Abstract
Two experiments were carried out to examine the effects of a single intramuscular (im) injection with prostaglandin F2α (PGF2α) or an analogue of it (ONO-1052) on regression of the corpus luteum, synchronization of estrus and ovulation, and fertility at synchronized estrus in the cow.
Exp. 1 was performed with three groups of 3 cows each. Group I was given an im injection with 15 mg of PGF2α, Group II with 500 μg and Group III with 800 μg of ONO-1052 7 to 11 days after ovulation. The number of cows having exhibited standing estrus within 2-4 days after treatment was 2, 3 and 2, and the number of cows in which ovulation had occurred within 5 days after treat-ment was 2, 3 and 2 in Groups I, II and III, respectively. One cow in each of Groups I and III showed a quiet ovulation 6 and 8 days after treatment, respectively. Decrease of plasma progesterone concentration during a 48 h period after treament was in the order of Group III>II> I.
Exp. 2 was carried out using 151 heifers in the luteal stage of the estrous cycle. They were divided into three groups; i.e., Group I of 50 heifers received 15 mg of PGF2α, Group II of 50 heifers 500 μg of ONO-1052, and Group III of 51 heifers 800 μg of ONO-1052 by the intramuscular route. The number of heifers exhibiting standing estrus within 5 days after treatment was 42 (84. 0%), 49 (98. 0%) and 46 (90. 2%), and the number of heifers in which ovulation occurred within 6 days was 45 (90. 0%), 50 (100. 0%) and 48 (94. 1%) in Groups I, II and III, respectively. Mean plasma proge-sterone concentrations were 5. 6, 5. 2 and 5. 5 ng/ml immediately before treatment and declined to 0. 9, 0.8 and 0. 7 ng/ml by 48 h after treatment in Groups I, II and III, respectively. The conception rate by the insemination at synchronized estrus was 73. 9% (34/46), 57. 1% (28/49) and 68. 0% (34/50) in Groups I, II and III, respectively.
These results indicate that with a single im injection, 500 or 800 μg of ONO-1052 was a little more effective than 15 mg of PGF2α in causing luteal regression and synchronizing estrus, though the conception rate was practically the same.
