Superphosphate requirements for maintenance of cattle production from grass-clover pastures in a subtropical environment

Abstract

Maintenance dressings of superphosphate were applied at 500,250, 125 or 0 kg/ha each year between 1972 and 1976 to a carpet grass (Axonopus affinis)-white clover (Trifolium repens) pasture in a subtropical environment at Grafton, New South Wales. The pasture had previously received a total of 1500 kg superphosphate/ha in four annual dressings between 1968 and 1972 and was in the maintenance phase, in contrast to the development phase, of fertilizer requirement. The topdressed pastures were grazed by weaner Hereford steers for 12 months during each of five years 1972 to 1977. Two stocking rates (1 .67 and 2.5 steers/ha) were applied in 1972 to 1974 and three (1.67, 2.5 and 3.3 steers/ha) in 1974 to 1978. The effects of superphosphate maintenance rate and stocking rate on dry matter and nitrogen in the available herbage components were small and inconsistent and suggested large residual effects of previously applied superphosphate. The effect of superphosphate maintenance rate on phosphorus in the available herbage components was more consistent but was also small and did not appear to influence pasture or animal production. Reduced maintenance dressings of superphosphate decreased (P< 0.05) annual liveweight gains of steers during only two of the five years, 1974-75 and 1975-76. The effect, which was small and dependent on stocking rate and seasonal conditions, indicated large residual effects of previously applied superphosphate which could be used to advantage when low beef prices preclude the application of maintenance dressings of superphosphate. Liveweight change was related to available herbage components (P<0.05) but the nitrogen content and digestibility of the herbage selected by cattle fistulated at the oesophagus provided more precise estimates of liveweight change. Superphosphate influenced steer liveweight change through its effect on pasture quality rather than quantity.