Effects of Zeranol on Growth and Ease of Calf Delivery in Beef Heifers

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Abstract
At approximately 100 days of gestation, 45 Angus ✕ Hereford and 10 Hereford heifers were assigned either to be implanted with 36 mg zeranol or to serve in a control group. Monthly measurements revealed no effect of treatment on body weight. Increased pelvic area was observed during the first (P<.05), third (P<.05) and fourth months (P<.01) after zeranol treatment. Calf birth and weaning weights were not affected by treatment of the dam. Ease of calf delivery was adversely affected (P<.05) by zeranol treatment. In a trial conducted 1 year later, 57 Angus ✕ Hereford and 23 Hereford heifers either received no implant (C) or were implanted with zeranol (36 mg) once (l×) twice (2×) or three (3×) times during gestation. Zeranol implants were administered on days 100, 170 and 225 of gestation. Monthly body weight and pelvic area measurements were not affected by zeranol treatment. Zeranol tended (P=.13) to lower calf birth weight. Treatment of the dam with zeranol increased calving ease (P<.10), as indicated by calving scores (2.50, 2.23, 1.94, 1.70 for C, l×, 2×, 3×, respectively). The incidence of abortion or reabsorption of fetuses was higher among heifers given zeranol once (P<.05) and three times (P<.01) than among control animals. Progesterone concentrations in serum were not affected by treatment. Estradiol concentrations in serum were lower during the first (P<.10), second (P<.05) and third (P<.05) months following the initial zeranol treatment. In conclusion, the administration of zeranol to pregnant heifers did not result in increased body weights or pelvic areas within 1 month of parturition. Although multiple administration of zeranol increased calving ease, this treatment also increased the incidence of abortion. Copyright © 1981. American Society of Animal Science . Copyright 1981 by American Society of Animal Science.

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