Factors associated with pregnancy following transfer of Japanese Black embryos into Japanese Black, Holstein, and crossbred heifers

We investigated factors associated with pregnancy approximately 60 days after transfer of Japanese Black (JB) embryos into JB, Holstein (HO), and their crossbred (JB×HO) heifers. A total of 26,196 embryo transfer (ET) records collected between 2008 and 2023 were analyzed. Recipient-, embryo-, and donor-related factors were initially evaluated using Akaike information criterion (AIC)-based variable selection, and the retained fixed-effect structure was subsequently evaluated in a mixed-effects logistic regression model with donor as a random intercept. The overall pregnancy rate was 70.7%. Pregnancy rates for JB, HO, and JB×HO recipients were 62.0%, 71.1%, and 71.6%, respectively. Embryo quality, embryo type, ET technician, farm, and ET attempt number showed prominent associations with pregnancy. Donor-related associations were statistically detectable but small. Embryo stage was retained in the AIC-selected model, but its overall effect was not statistically significant in the likelihood-ratio test ( P = 0.1222). These findings indicate that embryo- and recipient-related factors were more strongly associated with pregnancy rate than donor superovulatory response traits.