Читать книгу Bovine Reproduction - Группа авторов - Страница 367

Growth‐promoting implants are widely used in the beef industry. However, in heifers to be retained for breeding, implants containing estradiol, zernol, or trenbolone acetate inhibit the development of a mature reproductive endocrine system when administered to suckling beef heifers and consequently delay onset of puberty. For example, zeranol implants at birth delay the onset of puberty and decrease uterine horn diameter. Furthermore, zeranol‐implanted heifers have lower pregnancy rates and higher rates of abortion than non‐implanted herdmates [68]. In contrast, Rosasco et al. [69], in a study comparing performance of crossbred Angus heifers implanted with Synovex^® C (Zoetis, Parsippany, NJ, USA) at three months of age, reported no deleterious effects on reproductive performance through four calving seasons (note: product label specifies for heifers 45 days and older up to 400 pounds). The implanted calves were heavier at weaning than control heifers such that producers who do not make selection of replacement heifers until weaning may expect some profit advantage from heifers not retained as replacements. Replacement heifers that are identified early in life should not be implanted as there is no benefit in terms of age at puberty or incidence of dystocia.