Aspiration of the dominant follicle in mares and its effect on subsequent follicular development

An attempt was made to increase the number of small- and medium-sized follicles present for ultrasound-guided oocyte recovery by aspirating the dominant follicle prior to its inhibition of growth of other follicles. Cyclic mares were randomly assigned to one of four groups on day of ovulation. Each mare was examined daily with ultrasound beginning 8 days after ovulation to determine growth and regression of individual follicles at least 15 mm in diameter. Once a dominant follicle was identified, the dominant follicle in Group 1 mares was aspirated and all other follicles (at least 15 mm) aspirated 3 days later; the dominant follicle in Group 2 mares was identified but not aspirated and all follicles (at least 15 mm) aspirated 3 days later; the dominant follicle in Group 3 mares was aspirated and all other follicles (at least 15 mm) aspirated 6 days later; and the dominant follicle in Group 4 mares was identified but not aspirated and all follicles (at least 15 mm) aspirated 6 days later. A dominant follicle was defined as the largest follicle present for 2 consecutive days beginning 8 days after ovulation. Identification did not occur until the dominant follicle was at least 25 mm in diameter. The dominant follicle was identified 15.1 ± 3.4 days after ovulation. Mean number of small (3–17 mm) and medium (18–27 mm) sized follicles 3 or 6 days after identification/aspiration of the dominant follicle were similar among all treatment groups (P > 0.05). Mares in Group 2 had a greater (P < 0.05) number of large follicles (at least 28 mm) 3 days after identification of the dominant follicle than Group 1 mares (1.3 vs. 0.5, respectively); means for Groups 3 and 4 were similar. Based on linear regression slopes of growth patterns for individual follicles, a higher rate of atresia occurred in subordinate follicles within 6 days following identification, but no aspiration of the dominant follicle (−0.63 mm day−1), than for those follicles present following aspiration of the dominant follicle (−0.10 mm day−1, P < 0.05). A greater percentage of follicles were growing (+ slope) after aspiration of the dominant follicle (47137 or 34.3%) than those follicles present without aspiration of the dominant follicle (15133 or 11.3%, P < 0.05). In summary, aspiration of the dominant follicle did not increase the number of small- or medium-sized follicles 3 or 6 days later. However, a smaller proportion of follicles underwent atresia following aspiration of the dominant follicle compared with those follicles present with the dominant follicle still intact. Minimizing the number of subordinate follicles undergoing atresia may be a benefit for recovering viable oocytes from these follicles.