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Effects of Zeranol and Trenbolone Acetate on Testis Function, Live Weight Gain and Carcass Traits of Bulls¹

North Carolina State University⁵, Raleigh 27695-7621

Abstract

The ability of zeranol and trenbolone acetate (trenbolone) to alter testis function, weight gain and carcass traits of young bulls was studied. In Exp. 1, the effects of age at initial zeranol implantation was determined. After a 235-d experimental period, sequential implantation (56-d intervals) beginning at 100 or 150 d of age had reduced testis growth (P<.01), sperm production (P<.01) and serum testosterone concentration in response to gonadotropin releasing hormone (GnRH; P<.01). The 200-d age group was partially suppressed, while the 250-d age group was not affected. Body weights were similar to controls in all groups. In Exp. 2, bulls previously implanted with zeranol at 175 and 231 d of age received single implants of zeranol, trenbolone or trenbolone plus zeranol at approximately 300 d of age. At slaughter (135 d later), body weight and carcass characteristics in all treatments were similar to controls. However, trenbolone reduced sperm production (P<.05), zeranol reduced sperm production and testes weight (P<.05), but trenbolone plus zeranol was similar to controls. Mean testosterone response to GnRH was suppressed in all implant groups on d 65 (P<.01), but only in trenbolone or trenbolone plus zeranol groups on d 112 (P<.05). Results indicate that zeranol suppresses spermatogenesis and testosterone production if implanted before approximately 200 d of age. Reduction of endogenous testosterone without alteration of weight gain or carcass characteristics may be of benefit if behavioral or masculinity traits of bulls are altered. Also, it appears that no benefit is derived from implanting bulls with both trenbolone and zeranol.

¹ Paper No. 9195 of the Journal Series of the North Carolina Agr. Res. Serv., Raleigh. The use of trade names in this publication does not imply endorsement by the North Carolina Agr. Res. Serv. of the products named, nor criticism of similar ones not mentioned.

² Present address: Dept. of Anim. Sci., Texas A&M Univ., College Station 77843.

³ Address reprint requests to this author.

⁴ The authors express their appreciation to Dr. William Olson (Center for Regulatory Services, Reston, VA) for the trenbolone acetate implants utilized, Dr. Barry R. Zirkin (Johns Hopkins Univ., Baltimore, MD) for electron microscopy, and to Betsy Young for typing the manuscript. We appreciate the expert technical assistance of Ms. Z. Turner and Mr. E. B. Collins.

⁵ Reprod. Physiol. Res. Lab., Dept. of Anim. Sci.