EFFECT OF ZERANOL IMPLANTS ON AGE AT ONSET OF PUBERTY, FERTILITY AND EMBRYO FETAL MORTALITY IN BEEF HEIFERS

Thirty-three cross-bred beef heifer calves were implanted with zeranol (36 mg) at birth, 100 and 300 d of age and 40 heifer calves remained as untreated controls. Heifers were examined rectally by diagnostic ultrasonography every 14 d from 10 to 14 mo of age and then, every 21 d until the end of test (100 d after exposure to bulls). The presence of a CL, diameter of the largest follicle, total number of follicles greater than or equal to 3 mm, diameter of the right uterine horn and age at puberty were recorded. At 15 mo heifers were exposed to bulls for 63 d. Mean (+/- SEM) age at onset of puberty in zeranol-implanted heifers (420 +/- 8.9 d) was greater than control heifers (362 +/- 6.2 d; P < 0.05). The diameter of the largest follicle was smaller (P < 0.05) in pre-pubertal zeranol-implaned heiferes than in pre-pubertal control heifers between 10 and 14 mo of age and at puberty. Within the zeranol-implanted heifers, the diameter of the largest follicle was smaller in pre-pubertal (P < 0.05) than pubertal heifers until 14 mo of age. The uterine diameter of zeranol-implaned heifers was smaller (P < 0.05) than that of control heifers during all examinations prior to breeding. Pubertal heifers had a larger (P < 0.05) uterine diameter than pre-pubertal heifers. Pregnancy rate after the first 21-d period of the breeding season and at the end of test was higher (P < 0.001) for control than for zeranol-implanted heifers (82.5 and 100%, respectively and 24 and 42.5%, respectively). Pregnancy loss in zeranol-implanted heifers was 37.5% compared with OI for control heifers (P < 0.001). Heifers that became pregnant during the first 21 d of the breeding season had reached puberty earlier (396.9 +/- 12.5 and 358.7 +/- 6.4 d of age for zeranol-implanted and control heifers, respectively) than heifers not conceiving in the first 21 d (472.8 +/- 10.8 d and 380.0 +/- 17.7 d for zeranol-implanted and control heifers, respectively; P < 0.05). Heifers that became pregnant during the first 21 d of the breeding season had a larger (P < 0.001) uterine diameter (18.17 +/- 0.21 mm) than non-pregnant heifers (16.28 +/- 0.36 mm). Within the zeranol-implanted group, pregnant heifers had a larger (P < 0.05) uterine diameter (17.75 +/- 0.05 mm) than non-pregnant heifers (15.78 +/- 0.38 mm). Heifers that were not pregnant at the end of test began cycling later and had a numerically smaller uterine diameter than pregnant heifers (430 +/- 17.5 d vs. 374.7 +/- 6.2 d; and 15.1 +/- 0.6 mm vs. 17.3 +/- 0.24 mm, respectively), however these differences were not significant (P > 0.05). It was concluded that zeranol implants from birth delay the onset of puberty and decrease uterine horn diameter. Furthermore, the lower pregnancy rate in zeranol-implanted heiferes 100 d after exposure to bulls was caused by failure to cycle early in life, and in those that were cycling, failure to conceive and abortions between 25 and 45 d of gestation.