Effect of Zeranol on Sexual Development of Crossbred Bulls

Three groups of 1/2 Simmental × 1/4 Brahman × 1/4 Hereford bull calves were used during two different years to study effects of zeranol on sexual development. At 154 d of age, half the calves were implanted with 36 mg zeranol and half, not implanted, served as controls. Implanted calves were reimplanted at 90-d intervals throughout the trial (9 mo) each year. Trial 1 was conducted with 24 calves and Trial 2 was conducted the following year with 10 bulls. Twenty-four days after weaning (200 d of age) and at 28-d intervals thereafter, bulls in drylot in Trial 1 were weighed, scrotal circumference (SC) was measured and an ejaculate of semen was collected by electroejaculation to determine puberty. At these times, bulls were given 200 µg of GnRH i.m. and blood was collected at 0, 1, 2, 3, 4 and 5 h after GnRH. Serum concentrations of LH and testosterone (TEST) were determined. At slaughter, testis weight, length and circumference and pubertal status were recorded. Bulls implanted with zeranol had smaller SC than control bulls during the entire 9-mo period (P< .0001). More control bulls reached puberty than did implanted bulls (82.4 vs 23.5%, respectively; P< .001). Control bulls had larger testis measurements at slaughter (P< .0001). Implants did not alter total weight gain or ADG (P> .10). Overall, the LH response to GnRH was greater in implanted bulls than in control bulls (P< .02), but testosterone response to GnRH-stimulated LH release was greater in control than in implanted bulls (P< .02). At the end of the 9-mo period, however, there were no differences in LH or TEST responses to GnRH between control and implanted bulls (P> .10). Zeranol administered at 150 d of age inhibited testicular growth and function in bulls, but the effects were overcome with increasing age.