Effects of additional injection of hCG on the formation of the corpus luteum and the fertility of estrous synchronized dairy heifers by simultaneous injection with prostaglandin F<SUB>2α</SUB>, and estradiol ben-zoate

On one pasture and two raising farms, a total of 150 Holstein heifers in the luteal stage of the estrous cycle received simultaneous intramuscular (im) injection with 15 mg of prostaglandin F_2α (PGF_2α) and 0.5 mg of estradiol benzoate (E₂B). Of them, 119 heifers which showed standing estrus during a period of 25-72 h after the treatment were assigned to three experimental groups, A, B and C, in such manner that there might be no large difference in distribution of the time of occurrence of estrus among the groups. Group A comprised of 40 heifers received an im injection of 1, 500 IU of hCG at the time of insemination, Group B of 40 heifers an im injection of 1, 500 IU of hCG either 2 or 3 days after ovulation, and Group C of 39 untreated heifers served as the control.<BR>All the heifers were inseminated artificially 6-25 h after having shown estrus. The time of ovula-tion was examined at intervals of 6-12 h after insemination by rectal palpation. Ten to 12 days after insemination, the presence, number and size of the corpus luteum were ascertained by rectal palaption, and blood samples were collected from the jugular vein to measure the progesterone con-centration in plasma by radioimmunoassay. Pregnancy diagnosis was performed by rectal palpation nearly 50 days after insemination.<BR>The percentage of heifers showing standing estrus during a period of 25-48 h after PGF_2α and E₂B treatment was 85. 0, 87. 5 and 84. 6% for Groups A, B and C, respectively. The percentage of heifers in which ovulation occurred during a period of 49-84 h after the treatment was 95. 0, 95. 0 and 94. 9% for Groups A, B and C, respectively. The corpora lutea formed after ovulation were normal in size in most of the heifers of all the groups. They were small in size and regressed by 8 to 16 days after insemination in 2 heifers (5. 0%) of Group A and 7 heifers (18. 0%) of Group C. They were cystic in 4, 2 and 3 heifers of Groups A, B and C, respectively. In addition it was observed that 33 heifers of Group B had 1 or 2 additional corpora lutea each. The mean plasma progesterone concentration was significantly higher (P<0. 01) in Group B (6. 5 ng/ml) than in Group A (5. 2 ng/ml) and Group C (4. 8 ng/ml). The conception rate by the insemination at synchronized estrus was higher in Groups A (57. 5%) and B (55. 0%) than in Group C (41. 0%).<BR>These results indicate that an additional im injection of hCG at the time of insemination, or either 2 or 3 days after ovulation, following simultaneous im injection with PGF_2α and E₂B may have an effect on stimulating the formation of the corpus luteum and increasing the conception rate in cows.