# In Vitro Developmental Competence Predicts Pregnancy Outcomes Following Transfer of Beef Embryos to Dairy Recipients: A Retrospective Study

## TL;DR

This study shows that the quality of lab-produced beef embryos is the main factor affecting pregnancy success when transferred to dairy cows, more than the recipient cow's condition or environmental factors.

## Contribution

The study demonstrates that embryo developmental competence in the lab is a stronger predictor of pregnancy success than recipient or environmental factors in cattle embryo transfer.

## Key findings

- Embryo developmental competence, measured by blastocyst formation and degeneration rates, significantly predicts pregnancy outcomes.
- Recipient and environmental factors had little impact on pregnancy success compared to embryo quality.
- Optimizing lab conditions to improve embryo quality is more effective for reproductive efficiency than managing recipient cows.

## Abstract

In cattle breeding, transferring embryos produced in a laboratory into recipient cows is a popular method to improve genetics and beef production efficiency. However, success rates vary, and it is often unclear whether the condition of the recipient cow, the environment, or the quality of the embryo itself is most important for a successful pregnancy. This study analyzed records from 462 transfers of beef cattle embryos into dairy cows to identify the main factors predicting pregnancy success. We examined three categories: the recipient cow’s health and reproductive status, the laboratory performance of the embryos (such as how well they developed in the incubator), and environmental conditions like heat stress. Our results showed that the cow’s condition and weather had little effect on pregnancy outcomes. Instead, the specific developmental ability of the embryos, measured by their growth rates and quality grades in the laboratory, was the strongest predictor of success. Specifically, embryos from groups that grew better and had lower death rates in the lab resulted in significantly higher pregnancy rates. These findings suggest that to improve breeding efficiency, producers should focus primarily on optimizing laboratory systems to produce high-quality embryos rather than focusing solely on managing the recipient cows.

In bovine embryo transfer (ET) using in vitro-produced (IVP) embryos, recipient factors and embryo grade are well-established predictors of pregnancy success, but the impact of the laboratory-level developmental competence of IVP embryos remains insufficiently characterized. This retrospective study evaluated factors affecting pregnancy rates following the transfer of IVP beef embryos to dairy recipients. Medical records from 462 ETs were analyzed across three categories: (1) recipient-related factors (parity, body condition, estrus synchronization, corpus luteum characteristics); (2) laboratory factors (cleavage, blastocyst formation, degeneration, embryo grade, developmental stage, cryopreservation); and (3) environmental factors (temperature–humidity index, transport time). Mean comparison and chi-square analyses revealed significant differences in pregnancy rates based on corpus luteum volume, cleavage rates, blastocyst formation rates, degeneration rates, and embryo grade. In binary logistic regression, categorized increases in blastocyst formation rate, degeneration rate, and embryo grade were associated with a 1.45-fold increase, 0.74-fold decrease, and 0.56-fold decrease in pregnancy odds, respectively; no recipient or environmental variables were independent predictors. These findings indicate that developmental competence of IVP embryos is more critical for pregnancy success than recipient or environmental factors, suggesting that optimizing IVP systems to maximize embryo quality is the most effective strategy to improve reproductive efficiency in ET.