Abstract

Repeat breeding of a high yielding cow causes enormous economic loss to the dairy producers. In this study, we examined the effect of estradiol benzoate (EB) or GnRH on follicular wave emergence and follicular development, and on pregnancy rates for a CIDR-based timed AI (TAI) protocol in repeat breeder dairy cows. Repeat breeder cows received a CIDR device (InterAg, Hamilton, New Zealand), with an injection of 1 mg EB (SY Esrone^®; Samyang, Seoul, Korea) plus 50 mg progesterone (P4; SY Ovaron^®; Samyang) (EB + P4 group, n = 36) or with an injection of 250 µg Gonadorelin (GnRH, Fertagyl^®; Intervet, Boxmeer, The Netherlands) (GnRH group, n = 35) at the beginning of the experiment (Day 0). On Day 7, CIDR devices were removed and all cows received PGF_2± (Lutalyse^®; Pharmacia & Upjohn, Puurs, Belgium). Thereafter, cows in the EB + P4 group received an injection of 1 mg EB on Day 8, and TAI 30 h later; those in the GnRH group received an injection of 250 µg GnRH on Day 9, and TAI 19 h later. In a subset of cows for both groups (15 cows from each group), the ovaries of the cows were examined by transrectal ultrasonography (Sonoace 600 with 7.5 MHz linear-array transducer; Medison Co., Seoul, Korea) every 24 h from Days 0 to 8, on Day 10, and on Day 13 for the EB + P4 group, and every 24 h from Days 0 to 9, on Day 11, and on Day 14 for the GnRH group, to observe changes in ovarian structures (follicles and corpora lutea). Pregnancy was diagnosed 35 to 60 days after TAI, using both ultrasonography and rectal palpation. Data were analyzed using Fisher's exact, Student's t-, and chi-square tests. Follicular wave emergence occurred within 7 days in 12/15 EB + P4-treated and 13/15 GnRH-treated cows (P > 0.05). The mean diameters of preovulatory follicles were smaller (P < 0.05) in the EB + P4 group (12.1 ± 0.1 mm) than in the GnRH group (13.8 ± 0.1 mm). However, the proportion of cows with synchronized ovulation and pregnancy rates to TAI did not differ (P > 0.05) between the EB + P4 (11/15 and 19.4%, respectively) and the GnRH (13/15 and 34.3%, respectively) groups. In conclusion, treatment with either EB or GnRH for a CIDR-based TAI protocol in repeat breeder dairy cows results in synchronous follicular wave emergence, follicular development, and ovulation, and thus similar pregnancy rates to TAI.