Estrus expression following synchronization protocols in dairy cattle: Associations with fertility, periovulatory hormones, and endometrial receptor gene expression

Abstract

Reproductive efficiency is a major determinant of the profitability and sustainability of modern dairy production systems. Although synchronization protocols have revolutionized reproductive management by allowing timed artificial insemination (TAI) and timed embryo transfer (TET) without reliance on estrus detection, fertility remains highly variable among synchronized animals. Numerous studies have demonstrated that cows expressing estrus during synchronization programs consistently achieve greater fertility than cows that fail to express estrus. These observations suggest that estrus expression represents more than a behavioral event and may instead reflect a favorable periovulatory endocrine environment associated with improved reproductive competence. However, the physiological mechanisms responsible for these associations remain poorly understood. The overall objective of this dissertation was to determine whether estrus expression serves as a biological indicator of the periovulatory endocrine environment within synchronization protocols and to investigate how variation in this endocrine milieu influences subsequent reproductive events in dairy cattle. Specifically, this work evaluated the relationships between estrus expression, periovulatory ovarian and endocrine characteristics, endometrial programming, and fertility under synchronized reproductive management. The findings of this dissertation demonstrated that cows expressing estrus before TAI achieved greater pregnancy per artificial insemination and were more likely to subsequently calve than cows that failed to express estrus. In contrast, estrus duration within TAI protocols was not associated with fertility. Although these observations established estrus expression as a robust indicator of reproductive success within TAI, the retrospective nature of the study did not allow for the determination of whether greater fertility in cows expressing estrus reflected a more favorable hormonal milieu for pregnancy or was just the result of asynchronous animals in the no estrus group. Experimental manipulation of the periovulatory endocrine environment by extending the duration of proestrus before inducing ovulation increased estrus expression, altered periovulatory ovarian characteristics, and reduced pregnancy loss following embryo transfer. Because embryo transfer eliminates variation associated with fertilization and early embryonic development, these findings demonstrated that the preovulatory endocrine milieu influences reproductive performance independently of fertilization. Further evidence for this concept was provided by evaluating natural variation in the periovulatory endocrine milieu and its associations with subsequent luteal function and endometrial receptor dynamics throughout the estrous cycle. Periovulatory endocrine characteristics were associated with subsequent luteal progesterone production, the timing of luteolysis, and changes in endometrial receptor abundance, demonstrating that endocrine conditions established before ovulation continue to influence both ovarian and uterine function after ovulation. These findings suggest that the periovulatory endocrine environment coordinates postovulatory luteal function and endometrial programming through interconnected physiological pathways, providing a mechanistic explanation for the superior fertility consistently observed in cows expressing estrus. Collectively, these findings support a unifying biological concept in which estrus expression represents an external manifestation of a favorable periovulatory endocrine environment rather than an isolated behavioral event. However, the results of this dissertation also demonstrate that important physiological variation exists among cows that express estrus, indicating that estrus expression alone does not fully capture the complexity of the periovulatory endocrine milieu. Instead, endocrine conditions established during the periovulatory period continue to influence multiple physiological processes extending beyond ovulation, including follicular maturation, corpus luteum development, luteal progesterone secretion, endometrial programming, and ultimately pregnancy maintenance. Together, these findings support a conceptual model in which the periovulatory endocrine milieu functions as a central regulator of reproductive success in dairy cattle, emphasizing the importance of optimizing endocrine conditions before ovulation to improve fertility under synchronized reproductive management.