Effects of zeranol on reproduction in beef bulls: Luteinizing hormone, follicle-stimulating hormone, and testosterone secretion in response to gonadotropin-releasing hormone and human chorionic gonadotropin
Authors: Deschamps J. C. , Ott R. S. , Weston P. G. , Shanks R. D. , Kesler D. J. , Bolt D. J. , Hixon J. E. 
Published: February 2025

Effects of zeranol on the maturation of the adenohypophyseal-gonadal axis were studied in beef bulls. Calves were implanted with 36 mg of zeranol at 3-month intervals from birth through 6 months of age (group 2, n = 10) or were not treated (control group 1, n = 10). After 9 months, group-2 calves were given implants of 36 mg of zeranol at 3-month intervals through 18 months of age (group 2B, n = 5) or were not reimplanted (group 2A, n = 5). Areas under the curves outlined by concentrations of luteinizing hormone (lh), follicle-stimulating hormone (fsh), and testosterone for 6 hours after the administration of 100 μg of gonadotropin-releasing hormone (GnRH) were calculated. Gonadotropin-releasing hormone was administered at 3-month intervals from 1.5 through 19.5 months of age. Areas under the curves for concentrations of testosterone for 4 hours after the administration of 10,000 IU of human chorionic gonadotropin (hcg) at 4.5, 7.5, and 10.5 months or 1,000 IU at 13.5 and 16.5 months of age also were calculated.

The amount of fsh released was greater (P < 0.05) for group-2 than for group-1 calves at 4.5 and 7.5 months of age. The amount of fsh released in groups 2A and 2B tended (P < 0.10) to be greater than that for group 1. Significant differences between groups 2A and 2B were not observed. The amount of lh released at 7.5 months of age was less for groups 1 and 2 than that at earlier ages, and the decrease was greater (P < 0.05) for group 2. There was a tendency (P < 0.10) for lh release to be greater in group 1 than for the average of groups 2A and 2B. This difference was caused mainly by differences that existed between groups 1 and 2 at 10.5 and 13.5 months, because differences among groups were not evident in bulls at 16.5 or 19.5 months of age. Amounts of lh released in groups 2A and 2B were similar. The secretion of testosterone was greater (P < 0.01) for group 1 than that for group 2 after GnRH or hcg. The secretion of testosterone in response to GnRH tended to be greater (P < 0.10) for group 1 than for groups 2A and 2B. This difference was because of the smaller responses observed for groups 2A and 2B at 10.5 and 13.5 months of age. Administration of hcg at 10.5 months resulted in a greater response for group 1 than that for the average of groups 2A and 2B. There was a tendency for the response to 1,000 IU of hcg to be greater for group 1 than for groups 2A and 2B, and this difference was associated with the smaller amounts of testosterone secreted by groups 2A and 2B calves at 13.5 months. The response at 16.5 months was similar for all groups. Seemingly, sequentially implanting bull calves with zeranol from birth suppressed GnRH-induced release of lh, increased the amount of fsh released by GnRH, and suppressed the secretion of testosterone in response to lh or hcg. Restoration of GnRH-induced release of lh and hcg-induced secretion of testosterone was observed in zeranol-implanted bulls by 16.5 months, indicating that these effects were of a transient nature.