80 Randel Lectureship: Practical Applications for Developmental Programming of the Bovine Ovary.

Mammalian females are born with a finite supply of follicles within their ovaries that may be related to fertility and reproductive longevity. The number of microscopic follicles in the ovarian cortex is positively correlated to the number of antral follicles. In cattle, the number of antral follicles can be determined by ultrasonographic examination of the ovaries, is repeatable within a female, and has a moderate heritability. Studies in rodents suggest that nutritional status early in life can alter structure and function of the ovarian reserve, resulting in changes in follicle numbers, estrous cycle parameters, and reproductive longevity. This raises questions about how early life events might influence the ovarian reserve and reproductive performance in bovine females. A central hypothesis of our research is that the number of follicles that a heifer has when entering her first breeding season is influenced by external factors such as maternal nutrition during pregnancy, maternal stress during pregnancy, peri-pubertal nutrition, and health events. Understanding how these factors influence the structure and function of the ovarian reserve in replacement heifers may allow us to translate this knowledge into management practices that developmentally program the bovine ovary prior to the first breeding season to improve reproductive performance in the cow herd. Accomplishing this is made difficult because our understanding of the mechanisms controlling pre-antral follicle development in cattle is limited and growth of pre-antral follicles is occurring at the microscopic level within the ovarian cortex where it is difficult to monitor changes in real time. There has been some success, but variation in response across studies indicates that further research to understand the causes of this variation is required before we will be able to apply developmental programming in beef production systems to improve ovarian function in replacement heifers. USDA is an equal opportunity provider and employer. [ABSTRACT FROM AUTHOR]