highly significant increases in bone phosphorus occurred in all fertilizer treatments. In a third experiment, changes in bone phosphorus were measured in animals grazing pasture dressed with superphosphate for the first time. After the first year, F2 produced higher levels of bone phosphorus than did F1, but no difference occurred in the second year. The results suggested that for the promotion of bone phosphorus storage, the higher rate of fertilizer application was of only short-term advantage.