Embryo transfer in dairy herds: a comprehensive review of factors affecting pregnancy success from donor to embryo and recipient.

Embryo transfer (ET) is a fundamental biotechnology in modern dairy production, utilized to maximize the propagation of superior genetics and circumvent fertility limitations associated with high milk production. The physiological, environmental, and managerial elements that together determine pregnancy per embryo transfer (P/ET) are systematically analyzed in this paper. In order to increase the output of transferable embryos, healthy, non-lactating females are chosen, and appropriate nourishment is ensured. Evidence indicates that stage 7 embryo (expanded blastocyst), especially those transferred fresh, have better viability than frozen or vitrified in vitro produced (IVP) embryos, which are more vulnerable to cryoinjury. Embryo quality is still a major predictor of success. Importantly, field outcomes are frequently determined by the physiology of the recipient animal. Heifers are continuously more fertile than lactating cows, as implantation in cows is often compromised by uterine health problems such as endometritis and metabolic stress. The review underlines the negative effects of heat stress (high temperature humidity index [THI]) on conception and the need for stringent synchronization methods to guarantee a functioning, progesterone-producing corpus luteum. As essential elements of success, procedural variables such as technician skill and atraumatic deposition in the cranial uterine horn are also emphasized. Finally, the article explores future trajectories, specifically the integration of artificial intelligence for objective morphological grading and the discovery of non-invasive omics biomarkers to accurately predict recipient receptivity.