MAJOR AND MINOR FOLLICULAR WAVES DURING THE EQUINE ESTROUS-CYCLE

Follicular waves were characterized by a mathematical method for estrous cycles occurring during the fall (n=79) and spring (n=20) in mares of primarily Quarter Horse and Appaloosa breeding. Waves were classified as major (divergence of follicles of a wave into dominant and subordinates) and minor (no divergence). The major waves were subgrouped into secondary waves (emerging during estrus or early diestrus and giving origin to a dominant anovulatory follicle or a diestrous ovulation) and primary waves (emerging during diestrus and giving origin to the primary or estrous ovulation). The primary waves originated on mean Day 7.4+/-0.5 (ovulation=Day 0) when the follicles were a mean of 12.1+/-0.5 mm. Day of emergence of primary waves was positively correlated (P<0.05) with length of interovulatory intervals The mean intervals from emergence to divergence and from divergence to ovulation were 7.4+/-0.5 and 7.2+/-0.4 days, respectively. Secondary waves and minor waves each occurred in 8 of 34 cycles and occurred more commonly (P<0.05) during the spring than during the fall. Six of the 8 secondary waves emerged before ovulation, at the time of reported FSH increases in some mares. Divergence into a dominant and subordinate follicle occurred 3 days sooner (P<0.05) after wave emergence for secondary waves than for primary waves. In addition, the maximal diameter of the dominant follicle for secondary waves was significantly smaller than for primary waves and maximal diameter was reached earlier mean maximal diameter of largest follicle: 37.2+/-1.0 and 45.8+/-1.0 mm, respectively. Values for all follicular end points, except diameter of follicles on day of wave emergence, were smaller (P<0.05) for minor waves than for primary waves. The minor and secondary waves differed only in the occurrence of divergence with formation of a large dominant follicle in the secondary waves. The length of the interovulatory interval and all examined characteristics of primary waves were not different between fall and spring estrous cycles; the greater follicular activity previously reported for the spring was attributable to more secondary and minor waves. Minor waves were sometimes located close to the day of emergence of a primary wave, indicating that abroad base of follicular activity is sometimes associated with the emergence of primary waves.