The effect of implantation with zeranol or oestradiol benzoate alone or with progesterone on growth of steers and growth, carcass weight and testicular weight of bulls

O.C.llaghan, D.; Quirke, J.F.; Bourke, S.; Roche, J.F.

Irish Journal of Agricultural Research 27(1): 13-20


ISSN/ISBN: 0578-7483
DOI: 10.2307/25556200
Accession: 001705993

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The effects of implantation of bulls and steers with zeranol, or bulls with oestradiol plus progesterone, on growth, carcass and testicular parameters were examined. In Experiment 1, the performance of untreated bulls and steers (approximately 125 kg initial liveweight) was compared with that of animals implanted every 92-95 days with 36 mg zeranol (n = 35-38 per treatment). The weight gain of the bulls and steers was similar (257 and 244 kg respectively, p > 0.05) over a 269-day test period, and zeranol increased growth rate by 23 and 22 kg in bulls and steers, respectively, (p < 0.05), over the same period. In Experiment 2, groups of bulls (approximately 125 kg initial liveweight) were either kept as untreated controls or implanted every 70-72 days or 105-106 days with either 10 mg oestradiol benzoate (OB) plus 100 mg progesterone (P) or 20 mg OB plus 200 mg P (n = 16-17 per treatment). Steroid treatment increased liveweight gain by an average of 34 kg (p < 0.05) over a 239 kg gain by controls during a 279-day period. The dose level of steroids and frequency of steroid administration were without significant effect. In Experiment 3, intact bulls (approximately 140 kg initial liveweight) were allocated to the following treatments: (i) control; (ii) 20 mg OB plus 200 mg P (P-Syn); (iii) 40 mg OB plus 400 mg P; (iv) 130 mg OB in silastic rubber; (v) 20 mg OB plus 200 mg P (P-Tor) (n = 21-22 per treatment). Bulls allocated to treatment (iv) were implanted on day 1 only, while bulls in treatments (ii), (iii) and (v) were implanted every 97-103 days. Steroid treatment increased liveweight gain by 28 to 32 kg over a 233 kg gain by controls during a 300-day trial period (p < 0.05) and carcass weight was increased by 8 to 15 kg (p < 0.01). Conformation scores were improved and the degree of fat cover on the carcass increased (p < 0.05). Oestradiol plus progesterone administration reduced mean testicular weight to 226 g (20 mg OB plus 200 mg P-Syn), 213 g (20 mg OB plus 200 mg P-Tor) and 174 g (40 mg OB plus 400 mg P) compared with control weight of 283 g (p < 0.05). The magnitude of this response was dose dependent.