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

• Published in: Journal of animal science Vol. 96; pp. 43 - 44
• Main Authors: Smith, M F, Geisert, R D
• Format: Journal Article
• Language: English
• Published: Champaign Oxford University Press 01.03.2018
• Subjects: Anatomy; Anestrus; Animal reproduction; Artificial insemination; Biology; Bovidae; Breeding; Cattle; Cryopreservation; Domestic animals; Embryos; Estrus cycle; Feedback; Fertility; Fertilization; Gametes; Genetic improvement; Livestock; Livestock industry; Material requirements planning; Molecular chains; Mortality; Negative feedback; Organs; Ovulation; Physiology; Pituitary; Pituitary gland; Postpartum; Pregnancy; Progestin; Puberty; Reproduction (biology); Reproductive organs; Semen; Sex hormones; Sheep; Steroids; Uterus
• ISSN: 0021-8812; 1525-3163

Reproductive physiology has a long and distinguished history, beginning with the detailed anatomical study of the male and female reproductive organs in the 1600's and progressing to the use of "ablation and replacement" as a tool for elucidating endocrine mechanisms. The study of reproductive physiology is rapidly traversing the continuum from the whole animal to the molecular level. Due to the voluminous subject matter; this presentation focuses on domestic ruminants and is organized around the development of the artificial insemination technique and efforts to alleviate two major limitations to reproductive efficiency: anestrus and embryonic mortality. 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 then major advances in methods of semen collection, evaluation of male fertility, cryopreservation of sperm, gender-selected semen, and estrous cycle control have occurred. The preceding advances led to the widespread use of AI and contributed to our fundamental understanding of gamete biology, cryopreservation of gametes, ovulation control, and timing of insemination. However, continued efforts directed toward increasing pregnancy rates to fixed-time AI in tropically adapted cattle are needed. In regards to anestrus, our understanding of the neuroendocrine control of the pituitary gland and the role of steroid feedback led to the Gonadostat Theory, which proposes that onset of puberty is due to a decrease in the negative feedback of gonadal steroids over time. Subsequent studies in prepuberal and postpartum sheep and cattle established that a short luteal phase frequently precedes the first normal length cycle that is accompanied by estrous expression. This observation led to the common practice of treating prepubertal heifers and anestrous postpartum cows with a short term progestin treatment (e.g. CIDR) to induce normal estrous cycles. In domestic ruminants, fertilization rate is high (85 to 95%); 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 toward 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 summary, major advancements in our understanding of reproductive biology have stemmed from efforts to establish the AI technique and reduce the negative impact of anestrus and embryonic mortality in domestic ruminants.