17 Does the stage of in vitro-produced embryos transferred on different days of estrus affect pregnancy outcomes?

Morphological evaluation has been the method of embryo selection for many years and remains the primary approach to assess embryo viability. Embryo quality grading is directly associated with pregnancy outcomes. The objective of this study was to determine whether the stage of in vitro-produced embryos (IVPE) transferred different days after oestrus can affect pregnancy rates (PR) in Holstein recipients. Data from 8597 IVPE transfers performed between January 2020 to May 2023, at RuAnn Dairy, located in Riverdale, California, were collected and analysed. All IVPE were produced at the RuAnn Genetics laboratory as described by Demetrio et al. (2020 Anim. Reprod. 17, e20200053). Embryos from Holstein donors were classified based on developmental stage (4-morula, 5-early blastocyst, 6-blastocyst, 7-expanded blastocyst, 8-hatching blastocyst, and 9-hatched blastocyst) and quality (grade 1 or 2), according to the IETS guidelines, when removed from culture for transfer 6 or 7 days after fertilization. The embryos were transferred fresh, to healthy virgin heifers (natural oestrus, 13.5–15 months of age) or lactating cows (Presynch-Ovsynch, 86 days in milk, first or second lactation), 6 to 9 days after oestrus. Pregnancy diagnosis was done by dosing glycoprotein associated to gestation (IDEXX PAG) 30 to 40 days after expected oestrus. Pregnancy results by embryo stage and grades transferred 6 to 9 days after oestrus are shown below. Embryos transferred 7 and 8 days after oestrus had higher pregnancy rates than 6 and 9 days. Younger embryos removed Day 6 (stage 4) and Day 7 (stage 4 and 5) after fertilization, had a lower PR than the older embryos. Grade 2 embryos had a lower PR when compared to grade 1 embryos. High pregnancy rates can be achieved with good-quality embryos selected at optimal time and stage of development, transferred to well managed recipients 7 or 8 days after oestrus (56.5%, n = 2096). In vitro embryo quality assessment should take into consideration the grade and stage of embryos, since fast developing embryos are more likely to establish pregnancy. This type of descriptive analysis is a tool to be used by practitioners to identify optimal points to obtain better pregnancy outcomes. More studies are necessary, with comprehensive statistics, to verify the individual effects of sires and dams on embryo development and pregnancy outcomes.