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Relationship Between Ovarian Reserve and Preovulatory Estradiol During a Fixed-Time AI Protocol in Beef Heifers
Estradiol production is essential for reproductive efficiency. This study compared numbers of follicles in beef cows that did or did not have elevated preovulatory estradiol during a fixed-time AI (FTAI) protocol. In experiment 1, 5 low estradiol (LowE2) and 5 high estradiol (HighE2) cows were slaughtered at time of FTAI. In experiment 2, 17 LowE2 and 12 HighE2 cows were slaughtered 16 d after FTAI. Ovaries were collected at slaughter and all visible surface follicles (AFC) were counted. A representative cross section of the ovary was fixed in paraformaldehyde and embedded in paraffin for histological evaluation. Ovaries were sectioned and five sections (six μm thick) were collected with a minimum of ten sections between them. Sections were stained with hematoxylin and eosin. Follicles were counted and classified as primordial (oocyte surrounded by a single layer of squamous pre-granulosa cells); primary (oocyte surrounded by a single layer of cuboidal granulosa cells); secondary (oocyte surrounded by two or more layers of granulosa cells) or antral. Follicle classification and AFC were analyzed using MIXED procedure (SAS) with estradiol group, experiment, and their interaction as fixed effects and animal as random effect. There was a tendency (P = 0.06) for an estradiol and experiment interaction on AFC. HighE2 in experiment 1 had the greatest AFC (57 ± 5.3), LowE2 in experiment 1 were intermediate (37.8 ± 4.3). Both HighE2 (20.2 ± 3.1) and LowE2 (16.4 ± 2.7) in experiment 2 were similar and had fewer AFC. There were no effects of treatment, experiment or interaction on number of primordial follicles (P > 0.45). There tended (P = 0.06) to be an effect of experiment on number of primary (6.2 ± 1.5 vs. 3.1 ± 1.0; experiment 1 and 2) and secondary (0.29 ± 0.32 vs. 0.95 ± 0.20; experiment 1 and 2) follicles. There was an estradiol by experiment interaction on number of antral follicles (P < 0.01). Antral follicles were greater in HighE2 in experiment 1 (9.1 ± 1.2), LowE2 in experiment 1 were intermediate (5.4 ± 0.68). Both HighE2 (2.4 ± 0.5) and LowE2 (0.8 ± 0.7) in experiment 2 had fewer antral follicles but HighE2 tended (P = 0.07) to have more than LowE2. These results indicate that cows with elevated concentrations of preovulatory estradiol have a greater number of antral follicles that is not associated with an increase in the ovarian reserve. USDA is an equal opportunity provider and employer.
Keywords: Fertility, Ovarian Reserve, Estradiol