Abstract
The relationship of blood progesterone (P) and estradiol- 17β (E2) concentrations to the pregnancy rates in Japanese Black beef cattle recipients was investigated by determining blood P and E2 concentrations on the day before (day 6; day 0=onset of estrus) and the day of embryo transfer (day 7). Embryos were thawed by the stepwise method and transferred on day 7. The pregnancy rate showed a tendency toward increase with increasing P concentrations on both days 6 and 7 of embryo transfer. There was no significant correlation between the blood E2 concentration on day 6 and the pregnancy rate, wthereas pregnancy rate tended to rise when the blood E2 concentration on day 7 was low. The relationships of the blood E2/P ratio (× 103) on days 6 and 7 to the pregnancy rate were investigated. When the E2/P ratio was ≥1.0, the pregnancy rate tended to decrease. The bovine recipients, which showed poorly development CL, high blood E2/P ratio and coexistence of follicle ≥1.0 in diameter with CL, showed a tendency toward decrease in the pregnancy rate. From these observations, it is supposed to be important to select bovine recipient with the high bood P concentration and with the low blood E2 concentration on days 6 and 7 as bovine recipients for improvement of the pregnancy rate in bovine frozen-thawed embryo transfer.
