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The effective dosage of intramuscular injection of PGF2α and the effectiveness of intramuscular injection with PGF2α followed by additional HCG on the synchronization of estrus and ovulation were investigated in the cow. 1. In the Exp. 1, 4 cows were injected intramuscularly (IM) with 1 of 3 doses of PGF2α (10, 15 or 20 mg) during the luteal phase of the estrous cycle. Out of 4 cows in each group, 2 cows were given 2, 000 IU of HCG IM 42 hr later (HCG-42) and the other 2 cows 72 hr later (HCG-72), respec-tively after the PGF2α injection. The number of cows showed sexual behavior after the treatment was smaller in HCG-42 (2 of 6 cows) than in HCG-72 (4 of 6 cows). The time intervals from the injection of HCG to ovulation ranged from 33 to 37 hr and 16 to 38 hr in HCG-42 and HCG-72, respec-tively. Blood progesterone level during the period of 24 hr after the PGF2α injection in the cows given 20 mg of PGF2α decreased dramatically, compared with those in the cows given 10 mg and 15 mg of PGF2a. In the cows given 15 mg of PGF2α, blood progesterone level fell gradually by 48 hr after the PGF2α injection and thereafter the pattern and the concentration of the blood progesterone were coincided with those in the cows given 20 mg of PGF2a. In the cows given 10 mg of PGF2α, blood progesterone level decreased slowly and had maintained as much level at 24 hr after the PGF2α in-jection during the period of 6072 hr after the PGF2α injection. 2. In the Exp. 2, 89 grazing Holstein cows, which were considered to be on 5 days or more after ovulation, were divided into four groups; i. e. Group A (26 cows) and Group B (19 cows) received the injection of PGF2α plus 2, 000 IU of HCG IM, Group C (25 cows) and Group D (19 cows) received the injection of PGF2α IM alone. The dosage of PGF2α injected IM were 15 mg in Groups A and C, and 20 mg in Groups B and D. The time intervals from the PGF2α injection to the injection of HCG in Groups A and B were 60 hr and 7374 hr, respectively. In Groups C and D, the occurrence of estrus after the PGF2α injection was found to be synchronized effectively. In Groups A and B, however, the percentage of the cows in estrus by 96 hr after the PGF2α injection was some-what low, but ovulation could be synchronized effectively, as compared to those in Groups C and D. There was no marked differences between Groups A and C, Groups B and D in the conception rates at the synchronized estrus. From these results, it was concluded that the effective dosage of PGF2α for regressing the corpus luteum rapidly and completely in the cow is about 15 mg or more, and the most efficient time interval from the injection of PGF2α to the additional injection of HCG for the synchronization of estrus and ovulation may be between 42 hr and 72 hr, viz. 60 hr.
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