Effects of Zeranol® implants and dietary supplement on growth rate, endocrine status and blood metabolite levels of growing lambs at pasture

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Abstract
In a 2 × 2 factorial experiment, the effect of implants of the growth promoter Zeranol, with and without supplementary food, on the growth rate, blood metabolite and hormone profiles of lambs finished at pasture was examined. For lambs fed no supplement, Zeranol implants increased growth rates from 166 to 185 g/day (s.e. of difference = 11·4) whereas for animals fed supplement the response to Zeranol was much greater, the growth rates being 167 and 234 g/day (s.e. of difference = 12·8). Supplement given to lambs not treated with Zeranol replaced a proportion of the herbage intake while, in lambs given supplement and treated with Zeranol, herbage intake was greatly increased (P< 0·001). Plasma glucose and urea concentrations were not affected by treatment but non-esterified fatty acids (NEFA) were depressed and albumin levels increased by Zeranol implants, while supplementary food caused reductions in plasma NEFA and urea concentrations. It is suggested that Zeranol treatment and supplementary food acted synergistically through changes in insulin and growth hormone concentrations to induce a much larger increase in growth rate in the treated animals given supplementary food than in those given either Zeranol or supplement alone. The response to Zeranol may also have been altered by differences in thyroid hormone status associated with supplementary food.