Abstract
Background: In vitro production of dairy cattle embryos, complemented by embryo transfer, is one of the main reproductive biotechnologies employed globally. However, factors such as the embryonic development rate (EDR) and the temperature-humidity index (THI) affect their effectiveness. The aim of this study was to evaluate the influence of EDR, represented by blastocysts obtained on in vitro culture days 7 and 8 and THI on pregnancy rate (PR) in Holstein cows using the embryo transfer technique. Methods: An 800 recipient Holstein cows, which received a single quality 1 expanded blastocyst. The recipient cows were classified by the embryonic development (ED) day (day 7, n=356; day 8, n=444) and thermal conditions at the time of transfer, defined by THI as normal (≤72) or heat stress (HS) ( greater than 72). The PR was determined through ultrasonography 60 days after embryo transfer. To evaluate the association between the variables considered, a Pearson’s chi-squared test was performed. The association between the variables was estimated using the odds ratio and Cramer’s V. A logistic binomial regression model was adjusted to evaluate the individual effects of the variables and their interaction on the probability of pregnancy in cows. Result: The blastocysts transferred on the 7th day reported a higher PR than those transferred on the 8th day (52.0 vs 44.6%; P=0.038). Furthermore, cows exposed to a normal THI showed a higher PR than those under HS (64.7 vs 41.7%; P less than 0.001). The interaction between variables was significant, revealing that under HS conditions, blastocysts transferred on the 7th day showed a higher PR than those transferred on the 8th day (52.2 vs 30.9%; P less than 0.001). The early developmental blastocysts exhibited a greater capacity to maintain pregnancy under HS conditions, suggesting that EDR may serve as a complementary criterion for morphological evaluation to identify embryos with a higher likelihood of establishing pregnancy.