SUPERPHOSPHATE MAINTAINS SOIL FERTILITY AND BEEF-PRODUCTION ON GRAZEDWHITE 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 c lover (Trifolium repens) and carpet grass (Axonopus affinis) pasture r eceiving 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 stee rs/ha. Linear responses to superphosphate and stocking rate were signi ficant (P<0.001). The model estimating steer liveweight response to su perphosphate comprised a long-term quadratic trend with seasonal varia tions. The liveweight response to superphosphate application was great est in spring and least in autumn each year. Seasonal liveweight respo nse to superphosphate applied at 12 kg P/ha increased from 6.1 kg/stee r.45 days in year 1 to 10.6 kg/steer.45 days in year 7. Average livewe ight 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 decl ined 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 liveweigh t gain was negatively related to stocking rate, averaging -31.3 kg/ste er for each increment in stocking rate. Calculated stocking rates givi ng maximum liveweight production (122-469 kg/ha) varied between 2.2 an d 3.6 steers/ha. Annual liveweight gain of steers was related to bicar bonate-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.