| Abstract: | SUMMARY Eight experiments involving a total of 480 castrated cattle grazing Australian pastures were conducted to assess the effect of subcutaneous implants of 36 mg zeranol on weight gain. A wide range of cattle ages, bodyweights and environmental and nutritional conditions was involved. Growth rates of untreated cattle ranged from minus 0.15 kg/day over 90 days to 1.23 kg/day over 52 days. In 5 experiments treated cattle were implanted at the commencement of the experiment only. The other 3 experiments compared responses to early, late and early plus late implantation. Weight gains from a single implantation ranged from 4.6 kg over 70 days to 16.8 kg over 60 days—an average increase in growth rate of 13.5%. Repeating implantation at a later date increased growth rate further and provided greater gains than a single implantation whether applied early or late. Carcase weights were measured in 2 experiments. In one, weight was increased by 7.2 kg (P < 0.01). In the other, cattle in both implanted and control groups lost weight over the duration of the experiment. Whilst the bodyweight of implanted cattle was less than that of untreated cattle the carcase weight of implanted cattle was 2.4 kg greater than for control cattle. Neither of these differences was significant (P > 0.10). No side effects were noted in any of the treated cattle. |
