329 Ovarian reserve and fertility: Bridging basic science and reproductive management.

Abstract The ovarian reserve in cattle is established before birth and exhibits moderate heritability. Assessment of ovarian reserve typically relies on antral follicle count (AFC) or circulating anti-Müllerian hormone (AMH) concentration, a biomarker for AFC. The large variation in AFC/AMH among cattle is accompanied by marked physiological differences involving cellular and endocrine changes, which can influence reproductive performance. It is well established that AMH is positively associated with ovarian response to FSH stimulation and in vitro embryo production (IVEP) in cattle, albeit FSH stimulation treatments are generally applied without accounting for the physiological differences between AFC/AMH phenotypes. First a series of experiments aimed at evaluating the effect of various alterations to FSH-based superstimulatory treatments on ovarian response and IVEP in Holstein heifers with different AFC/AMH phenotypes. Primary findings indicate that administration of FSH before OPU enhances IVEP in both high- and low-AMH heifers in a dose-dependent manner, with embryo yield increases, relative to a non-stimulated control, of ~146% and ~133% for low and high AMH heifers, respectively. However, maximal oocyte developmental competence was attained at a lesser FSH dosage in low than high AMH phenotype heifers, suggesting that the optimal FSH dosage is less for low than high AMH phenotype heifers. FSH dose schedule – decreasing, constant, or increasing – did not affect ovarian response, or embryo yield, irrespective of AMH phenotype. Extending the superstimulatory regimen from 2 to 3 days increased the percentage of large sized follicles; however, FSH treatment duration was without effect on number of total follicles, oocyte developmental competence, or embryo production among AMH phenotypes. A single FSH administration diluted in 0.5% hyaluronan or saline resulted in a lesser ovarian response in all AMH phenotypes compared to a multiple administration regimen using the same total FSH dose. Furthermore, in high AMH heifers, use of a multiple administration regimen resulted in greater oocyte developmental competence, leading to ~115% and ~70% more embryos compared to a single administration in saline and a single administration in hyaluronan, respectively. Conversely, in low- and intermediate-AMH heifers a single administration of FSH, regardless of the vehicle, led to comparable oocyte developmental competence and embryo yield. Furthermore, we evaluated whether AFC/AMH phenotype affected fertility in embryo transfer recipient heifers. Preliminary data from 750 heifers in a fixed-time embryo transfer program suggest that AFC/AMH phenotype does not affect expression of estrus, recipient utilization rate, pregnancy per embryo transfer nor pregnancy loss. In summary, while AFC/AMH phenotype appears to have minimal impact on embryo recipient reproductive performance, it plays a significant role in shaping reproductive outcomes during FSH-stimulation for in vitro embryo production. Furthermore, these findings underscore the opportunity for developing targeted superstimulation regimens, based on AFC/AMH phenotype.