EFFECT OF REPEATED IMPLANTATION WITH ZERANOL FROM BIRTH OR WEANING ON GROWTH AND REPRODUCTION IN BEEF HEIFERS

Liveweight, pregnancy rate, skeletal development and carcass characteristics were measured on 52 crossbred beef heifers implanted with zeranol at birth then at 110, 204 and 314 d of age (B); at weaning (204 d of age) and at 314 d of age (W); or not implanted (C). At 14 mo of age, the heifers were bred for 6 wk in corrals by natural service. Six weeks later they were slaughtered and their reproductive tracts collected. Height at the withers and pelvic area were measured before slaughter. Mean weaning weights ± SD, corrected to 200 d and for age of dam, were 249 ± 20 kg for group B and 226 ± 22 kg for groups W and C (P < 0.001). At breeding, the heifers weighed 434 ± 24, 416 ± 29 and 392 ± 31 kg for groups B, W and C, respectively (P < 0.001). At slaughter, their liveweights were 531 ± 26, 508 ± 32 and 483 ± 36 kg, respectively (P < 0.001) and carcass weights were 304 ± 17, 286 ± 21 and 270 ± 20 kg, respectively (P < 0.001). Rib eye area was 84.3 ± 8.47, 76.5 ± 8.89 and 72 ± 7.81 cm 2 , respectively (P < 0.001) but there were no significant differences between groups for cutability (58.2 ± 1.85%), average fat cover (9.4 ± 0.84 mm) or grade (90.3% ± A1/A2 and 9.7% A3/A4). Height at the withers did not differ between groups (46.7 ± 1.61 cm) but zeranol increased pelvic area (186.4 ± 19.54, 178.6 ± 21.84 and 165.3 ± 21.14 cm 2 zeranol for groups B, W and C, respectively; (P < 0.05). Pregnancy rate was significantly reduced (P < 0.05) in group B (42.9%) compared with groups W and C (84.2 and 77.8%, respectively). However, examination of the reproductive tracts indicated that only one nonpregnant heifer in each of groups B and C was not cycling and no other abnormalities were found. It was concluded that zeranol will increase live and carcass weights, rib eye area and pelvic area of heifers but that four implants given at birth and at approximately 100-d intervals to 314 d of age will reduce reproductive rate in comparison to heifers implanted twice postweaning or not implanted. Key words: Zeranol, heifer, growth, reproduction, pelvimetry, carcass