Superphosphate maintains soil fertility and beef production on grazed white clover pastures in the subtropics. 2. Liveweight responses of beef cattle

Abstract

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, and48 kg P/ha annually. Hereford weaner steers grazed 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 liveweight response to superphosphate applied at12 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% spring and least in autumn each year. Seasonal 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.