Factors affecting oocyte quality in New Zealand dairy cows
Oocyte quality is an important factor affecting the reproductive performance of dairy cows. Good oocyte quality is essential for embryonic survival through early pregnancy as maternal transcripts, cellular machinery, and regulatory factors required by the embryo are derived from the oocyte. Early embryonic development coincides with a high rate of pregnancy loss in New Zealand dairy cows, suggesting that poor oocyte quality is limiting their reproductive efficiency (Berg et al. 2017). The research presented in this thesis investigated the effect of genetic merit for fertility and uterine infections on oocyte quality in New Zealand dairy cows. I also sought to determine the characteristics of a good follicular microenvironment that is capable of supporting the development of a high-quality oocyte, and potentially identify biomarkers associated with fertility.
To achieve the objective of this thesis, I assessed oocyte quality and the corresponding follicular microenvironment in dairy cattle with divergent fertility breeding values (FBV) across multiple life stages. To this end, cumulus-oocyte complexes (COC) and follicular fluid (FF) were recovered from heifers, lactating cows, and mature non-lactating cows of High-and Low-FBV. Molecular biomarkers within individual cumulus cell masses and oocytes were used to assess oocyte quality. Follicular concentrations of amino acids, metabolites, and steroids were quantified to elucidate the follicular microenvironment. While oocyte quality and follicular microenvironment appeared to be subtly different between the High-and Low-FBV animals throughout their life, these differences were at their greatest during lactation when oocyte quality is clearly superior in High-FBV cows. This result was not unexpected as FBV is calculated solely from phenotypes measured in lactating cows. Furthermore, previous research has indicated that the advantage of a higher FBV is greatest when the cow’s fertility is challenged, as occurs during lactation (Craig et al. 2018).
Another objective of this thesis was to identify biological predictors of fertility, particularly those apparent in early life. Altered adrenal function appeared to be related to the sub-fertility of Low-FBV cows at all life stages investigated. As such, differences in circulating adrenal steroids could potentially provide a marker of poor fertility. I also compared oocyte quality and the follicular microenvironment between lactating cows that did or did not conceive on the subsequent oestrus, regardless of FBV. This facilitated the identification of novel markers of fertility that would not have been captured by the current FBV. The preliminary evidence of biological markers associated with fertility presented in this thesis may direct future research.
This thesis also investigated the contribution of oocyte quality to the sub-fertility of cows experiencing an acute uterine infection. Oocyte quality was not different between cows with or without a uterine infection despite changes in the follicular microenvironment. This may have been partially due to the follicular stage sampled, the model of uterine infection, or the small number of animals sampled. It is possible that had the oocyte been allowed to fully mature within the altered follicular microenvironment of the infected cows, differences in oocyte quality would have been identified.
Overall, the results presented in this thesis extend our understanding of oocyte quality and the follicular microenvironment of growing dominant and preovulatory follicles in pasture-fed dairy cows.
