THE EFFECTS OF ZERANOL ON THE FEEDLOT PERFORMANCE OF BEEF BULLS

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Abstract
Fifty-three cross-bred bulls, aged 5–7 mo, were divided at random into a nonimplanted control group of 26 bulls and a treatment group of 27 bulls, which were each implanted with 36 mg of zeranol on the 1st, 81st, and 155th day of the experiment. They were fed a high energy diet ad libitum for 168 days during which time liveweight and feed consumption were recorded at 2-wk intervals. Occasional measurements of scrotal circumference and serum T3 and T4 concentration were taken on 26 bulls (12 control, 14 implanted). At the end of 168 days the same 26 bulls were halter-broken in preparation for libido testing and semen evaluation. All bulls were slaughtered 236 days after the initial implanting, at which time a study was made of the behavior of 20 control and 20 treated bulls during the 6–7 h leading up to slaughter. There was a large (6.1%) but nonsignificant difference in rate of gain, with no difference in feed efficiency due to the implant. Zeranol was found to have no significant effect on sexual development, thyroid hormone concentration, behavior or carcass characteristics. There was, however, a four-times greater incidence (nonsignificant) of dark-cutting carcasses among the control bulls, than the treated bulls. Key words: Zeranol, beef cattle, feedlot performance, sexual development, behavior