Kirk and Cooper (1983) reported two experiments in which the growth performance and carcass conformation of intensively fed dairy-bred bulls was improved by sequential implantation with Zeranol®. Two further studies have been conducted in which there has been repeat implantation of Zeranol® and detailed carcass dissection to measure precisely the effects on carcass quality.

In trial A, 16 Holstein bull calves were purchased from pedigree breeders and weaned at eight weeks of age, when they were divided into two groups of eight, balanced by weight. From 8 to 12 weeks they were given commercial rearing pellets followed by a. commercial pencil and hay was given ad libitum. From 12 weeks onwards a cereal mix was offered ad libitum, formulated to give 13 MJ/kg dry matter, 160 g/kg crude protein. Hay was withdrawn from 14 weeks of age. At eight weeks of age, half the animals were implanted with 36 mg Zeranol® (Ralgro, Crown Chemical Company, Kent), and re-implanted a further three times every 70 days. Five blood samples were taken from all animals over a 15h period at about 8, 14, 33 and 43 weeks of age.