Effect of zeranol implantation on body weight changes in zebu crossbred cattle grazing tropical pasture

Nine experiments on the effect upon bodyweight change of subcutaneous ear implantation of 36 mg zeranol in 605 Bos indicus crossbred cattle were conducted. Bullocks aged 3 to 4 years, steers aged 18 months and entire heifers aged 18 to 30 months were used over the period January to August, 1980. They were grazed on 6 commercial farms in tropical northern Australia. Seven of the experiments examined the results of single implantation after the initial 60 to 97 days. Growth rates of untreated cattle in the January to May period ranged from 0.43 kg/day over 97 days in heifers to 1.07 kg/day in bullocks. Bodyweight gains attributed to zeranol ranged from 1.8 kg (4.3% increase) over 97 days in heifers (NS) to 22.3 kg (28.9% increase) over 90 days in 18 month-old steers (P < 0.01). The significant bodyweight responses to zeranol treatment in all 5 experiments involving older 3-to 4-year-old bullocks have not been previously reported. Hot dressed weights of the zeranol-treated bullocks were significantly heavier than the untreated controls and dressing percentages were similar. Increases in bodyweight attributable to implantation with zeranol yielded 50 to 54% saleable carcase weight. Single-, repeat-, and non-implanted treatments were compared over 186 days from January to August. Both zeranol treatments significantly out-performed the controls (P < 0.05), and the repeat-implanted bullocks gained 9.2 kg more than the single-implanted bullocks (P<0.10) in spite of bodyweight tosses recorded in the 3 treatments over the final 105 days. In 2 experiments bullocks implanted once in January/February were weighed in August to monitor compensatory bodyweight changes after April/May. The cattle retained 72.4% and 92.6% of the original bodyweight advantage attributed to zeranol treatment, depending upon whether they lost or gained in bodyweight respectively during the April/May to August period. The commercial relevance of these results is discussed and suggestions are made for further work.