Effects of different doses of human chorionic gonadotropin at timed artificial insemination on the fertility of lactating dairy cows

The aim was to investigate the effects of administering GnRH and low (1650 IU) or high (3300 IU) doses of human chorionic gonadotropin (hCG) at timed artificial insemination (TAI) on the rates of ovulation and pregnancy in lactating dairy cows. Forty-seven multiparous lactating dairy cows (parity: 2 to 5; days in milk: 55±1) received a controlled internal drug release (CIDR) insert and 25 μg of GnRH intramuscularly on Day –10. On Day –3, the CIDRs were removed and 500 μg of cloprostenol sodium was administered intramuscularly. On Day –1, all animals were administered another dose of GnRH and inseminated 16-18 h later (Day 0). At TAI, the animals allocated randomly to the following groups: SALINE (n=12) which received 5 mL of sterile normal saline intramuscularly (i.m.), GNRH (n=11) which received 25 µg of GnRH i.m., L-HCG (n=12) treated with 1650 IU of hCG i.m., and H-HCG (n=12) treated with3300 IU of hCG i.m.. The animals were examined by transrectal ultrasonography on Days –1, 0 and 1 to identify and record follicle(s) ≥ 8 mm in diameter and ovulation, and on Days 30 and 60 to determine conception and pregnancy rates, respectively. Ovulation was defined as the disappearance (from one scanning session to the next) of a previously identified follicle ≥ 8 mm in diameter. Blood samples were collected from the jugular vein of all animals into vacuum tubes without anticoagulants on Days 0, 6 and 12 for progesterone assay. The results demonstrated no significant differences in the mean progesterone concentrations at days 0, 6 and 12, and also in conception and pregnancy rates to the first insemination among the groups, although administration of gonadotropins resulted in numerically greater conception and pregnancy rates by 16.741% compared to SALINE. However, ovulation rate was greater in H-HCG compared to SALINE (P<0.05). In conclusion, it was demonstrated that administering GnRH or hCG at TAI, regardless of low or high doses, resulted in nonsignificant improvement in conception and pregnancy rates in lactating dairy cows synchronised with the Ovsynch protocol combined with CIDR.