Abstract

The effect of pregnant mare serum gonadotrophin (PMSG) treatment on activation of quiescent ovaries was examined in heifers. Groups of thirteen, twenty and twelve heifers which showed ovulation within 2 d and corpus luteum (CL) development after injection with a luteinizing hormone releasing hormone analogue (LH-RH-A) were supplementally injected with 500 IU of PMSG (Group I); 500 IU of PMSG and 500 μg of Prostaglandin F 2α analogue (PGF 2α-A; Group II); and 500 μg of PGF 2α-A (Group III) on Day 6 after the injection of 200 μg of LH-RH-A (Day 0), respectively. Estrus appeared in 33.3 to 45.0% of the heifers of the respective groups after the treatment. Ovulation occurred at a significantly (P<0.01) higher rate in Groups I (100%) and II (90.0%) than in Group III (41.7%). The ovarian cyclic activity was initiated in all the heifers that ovulated. Plasma progesterone levels decreased significantly (P<0.05) to about 1 ng/ml on Day 8 and Day 7 in Group I and Groups II and III, respectively. Plasma estradiol-17β (E z) levels increased significantly (P<0.05), reaching a peak on Days 7 to 7.5 in Groups I and II but not in Group III. It is concluded that PMSG treatment stimulates maturation and E z secretion of a follicle, thus promoting ovulation and the onset of ovarian cyclic activity.

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