Effects of frequency of zeranol implantation on liveweight gain of steers grazing improved pastures in central Queensland

Abstract

Various combinations of single and repeat zeranol treatment were examined during two consecutive growing seasons, using Brahman x Murray Grey crossbred steers grazing improved pastures at Moura in central Queensland. Compared with untreated controls, the response (P<0. 05) to the initial implant of 36 mg zeranol varied from 0 . 166 to 0.208 kg Iiveweight/d during the 92-98 d period after implantation. Repeat implantations gave responses of 0.06 1 (P> 0.05) and 0.101 (P< 0.05) kg/d during the following 97 and 94 d. Steers that had only one implant gained more quickly than those on their repeat implant at the same time: 1.161 vs 1.014 (P<0. 05) and 0.658 vs 0.593 (P> 0.05) kg/d in the two seasons, respectively. Final liveweight was similar for steers given four implants over both growing seasons (594 kg), two implants to give response during the second growing season (592 kg), and one implant to cover the second half of the first growing season, and one implant to cover the latter half of the second growing season (589 kg). When managerial factors and cost of the drug are considered, the preferred strategy appears to be either two implants during the final growing season, or one implant during each of the two growing seasons.