Field study of the effects of zeranol, implanted at the time of breeding, on weight gain and pregnancy rate of beef heifers

Zeranol implants (Ralgro) were given to yearling heifers at the time of breeding to determine the effects of implanting on weight gain and pregnancy rate. Sixty 14 ½ month-old crossbred heifers were randomly divided into an implanted group which received 36 mg zeronol one day before exposure to bulls and a control group which was not implanted. Four yearling Angus bulls were used to service the heifers for a 60-day breeding period. Two bulls were assigned to each of two pastures containing 15 implanted and 15 control heifers. All heifers were weighed and slaughtered 120 days later and reproductive tracts were examined. Average weight of heifers (± SD) at time of implanting and at slaughter was 259 ± 28.8 kg and 378 ± 47.8 kg and 251 ± 26.8 kg and 371 ± 41.3 kg for the implanted and control heifers, respectively. Average weight gain was 119 kg for the implanted heifers and 12 kg for the controls.Fifteen heifers in the implanted group and 15 heifers in the control group were pregnant. Implanting heifers with zeranol prior to the breeding season did not increase weight gain and pregnancy rate was the same for each group.