Associations between FSH concentrations and major and minor follicular waves in pregnant mares

Abstract

Follicular waves were detected in 19 pregnant mares (Days 11 to 40) by a significant increase followed by a significant decrease in diameters of follicles after removing large (≥25 mm) follicles from the data sets. The waves were defined as major (largest follicle, ≥35 mm; n=18) or minor (largest follicle, <35 mm; n=17). Six mares (32%) had 2 successive major waves beginning on mean Days 15.2 and 26.8; 6 had a solitary major wave beginning on Days 11 to 20; and 6 had only minor waves occurring at irregular intervals. The mean interval between minor waves (7.8 days) was less (P<0.05) than for major waves (11.7 days). Mean divergence in diameters of the largest and second largest follicles of a wave began 4 days after the detected emergence of consecutive major waves, and was taken as the beginning of the expression of dominance by the largest follicle. The interval from emergence to divergence was several days longer (P<0.05) for solitary major waves than for consecutive waves. Dominance was not detected for the minor waves, using mean diameters of the 2 largest follicles, but was apparent on inspection of individual wave profiles in 5 of 17 (29%) minor waves. Minor waves, compared with major waves, had larger diameter of follicles on the day of wave emergence (15.0 versus 12.1 mm), and significantly greater variation in the day of attainment of maximal diameter of largest follicle and small follicles. A mean increase in FSH was temporally associated with the emergence of both major and minor waves. In mares with minor waves, concentrations of FSH were higher, on average, over Days 11 to 40, which seemed consistent with the origin of follicular waves from larger follicles in the basal populations. The lower overall FSH levels in mares with major waves seemed at least partly due to depression of FSH levels beginning at the time of divergence between the 2 largest follicles.