J. Anim Sci.
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J. Anim Sci. 1983. 57:527-534.
© 1983 American Society of Animal Science
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Growth and Reproductive Traits in Beef Heifers Implanted with Zeranol1

R. B. Staigmiller, R. A. Bellows and R. E Short2,3,

Livestock and Range Research Station, US Department of Agriculture-Agricultural Research Service, and Montana Agricultural Experiment Station, Miles City 59301

Abstract

Weaned replacement heifers were implanted with zeranol at 8 and 11 mo of age in two trials to evaluate growth and reproductive traits. Approximately 100 beef heifers were used in each trial, and the heifers were divided into categories of heavier than average (H) and lighter than average (L) weaning weight. Weight categories were further divided into control (HC and LC) and zeranol-implanted (HI and LI) groups. Heifers were kept in drylot from weaning to just before the start of the breeding season and were fed to reach a predetermined body weight by the start of the breeding season. Rates of gain in the drylot were greater in implanted than in control heifers in both trial 1 (.53 vs .48 kg/d; P<.09) and trial 2 (.70 vs .63 kg/d; P<.01). Pelvic areas were greater (P<.01) in implanted than in control heifers of both trial 1 (175 vs 159 cm2) and trial 2 (175 vs 164 cm2). This increase in pelvic size was still present at the end of the summer pasture period, which followed the drylot period. Zeranol had no major effect (P>.10) on age or weight at puberty. Fall pregnancy rate was 16% lower in implanted heifers than in control heifers in trial 1 (62 vs 78%, P = .08) but did not differ in trial 2 (88 vs 87%; I and C, respectively).

Footnotes

1 This study was a contribution to Western Regional Project W-112, Reproductive Performance in Beef Cattle. Publication has been approved by the Director of the Montana Agr. Exp. Sta., Journal Series No. 1319.

2 Livestock and Range Research Station, USDA, ARS, Montana Agr. Exp. Sta., Miles City 59301.

3 The authors express appreciation to IMC, Terre Haute, IN, for donating the zeranol.






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Copyright © 1983 by the American Society of Animal Science.