Mineral content and nutritive value of native grasses and the response to added phosphorus in a Pilbara rangeland.

We measured the seasonal variation and response to added P of concentrations of the major mineral nutrients plus nitrate, soluble sugars, starch and lignin and dry matter digestibility (DMD) of a range of native grasses and forbs on Hamersley Station in the Pilbara, Western Australia. Seasonal variations were far greater than the variation due to the application of P and were largely a reflection of patterns of growth, especially the cycle of new and rapid growth after rain followed by a slower period of senescence. Ca and Mg concentrations varied as much as 8-10 fold in one year with smaller variations in K, N and P. Concentrations of N, P, NO3, Mg and K increased after rain whereas that of Ca decreased. Lignin concentrations increased as new growth matured. The effect of adding P fertiliser was largely confined to the periods of active plant growth. In dry months, nitrogen concentrations and the DMD of the dominant grasses, Eriachne helmsii (Buck wanderrie grass) and Triodia basedowii (hard spinifex), declined to well below minimum requirements for the maintenance of animal weight. P concentrations were increased by P fertiliser but remained just below minimum requirements for the maintenance of animal weight. N:P ratios for all species were always less than 12:1 irrespective of treatment, suggesting a strong N limitation. Ca:P ratios of grasses were always greater than 18:1 except immediately after rain, whereas those of the forbs were between 3:1 and 6:1. provide P in satisfactory amounts for stock. The feeding of phosphorus supplements to cattle grazing these pastures seems warranted.