Equine chorionic gonadotropin improves the efficacy of a progestin-based fixed-time artificial insemination protocol in Nelore (Bos indicus) heifers

A total of 177 Nelore heifers were examined by ultrasonography to determine the presence or absence of a corpus luteum (CL) and received a 3 mg norgestomet ear implant plus 2 mg of estradiol benzoate i.m. On Day 8, implants were removed and 150 μg of d-cloprostenol i.m. was administered. At the time of norgestomet implant removal, heifers with or without CL at the time of initiating treatment were assigned equally and by replicate to be treated with 0 IU (n = 87) or 400 IU (n = 90) eCG i.m. All heifers received 1 mg of EB i.m. on Day 9 and were submitted to fixed-time artificial insemination (FTAI) 30–34 h later. The addition of eCG increased the diameter of the largest follicle (LF) at FTAI (10.6 ± 0.2 mm vs. 9.5 ± 0.2 mm; P = 0.003; mean ± SEM), the final growth rate of the LF (1.14 ± 0.1 mm/day vs. 0.64 ± 0.1 mm/day; P = 0.0009), ovulation rate [94.4% (85/90) vs. 73.6% (64/87); P = 0.0006], the diameter of the CL at Day 15 (15.5 ± 0.3 mm vs. 13.8 ± 0.3 mm; P = 0.0002), serum concentrations of progesterone 5 days after FTAI (6.6 ± 1.0 ng/ml vs. 3.6 ± 0.7 ng/ml; P = 0.0009), and pregnancy per AI [P/AI; 50.0% (45/90) vs. 36.8% (32/87); P = 0.04]. The absence of a CL at the beginning of the treatment negatively influenced the P/AI [30.2% (16/53) vs. 49.2% (61/124); P = 0.01]. Therefore, the presence of a CL (and/or onset of puberty) must be considered in setting up FTAI programs in heifers. In addition, eCG may be an important tool for the enhancement of follicular growth, ovulation, size and function of the subsequent CL, and pregnancy rates in progestin-based FTAI protocols in Bos indicus heifers.