EFFECT OF SUBCUTANEOUS IMPLANTS OF ZERANOL ON WEIGHT GAINS OF CASTRATED CATTLE AT PASTURE†

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Abstract
Eight experiments involving a total of 480 castrated cattle, grazing Australian pastures, were conducted to assess the effect of s.c. 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. Carcass 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. The bodyweight of implanted cattle was less than that of untreated cattle; the carcass weight of implanted cattle was 2.4 kg greater than for control cattle. Neither difference was significant (P > 0.10). No side effects were noted in any treated cattle.