Reproduction in Domestic Ruminants during the Past 50 Years: Discovery to Application

Reproductive physiology has a long and distinguished history, beginning with the detailed anatomical study of male and female reproductive organs in the 1600’s to the current elucidation of molecular mechanisms controlling critical reproductive events. We have chosen to focus on biological processes essential to the establishment and maintenance of pregnancy in domestic ruminants that resulted in management strategies to increase reproductive efficiency or genetic merit. To date, artificial insemination (AI) is the most powerful technique that reproductive physiologists and geneticists have provided the livestock industry for genetic improvement. Early efforts to establish AI as a tool were initiated in Russia around 1899 and since that time there have been major advances in methods of semen collection, evaluation of male fertility, cryopreservation of sperm, gender-selected semen, and estrous cycle control. The preceding advances not only led to the widespread use of AI, but also contributed to a fundamental understanding of gamete biology, cryopreservation of gametes, ovulation control, and timing of insemination. Purification of gonadotropins and steroids, development of radioimmunoassays (RIA), and the ability to monitor follicular waves and luteal function not only increased our understanding of estrous cycle regulation, but resulted in effective protocols that precisely synchronize ovulation in cattle. Furthermore, understanding the role of the short luteal phase in prepubertal heifers and anestrous postpartum cows resulted in the development of short-term progestin treatments (e.g. CIDR) to induce normal estrous cycles. In domestic ruminants, fertilization rate is high; however significant embryonic mortality before or around the time of maternal recognition of pregnancy (MRP) reduces the conception rate to a single breeding. Significant effort has been directed at determining the time of MRP, the signal for MRP, as well as elucidating the physiological, cellular, and molecular dialogue between the conceptus and uterine environment. In addition, efforts to elucidate mechanisms associated with oocyte maturation, fertilization and early embryonic development have led to in vitro production and transfer of embryos. Finally, progress in our understanding of molecular biology and genomics have resulted in the development of powerful tools for gene editing such as CRISPR/Cas 9 technology. In summary, during the past 50 years many of the advancements in our understanding of the reproductive biology of domestic ruminants have resulted from a concentrated effort to develop methods for the genetic improvement of these species (e.g. AI and embryo transfer) as well reduce the negative impact of anestrus and maximize the pregnancy rates.