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US Department of Agriculture7 and Montana Agricultural Experiment Station, Miles City, MT 59301 and Southern Agricultural Research Center, Huntley, MT 59037
Abstract
Two trials were conducted to study the effect of zeranol implants on growth and sexual development of bull calves. Trial 1 compared the effects of implanting with 72 mg of zeranol at 48 d of age (branding), at 215 d of age, or at both times with nonimplanted control bulls. Implanting at branding resulted in decreased scrotal circumference, testicle weight and proportion of bulls that could produce an ejaculate at 14 mo of age (P<.01). Implanting at 215 d of age had no effect on any of these traits. Growth rate was not increased by implanting at either time but was decreased (P<.02) in animals implanted at both times when compared with control bulls. In trial 2, both bulls and steers were implanted with zeranol and compared with nonimplanted control bulls and steers. Thirty-six-milligram implants were given at 21, 103, 260 and 343 d of age. Scrotal circumference, testicle weight and serum testosterone concentrations decreased (P<.01) and the occurrence of penis abnormalities increased (P<.01) in implanted bulls compared with control bulls. By the time of slaughter, however, testosterone concentrations were equal in control and implanted bulls; and the difference in scrotal circumference was diminishing. This is interpreted as evidence that as the bulls get older, they can overcome the effect of the implants. Carcass weights were heavier in implanted steers than in control steers but were lighter in implanted bulls than in control bulls (P<.02). Carcasses of implanted bulls had higher quality scores and more marbling than control bulls, but carcasses of implanted steers had lower quality scores and less marbling than control steers (both interactions, P<.01). Implanting bulls with zeranol at an early age resulted in restricted sexual development but not in total sterility. Repeated zeranol implants throughout the growing and finishing phase enhanced carcass quality in bulls slaughtered at 14 to 16 mo of age.
1 Publication has been approved by the Director of the Montana Agr. Exp. Sta., Journal Series No. 1331.
2 This study was made possible in part by a grant from International Minerals and Chemical Corp., Terre Haute, IN 47808.
3 Fort Keogh Livestock and Range Research Station, USDA-ARS, Route 1, Box 2021, Miles City, MT 59301.
4 Cooperative Extension Service, Montana State Univ., Bozeman, MT 59717.
5 Southern Agricultural Research Center, Huntley, MT 59307.
6 The authors express appreciation to Dr. B. G. England, Univ. of Michigan, Ann Arbor, for anti-testosterone serum.
7 Fort Keogh Livestock and Range Research Laboratory, Agricultural Research Service.
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