312 INFLUENCE OF LARGE NONOVULATED FOLLICLES ON ESTROUS INDUCTION AFTER EMBRYO COLLECTION IN SUPEROVULATED JAPANESE BLACK COWS

Abstract

Rapid oestrous induction after embryo collection shortens the calving interval of donor cows and improves the efficiency of embryo production. This study aimed to evaluate whether large nonovulated follicles present at the time of embryo collection affect oestrous induction after embryo collection in Japanese Black donor cows. Superovulation treatments were initiated during mid-cycle (9–12 days after oestrus) with 20 AU of FSH (Antrin; Kyoritsu Seiyaku Corp., Tokyo, Japan) administered intramuscularly through 6 injections of decreasing doses at 12-h intervals. On the third day, the cows received 0.5 mg of prostaglandin F_2α (PGF; Resipron-C, Asuka Seiyaku Corp., Tokyo, Japan) to induce regression of the corpus luteum and were artificially inseminated 60 to 72 h later. Embryos were collected 7 days after AI. All cows received 0.5 mg of PGF immediately after embryo collection and were allocated to 1 of 2 groups: cows (n = 6) with one or more large nonovulated follicles >18 mm in diameter (Group 1) and cows (n = 5) with no large follicles (Group 2). Oestrus detection was performed by visual observation conducted twice daily after embryo collection. Ovarian ultrasonographic examinations and blood collections (for serum progesterone [P4] and oestradiol [E2] measurements) were performed once daily until the next ovulation. Statistical significance was determined using Student’s t-test. There were no differences in the numbers of corpora lutea, ova/embryos, and transferable embryos or the percentages of transferable embryos between the 2 groups. The interval (mean ± SEM) from embryo collection to the appearance of the dominant follicle and to ovulation was longer (P < 0.05) in Group 1 (5.3 ± 0.7 and 11.0 ± 1.0 days, respectively) than in Group 2 (2.6 ± 0.7 and 8.2 ± 0.7 days, respectively). However, the interval from embryo collection to luteolysis (P4 <1 ng mL^–1) did not differ between the 2 groups (4.0 ± 0.7 v. 3.6 ± 0.7 days). In conclusion, the presence of large nonovulated follicles at the time of embryo collection may delay oestrous induction after embryo collection.