Control of ovarian function; effect of local interactions and environmental influences on follicular turnover in cattle: a review

Abstract

Future improvements in the reproductive performance of cattle will require a better understanding of the mechanisms controlling ovarian follicle development and the influence of environmental factors such as nutrition. There is a need to improve conception rates, reduce embryo loss and control superovulatory responses more precisely. In addition, there is increasing evidence that, even though fertilisation may be achieved, a significant percentage of subsequent embryo loss is due to the poor quality of the oocyte prior to conception. However, relatively recent developments in ultrasound technology, and cellular and molecular biology have improved our understanding of the complex interaction of local and systemic feedback mechanisms which ensure that only one follicle will ovulate in ≥91% of cows. The integrated use of these technologies has demonstrated that follicular growth in cattle occurs in a wave pattern, with usually two or three waves during each oestrous cycle. This pattern of follicular waves is closely associated with changes in circulating gonadotrophins since follicle growth >4 mm in diameter is dependent on FSH and LH. In vitro studies have also demonstrated that locally produced growth factors work in concert with gonadotrophins to control ovarian function. In addition, the recent demonstration that GH and nutritional treatments can alter follicular growth patterns and influence superovulatory responses, highlights the importance of extra-ovarian factors and provides an avenue through which to investigate nutritional influences. Only a complete understanding of the complex mechanisms involved in the control of bovine follicular growth and oocyte maturation will ensure that the reproductive potential of cattle are maximised.