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Effects of a β-agonist (clenbuterol) on growth, carcass composition, protein and energy metabolism of veal calves

1. Twenty-two British Friesian bull calves were used in a comparative slaughter experiment to determine the effects of a β-agonist (clenbuterol) on body composition and energy retention. Four calves were slaughtered at 18 d of age and constituted the initial slaughter group. Of the remaining calves, eight (group A, controls) were given milk replacer only, and ten calves (groups B and C, five calves per group) were given milk replacer plus clenbuterol(O.1 and 1.0 mg clenbuterol/kg milk replacer equivalent to approximately 2 and 20 μg/kg body-weight respectively over the 105±3 d of the experimental period). Calves were slaughtered over the weight range 146–177 kg. 2. Clenbuterol had no significant effect on dry matter (DM) intake, daily live-weight gain or feed conversion ratio. DM digestibility of the milk replacer was not affected by treatment. Nitrogen balance was measured on three separate occasions starting when the calves weighed approximately 60, 110 and 130 kg. N retention was increased over the experimental period in clenbuterol-treated calves, although the effect only achieved significance in calves weighing approximately 110 kg live weight (P < 0.05). 3. Clenbuterol (20 μg/kg body-weight) increased estimated mean daily N retention in the carcass of the calves from 22 to 25 g whilst N retention in the non-carcass components decreased from 10 to 8 g/d. Effects of clenbuterol on N retention occurred mainly in skeletal muscle. Fat in both carcass and non-carcass components was reduced by treatment with clenbuterol. The total energy content of live-weight gain was reduced from 1077 to 897 MJ in clenbuterol-treated calves and mean daily heat production was estimated to increase from 23.1 in controls to 25.9 MJ/d in calves in group C. 4. In calves of mean live weight during balance of 120 and 136 kg, clenbuterol significantly increased daily urinary creatinine excretion and in 120 kg calves NT-methylhistidine was significantly decreased (P < 0.05). Based on estimates of muscle mass from urinary creatinine and protein degradation fromN7-methylhistidine NT-methylhistidine excretion, the fractional breakdown rate of muscle protein in clenbuterol-treated calves was only 0.66 of that in the controls when the calves weighed 120 kg.