Abstract
Effects of zeranol upon testicular function in bulls were studied. In Experiment I, young bulls (104±1.7 days of age) were either implanted with 36 mg zeranol or served as controls with reimplantation on days 56 and 112 after first implant. After a 168-day treatment period, half the bulls were orchidectomized with the remaining bulls assigned to a 168-day recovery period, after which they were also orchidectomized. Zeranol inhibited testicular development during the treatment period. Testosterone (T) concentrations were lower (P<0.01) in treated bulls. Spermatozoa were absent in testes of treated bulls while control bulls averaged 27×10 6 sperm per gram of testis. At the end of the 168-day recovery period, testes weights and sperm concentrations were similar (P>0.10) between bulls previously treated with zeranol and controls. Implantation of older bulls with either 36 mg (267±3.4 days of age: Experiment II) or 72 mg (16 months of age: Experiment III) of zeranol had no effect (P>0.10) upon spermatogenesis or T production over a 168-day treatment period. The results indicate that the inhibitory effects of zeranol upon bovine testicular function are age-dependent and suggest that there may be an increased sensitivity of the hypothalmic-pituitary-testicular axis to zeranol in the prepuberal bull.
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