Abstract
Twenty Holstein heifers with ovarian quiescence were assigned to 4 treatment groups. Heifers of Groups I (n=6), II (n=6), III (n=5) and IV (n=3) were treated with a single intramuscular injection of 500, 1, 000, 2, 000 and 4, 000 IU of PMSG, respectively. Ovarian changes were inspected by rectal palpation after the treatment. Changes of plasma estradiol-17β(E2) and progesterone (P) levels in the peripheral blood plasma were examined by radioimmunoassay in 5 of the heifers. E2 level increased sharply and reached a peak 2 days after the treatment. 0vulation was induced 2-4 days after the treatment in 18 heifers (90%) except in 2 heifers of Group I. Three types of corpus luteum (CL) developed after the induced ovulation: hypoplastic and short-lived, well developed but short-lived, and well developed and normal life-spanned. The CL were accompanied by an obvious elevation of blood P level. Second ovulation occurred spontaneously in 6 heifers (30%) 8-22 days after the induced ovulation. Many follicles grew in some heifers of Group II and most heifers of Groups III and IV, accompanied by high E2 peak on the 7th day of posttreatment. It was indicated that PMSG stimulated the growth, maturation and active E2 secretion of follicle and induced the ovulation in ovarian quiescent heifers. It was suspected too that the increased E2 triggered the ovulatory luteinizing hormone surge so that ovulation occurred subsequently.
