Improvement of pregnancy rate in Japanese Black cows by administration of hCG to recipients of transferred frozen-thawed embryos.

Japanese Black primiparous and multiparous beef cows (n = 120) were selected as recipients and randomly divided into three groups (A, B, and C) of 40 recipients each. Group A received an intramuscular (i.m.) treatment of 1500 IU human chorionic gonadotropin (hCG) on day 1 (day 0 = onset of estrus), while Group B received an i.m. treatment of hCG on day 6. Group C received an i.m. treatment of 5 ml saline on day 6 as a control. On day 7, frozen-thawed embryo transfer was conducted in all groups, and pregnancy was diagnosed by palpated per rectum 40-50 days after the transfer. Twelve recipients were randomly selected from each group. Plasma progesterone (P) and estradiol-17beta (E2) concentrations were determined in these recipients on days 6, 7 and 14, and at the time of pregnancy diagnosis, and their ovaries were examined for a corpus luteum and follicles by palpated per rectum. The pregnancy rate in Group B was higher (67.5%. P < 0.05) than the rate in Group C (45.0%) and in Group A (42.5%). The plasma P concentration on day 14 tended to be higher although not significantly in Group B than in Groups C and A. At the time of pregnancy diagnosis, the blood P concentration of pregnant recipients in Group B was higher (P < 0.05) than that of those in Groups C and A. The plasma E2 concentrations on days 7 and 14 were lower (P < 0.05) in Group B than in Groups C and A. These results showed that administration of hCG 6 days after estrus improved the pregnancy rate for non-surgical frozen embryo transfer 7 days after estrus by enhancing luteal function and depressing E2 secretion.