The nutritional regulation of oocyte quality and early embryo development in cattle

The importance of nutrition on reproductive performance is well recognised. However,the mechanisms and specific factors regulating their interaction are still poorly understood. Interest has recently intensified, as fertility in dairy cows has declined. There is a body of evidence on the negative effects of reduced body condition, the high levels of energy and protein intake, however, the results are inconsistent between studies and the role of specific factors, such as insulin and leptin, in the regulation of ovarian function in cattle remains unclear. The main objective of this thesis, therefore, was to test the effects of animal body composition, planes of nutrition and diet composition with emphasis on endocrine and metabolic factors, including progesterone, oestradiol, insulin, IGF-I and leptin, on oocyte quality and early embryo development in cattle. This thesis has demonstrated that alterations in planes of nutrition (Maintenance (M) vs twice maintenance (2M)) affected oocyte quality, and that the effects were dependent on animal body condition score (BCS) (Low vs Moderate). Whilst high (2M) levels of feeding were beneficial for oocytes from animals in Low BCS, they were detrimental to oocyte quality for animals in Moderate BCS. Furthermore, the combination of feeding level and animal BCS created low (10 ull.J/ml), moderate (20 ).lIU/ml) and high (40).lIU/ml) plasma insulin concentrations. Inevitably, however, it also led to the creation of three corresponding levels of plasma leptin concentrations (low: 2 ng/ ml, moderate: 4 ng/ml, and high: 7 ng/ml), indicating that the effects of these metabolic hormones on ovarian function cannot be separated in vivo. However, there was no correlation between changes in metabolic hormones and oocyte quality, suggesting that other endocrine or metabolic factors, not recorded in this study, may be involved in determining the post-fertilisation developmental competence of oocytes. Animal body composition also played an important role in the dietary-induced modifications to oocyte developmental competence both in vivo and in vitro. Indicated by post-fertilisation development in vitro, oocyte quality was enhanced in Low BCS heifers offered the Fibre than the Starch based diets (43.6 vs 24.6% blastocysts for Fibre vs Starch diet), whereas inclusion of protected lipid reduced oocyte developmental competence (27.7 vs 42.9% blastocysts for presence vs absence of Megalac). However, in Moderate BCS heifers, diet composition had litde effect on oocyte quality. Although dietary treatments had little effect on endocrine profile, elevated plasma leptin concentrations were consistently correlated (P