What makes a good embryo recipient?

As the genetic merit of embryos increase, small management decisions carry greater consequences. Among those decisions, recipient selection remains one of the most influential factors affecting pregnancy outcomes in embryo transfer (ET) programs.

A successful ET program begins well before transfer day. For example, days postpartum is an important driver of fertility. Recipient cows must have adequate time after calving to resume cyclicity before entering an estrous synchronization protocol. Cows too early in their postpartum period often respond inconsistently to synchronization, and are less likely to become pregnant. 

In most operations, recipients that are at least 45 days postpartum at the start of the protocol, depending on nutritional status and overall management, represent more dependable candidates.

Response to the estrous synchronization protocol is another critical factor. Expression of behavioral estrus following the protocol reflects appropriate follicular development, endocrine function and uterine readiness. Cows that express estrus consistently achieve higher pregnancy rates than cows that do not exhibit estrus. Cows that express estrus are less likely to experience pregnancy loss after the establishment of pregnancy.

However, cows failing to express estrus can and do become pregnant, particularly in well-managed fixed-time ET programs. Lack of visible estrus should not automatically exclude a recipient from receiving an embryo. Instead, estrous expression should be viewed as a positive indicator that helps prioritize more fertile recipients among candidates.

History is paramount. Cows with a 365-day calving interval that breed back promptly, calve unassisted and maintain structural soundness are proven females. Even more valuable are cows that have previously carried an ET pregnancy to term. These females have already demonstrated uterine competence under ET conditions. In many herds, track record is more predictive of success than any technology applied on transfer day.

Evaluation of the corpus luteum (CL) is the final checkpoint before transfer. A functional CL is essential because it produces progesterone, the hormone that supports early pregnancy establishment. Traditionally, practitioners assess CL presence and size through palpation or conventional ultrasonography. While size provides useful information, it does not always reflect functional capacity.

Emerging tools such as color Doppler ultrasonography offer additional insight into CL function. Unlike conventional ultrasound imaging, color Doppler evaluates blood perfusion within the CL. Greater blood perfusion is generally associated with greater luteal activity and progesterone production (Figure 2). Research evaluating luteal blood perfusion in embryo recipients has demonstrated cows with greater CL blood perfusion at the time of transfer achieved greater pregnancy rates than cows with minimal perfusion, even when CL size was similar (Figure 3).