SUPERPHOSPHATE MAINTAINS SOIL FERTILITY AND BEEF-PRODUCTION ON GRAZED WHITE CLOVER PASTURES IN THE SUBTROPICS .2. LIVEWEIGHT RESPONSES OF BEEF-CATTLE

Growth of cattle was measured between 1974-75 and 1980-81 on a white clover (Trifolium repens) and carpet grass (Axonopus affinis) pasture receiving superphosphate applied at 0, 12, 24, and 48 kg P/ha annually. Hereford weaner steers -razed the pastures at 1.67, 2.5, and 3.3 steers/ha. Linear responses to superphosphate and stocking rate were significant (P<0.001). The model estimating steer liveweight response to superphosphate comprised a long-term quadratic trend with seasonal variations. The liveweight response to superphosphate application was greatest in spring and least in autumn each year. Seasonal liveweight response to superphosphate applied at 12 kg P/ha increased from 6.1 kg/steer.45 days in year 1 to 10.6 kg/steer.45 days in year 7. Average liveweight responses to 48 kg P/ha also increased from 34.0 kg/steer.45 days in year 1 to 78 kg/steer.45 days in year 7. Annual liveweight gain of steers at the low stocking rate (1.67/ha) without superphosphate declined from 165 kg/steer in year 1 to 38 kg/steer in year 7, while 24 kg P/ha maintained liveweight gain at 120-220 kg/steer. Annual liveweight gain was negatively related to stocking rate, averaging -31.3 kg/steer for each increment in stocking rate. Calculated stocking rates giving maximum liveweight production (122-469 kg/ha) varied between 2.2 and 3.6 steers/ha. Annual liveweight gain of steers was related to bicarbonate-extractable soil P using the exponential model, which accounted for 19% of the variation. It was concluded that maintenance dressings of superphosphate would be required when available soil P levels fell below 30 mg/kg, in order to maintain liveweight gains on white clover-Axonopus pastures. The linear response model will assist producers to decide on rates of superphosphate.