The effects of zeranol on growth and fattening in beef calves before weaning

Four-month-old beef calves were implanted with 36 mg zeranol, and their growth compared with untreated calves. Treated steer and heifer calves gained 10 kg more bodyweight to weaning at 7.5 months of age. The trial was conducted at 2 research centres and calves at both centres showed significant responses for the first 45 days (P less than 0.001) after treatment but during the following 30 days only calves at one centre showed a continued response (P less than 0.05). No differences in growth rate occurred between treated and untreated calves over the final 30 days before weaning at either centre. The backfat measurement of untreated animals increased more than that of the implanted animals (P less than 0.05) at one centre but not at the other.