Abstract 178

Kerridge,-PC; McLean,-RW

CSIRO, Division of Tropical Crops and Pastures, St. Lucia, Qld 4067, Australia.

Proceedings-of-the-Australian-Society-of-Animal-Production. 1988, 17, 426; 1 ref.

An experiment was designed to derive response functions of liveweight gain (LWG) in steers with variables that might be affected by inputs of fertilizer phosphorus. The experiment was at the Narayen Research Station on light textured yellow podzolic and red earth soils, low in available P. A buffel grass and Siratro pasture was planted on virgin country, P was applied as triple superphosphate and the experimental area grazed with Belmont Red weaners changed annually. As there was a strong LWG response to P fertilization and evidence of dietary P deficiency, the experiment was modified after 3 years to include a P supplementation treatment. Increasing P fertilization resulted in an increased proportion of legume in the pasture, increased available soil P and mineralizable soil nitrogen, better persistence of buffel grass and increased dietary P, calcium and N. LWG was related to soil P when available pasture was not limiting. The size of the response to inputs of P as fertilizer or supplement was greatest at low soil P concentrations and minimal above 10 æg P/g soil. Supplementation can reduce the input of fertilizer P at low soil P concentrations. For example, provision of P supplement at 2 kg/head per year to steers grazing pastures on unfertilized soils containing 4 æg P/g increased LWG from 75 to 125 kg. The same level of production was obtained by application of fertilizer P 2.5 kg/ha per year which increased soil P to 6 æg/g. It is concluded that these relationships are useful for determining the economic effects of fertilization and supplementation and for making decisions as to where legumes might be planted and supplementary P used. This approach can be used with legumes that have a lower P requirement than Siratro and for soils with lower concentrations of available P.