Efficacy of hCG and GnRH with Respect to Follicular Size and Presence of the Corpus Luteum in Cosynch Protocol Integrated with Norgestomet in Lactating Cows

Efficacies of hCG and GnRH with respect to follicular diameter in Cosynch protocol were investigated. Lactating cows (n=119) received injections of GnRH and PGF2 7 d apart. Norgestomet ear implant was inserted at GnRH administration and removed prior to PGF2 injections. Presence of the corpus luteum (CL) at GnRH and PGF2 injections, and diameter of preovulatory follicle [small (SPF) <8 mm or large (LPF) ≥8 mm] at PGF2 were determined with transrectal ultrasonography (USG). The animals were randomly treated with GnRH (0.01 mg; GPG, n=62) or hCG (1,500 IU; GPH, n=57) at timed artificial insemination (TAI) 56 h after injection of PGF2α. Pregnancies were diagnosed with USG 32-35 d after TAI. Pregnancies per TAI (P/TAI) were significantly (P<0.05) lower in GPH cows (28.1%; 16/57) compared to those in GPG animals (46.8%; 29/62). P/TAI with respect to follicular size did not differ between GPG and GPH cows, although P/TAI was numerically higher in LPF (30.0%; 15/50) than in SPF (14.3%; 1/7) in GPH cows. P/TAI in cows without CL at PGF2 was 2.5 (1.0-6.7) times higher (P<0.01) in GPG (20.0%; 3/15) compared to GPH (10.5%; 2/19). In conclusion, use of hCG in milieu of the second GnRH in Cosynch+progesterone insert protocol did not have any advantage in lactating cows.