CSIRO Division of Tropical Crops and Pastures, Townsville, Queensland, Australia
Pastures of Urochloa mosambicensis (Sabi grass), Stylosanthes hamata cv. Verano and S. scabra cv. Seca were established on a low phosphorus (P) soil. The P status of the soil and pasture was manipulated by applying superphosphate at different frequencies, viz: an unfertilised control; fertilised once-only 12 months after sowing; fertilised biennially; and fertilised annually. The pastures were set-stocked annually with yearling Droughtmaster heifers. Soil and pasture responses to fertiliser, and animal responses to soil and supplementary P, were determined over 9 years following sowing.
Sabi grass became the dominant species in all fertilised pastures whereas unfertilised pasture became progressively more stylo-dominant during the first 7 years. Subsequently, there was an increase in Sabi grass in unfertilised pasture. Fertiliser P increased dry matter yields and P concentration of pasture and diet but had no effect on the nitrogen concentration or digestibility of forage.
Liveweight gain responses to fertiliser were large (6–80 kg/heifer/annum) in the early years, irrespective of the amount of applied P. In later years, liveweight gain on once-only fertilised pasture declined relative to annually and biennially fertilised pasture, but liveweight gain on unfertilised pasture increased relative to fertilised treatments. These changes were considered to be due to differences in the quantities of Sabi grass on different treatments. Differences in total pasture yield were not an important determinant of animal performance except in a drought year. A dietary P supplement was able to substitute for fertiliser P in improving liveweight gain except in the drought year when there was an increasing dry matter limitation with decreasing amount of applied fertiliser. Liveweight gain responses to both fertiliser and supplementary P were mainly confined to the wet season and early dry season when dietary nitrogen and energy levels were sufficient to sustain growth.
Levels of plasma inorganic phosphate, faecal P concentration, thickness and chemical composition of cortical rib-bone were assessed as indicators of responsiveness to P supplementation. Plasma inorganic phosphate level during the autumn was the most reliable indicator of P deficiency with levels of 50 mg/L or less being indicative of a P-responsive situation.
Liveweight gains were much higher than predicted for such low-P diets indicating that the published P requirements for growing cattle overestimate the requirements of growing heifers grazing stylo-based pastures on low-P soils in north Queensland.