Ovarian hyperstimulation used in conjunction with embryo transfer techniques is an assisted reproductive technology that aids cattle producers in reaching their reproductive, genetic, and financial goals; however, viable embryo recovery results are inconsistent, possibly because the reproductive endocrine profile is poorly understood. The objective of these experiments was to evaluate the endocrinological and ovarian dynamics surrounding follicular deviation in hyperstimulated cows.

In Experiment 1, the perideviation period (PDP; largest follicle diameter from 8–10- mm) was examined in Hyperstimulated (50 mg FSH) or Control (saline) dry Holstein cows (n = 4/group). Daily blood samples began at the initiation of an engineered follicular wave and ended at ovulation. Jugular blood samples were collected every 15 min during the PDP. Trans-rectal ultrasonography was performed at 8 hr intervals prior to and 4 hr intervals during the PDP, and daily thereafter until ovulation. Serum samples were analyzed for LH and FSH concentrations. Results indicated that Controls experienced a longer PDP (P = 0.003) and more LH pulses in the PDP (P = 0.008) without increasing the overall concentration or the number of LH pulses per hour. Hyperstimulated cows tended to have more follicles > 7- mm at the initiation (P = 0.059) and the cessation (P = 0.054) of the PDP, experienced a faster growth rate (P = 0.018), and more ovulations (P = 0.087).

In Experiment 2, the PDP (6–10- mm) was evaluated in hyperstimulated beef cattle with 1 follicle ≥ 6 mm (Single – Follicle Group; n = 8) or all follicles ≥ 6 mm (Multi – Follicle Group; n = 7). Trans-rectal ultrasonography and blood collection were conducted to evaluate follicular dynamics and FSH and E2 concentrations. In the Single – Follicle Group, non-retained follicles were ablated upon reaching 6- mm (approximately 3.5 ablations per cow). When the largest follicle reached 10- mm in the Single – Follicle Group, that follicle was aspirated and the follicular fluid stored for future assay. When the largest follicle reached 10- mm in the Multi – Follicle Group, fluid from the 10 largest follicles (≥ 8 mm) was collected, pooled, and stored for future assay. The results of this study indicated that the presence of more than one follicle ≥ 6 mm in a hyperstimulated follicular wave gradually increased serum E2 and suppressed FSH when the largest follicle was detected to be 10- mm. Prior to the cessation of the PDP, the Multi – Follicle Group experienced a gradual increase in serum E2 and marked decrease in FSH over the Single – Follicle Group. Additionally, there was a trend towards increased total follicular fluid E2 in the Multi – Follicle Group (P = 0.076); however, there was not a direct relationship between the number of large follicles and E2 production. This may be indicative of a compensatory mechanism to produce a threshold amount of systemic E2 perhaps necessary to regulate FSH or that E2 production potential was inhibited by the sub-luteal Progesterone concentrations in the CIDR inserts used in this experiment.

In addition to providing a better understanding of the endocrine and follicular dynamics of hyperstimulation, these results indicated; (1) an apparent enhanced competition for dominance among follicles in hyperstimulated cows, (2) the largest follicles in a hyperstimulated follicular wave require less time to reach 10- mm and there was a tendency towards enhance ovulation rates (3) endogenous LH was sufficient to support one or multiple dominant follicles, and (4) endocrine deviation and follicular E2 production may be postponed in a hyperstimulated follicular wave. Future research is needed to understand the roles of individual follicles of various diameters and the impact of E2 production (in the face of varying Progesterone concentrations) on the hyperstimulatory response, capacity for oocyte fertilization, and embryo production.