Synchronization of ovulation in crossbred dairy heifers using gonadotrophin-releasing hormone agonist, prostaglandin F2 alpha and human chorionic gonadotrophin or estradiol benzoate

Girolando (Gir x Holstein) is a very common dairy breed in Brazil because i t combines the rusticity of Gir (Bos indicus) with the high milk yield of H olstein (Bos taurus). The ovarian follicular dynamics and hormonal treatmen ts for synchronization of ovulation and timed artificial insemination were studied in Girolando heifers. The injection of a gonadotrophin-releasing ho rmone (GnRH) agonist was followed 6 or 7 days (d) later by prostaglandin F2 alpha (PGF2 alpha). Twenty-four hours after PGF2 alpha injection either hu man chorionic gonadotropin (hCG, GPh-d6 and GPh-d7 groups) or estradiol ben zoate (EB, GPE-d6 and GPE-d7 groups) was administered to synchronize ovulat ion and consequently allow timed artificial insemination (AI) 24 and 30 h a fter hCG and EB injection, respectively. Follicular dynamics in Girolando h eifers was characterized by the predominance of three follicular waves (71. 4%) with sizes of dominant follicles (10-13 mm) and corpus luteum (approxim ately 20 mm) similar to those for Bos indicus cattle. In the GnRH-PGF-hCG p rotocol, hCG administration induced earlier ovulation (67.4 h, P<0.01) comp ared to the control group (GnRH-PGF) and a better synchronization of ovulat ion, since most of it occurred within a period of 12 to 17 h. Pregnancy rat e after timed AI was 42.8 (3/7, GPh-d6) to 50% (7/14, GPh-d7). In contrast, estradiol benzoate (GnRH-PGF-EB protocol) synchronized ovulation of only 5 of 11 heifers from the GPE-d7 group and of none (0/7) from the GPE-d6 grou p, which led to low pregnancy rates after timed Al (27.3 and 0%, respective ly). However, since a small number of Girolando heifers was used to determi ne pregnancy rates in the present study, pregnancy rates should be confirme d with a larger number of animals.