Quantifying the Effect of Pre-Ovulatory Exposure to Extreme (High or Low) Blood Urea Nitrogen Concentration on the Progression of Bovine Oocytes to Embryos In Vitro

This study investigated the impact of exposure of developing bovine oocytes in pre-ovulatory follicles to extreme blood urea nitrogen (BUN) concentrations in vivo on bovine oocyte development in vitro. In situations where pasture or ration components fall short of meeting dietary protein requirements, dietary supplementation is often necessary. Both high and low supplementation levels are linked to altered BUN concentrations, adversely affecting reproductive performance. The specific time of exposure to extreme BUN concentrations (7mg/dL > BUN > 20mg/dL) causing diminished reproductive performance remains elusive, as does the stage at which embryonic development is impacted. This research used 800 oocytes that were aspirated from cows with normal BUN concentration (7mg/dL < BUN concentration < 20 mg/dL) and 409 that were retrieved from cows with extreme BUN concentrations to investigate the effect of in vivo exposure of bovine oocytes to extreme BUN levels on the developmental competence of embryos in vitro. The BUN concentration status of donor cows affected the in vitro survival of oocytes and subsequent embryos, with oocytes from cows with extreme BUN concentration having a 1.18 times (95% confidence interval, 1.03-1.36) higher rate of failing to develop to the blastocyst stage. Exposing bovine oocytes to extreme (low or high) BUN concentrations before ovulation reduced their progression to embryos in vitro, which implies that extreme BUN concentrations might affect bovine fertility for extended periods of time, even after the normalisation of BUN concentrations in an animal.