| Summary: | 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.
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