Pregnancy Rate and Blood Progesterone Concentrations on the Previous Day and the Day of Frozen Embryo Transfer in Parous Recipient Cows of Japanese Black

Abstract

Blood progesterone (P) concentrations were determined on the previous day (6 days after onset of estrus; day 6) and the day of embryo transfer (7 days after onset of estrus; day 7) in recipients of Japanese Black beef cow to clarify the correlation between luteal function and pregnancy rate in the stepwise transfer method of frozen Japanese Black embryos. When the blood P concentrations on both day 6 and day 7 in the bovine recipients increased, the pregnancy rate also tended to increase. Pregnancy rates in the recipients with blood P concentration <2.5 ng/ml and ≥2.5 ng/ml on day 6 were 38.6% (17/44) and 60.7% (34/56), respectively, showing a significant (P<0.05) difference. The pregnancy rate in the recipients with blood P concentration <2.5 ng/ml on day 7 was 35.7% (10/28), while the corresponding rate was 56.9% (41/72) in the recipients with blood P concentration of ≥2.5 ng/ml, showing no significant difference. The pregnancy rate in the recipients in which blood P concentration was <2.5 ng/ml on day 6, and increased to ≥2.5 ng/ml on day 7 was low at 45.0% (9/20), while the rate in the recipients which showed blood P concentration of ≥2.5 ng/ml on both day 6 and day 7 was as high as 61.5% (32/52). The relationship between the blood P concentration and luteal development on day 6 corresponding to pregnancy rate was investigated by dividing corpus luteum (CL) into the following three groups by rectal palpation findings : favorably developing, poorly developing and cystic groups. The mean blood P concentration was 2.8 ng/ml in the favorably developing CL, whereas the levels were as low as 2.0 ng/ml and 2.1 ng/ml in the poorly developing CL and cystic CL, respectively. The pregnancy rate for the favorably developing CL was 55.3% (42/76) which tended to be higher than for the poorly developing CL, (44.4% (8/18)), and cystic CL, (16.7% (1/6)), respectively. These observations showed that a high pregnancy rate is achieved by selecting cows with blood P concentration of ≥2.5 ng/ml on day 6 and day 7 of embryo transfer.