Timing of human chorionic gonadotropin administration affects conception rates after embryo transfer in crossbred beef heifers.

This study aimed to determine the effect of the timing of human chorionic gonadotropin (hCG) administration to embryo transfer (ET) recipients on the reproductive outcome of ET in crossbred beef heifers. Heifers (n=1,887) were randomly assigned to the non-treatment group (n=1,160), and hCG was administered on day 5 (n=201), 6 (n=210), 7 (n=213), or 8 (n=103) after estrus. The chi-square test showed that the conception rates in the hCG administration on day 5 (79.6%, P=0.0001) and day 6 (75.2%, P=0.0047) were higher than those in the non-treatment group (65.2%). However, the conception rates with hCG administration on day 7 (70.9%, P=0.1103) and day 8 (70.9%, P=0.2499) were comparable to the non-treatment. A multiple logistic regression analysis, furthermore, showed that early treatment (hCG on day 5 or 6 after estrus) positively affected the conception rates compared to non-treatment (odds ratios (OR) = 1.723 [95% confidence intervals (CI) = 1.316-2.257]), but not late treatment (hCG on day 7 or 8 after estrus) (OR = 1.299 [95% CI = 0.981-1.719]). None of the confounding variables, including years, season at the time of ET, age in months, and number of ET, significantly affected the conception rate. In conclusion, hCG administration on day 5 or 6 after estrus for ET recipients improved the conception rate, but not on day 7 or 8, in beef heifers.