Synchronization Protocols Utilizing Progesterone Implants Associated with Estrus Expression, Vaginal Microbiome, and Immune Response in Beef Cattle

This thesis aims to provide a deep understanding of presynchronization strategies and the varying factors affecting their efficacy. Beginning with a literature review of the estrous cycle and the endocrinology driving it. Building on this foundation, the physiological basis for estrous synchronization is described, followed by the advancement of pre-synchronization protocols designed to increase estrus and conception. Further, we will focus on the influence of estrus expression on insemination time and conception rates. Finally, how the immune system interacts with the conception process. In addition, the review goes beyond exogenous endocrine manipulation and dives into the emerging role of the microbial communities of the reproductive tract and their influence on pregnancy and inflammation of the reproductive tract. Together, these interconnected fields underscore the multifactorial nature of fertility and highlight opportunities to improve reproductive management strategies in cattle. Two parallel studies were conducted to address the estrous expression and the microbiome related to synchronization protocols that utilize a 14-day implant. The first study focused on characterizing the onset of estrus in heifers, primiparous, and multiparous cows that are synchronized using the 7&7 synchronization protocol and variations. The second characterized the vaginal microbiome changes from the start to the end of the 7&7 synchronization protocol by utilizing 16S rRNA gene sequencing. The understanding of biological and management factors and their contribution to fertility is vital in increasing conception and reducing reproductive loss in cattle.