Temporal relationships between minor, preovulatory, or periovulatory FSH surges and the emergence and development of 2-mm follicles of wave 1 in Bos taurus heifers

An FSH booster surge for resurgence of the preovulatory follicle in heifers

Emergence of wave 1 during an interovulatory interval (IOI) in heifers is stimulated by FSH surge 1. A minor FSH surge with absence of a dominant follicle in the associated follicular wave occurs immediately before FSH surge 1. This minor surge is termed an FSH booster surge owing to its occurrence temporally during a resurgence of a preovulatory follicle that has been decreasing or lagging in growth rate for several days. The beginning nadir, peak, and ending nadir of the FSH booster surge occur at means of 7, 4, and 3 d before ovulation. The beginning nadir occurs at the beginning of a decrease in growth rate of the preovulatory follicle, and the peak occurs at the beginning of resurgence. The frequency of an FSH booster surge in 1 study was 10/17 (59%) and 4/18 (22%) in 2-wave and 3-wave IOI, respectively. The presence versus absence of a booster surge in 3-wave IOI is associated with a slower growth rate of the preovulatory follicle and a longer IOI. Most 3-wave IOI have rapid growth rate of the preovulatory follicle and do not have an FSH booster surge. Estradiol is a known FSH suppressor and begins to decrease at the beginning of the FSH booster surge and the beginning of a lag in growth rate of the preovulatory follicle. Concentration of LH increases significantly by the day of the peak of the booster surge. However, LH increases during waves with and without resurgence of the preovulatory follicle, whereas the FSH booster surge develops only in waves with resurgence in both 2-wave and 3-wave IOI. The beginning nadir of the booster surge is accompanied sometimes (eg, 6 of 10 surges) by the emergence of a minor follicular wave which does not develop a dominant follicle. Emergence of the booster wave and beginning FSH nadir occur earlier than the first progressive increase in LH indicating that the minor wave is attributable to the FSH booster surge. Based on temporality, the booster surge is used to stimulate recruitment or emergence of some of the small antral follicles (eg, 1 and 2 mm) that will become part of wave 1 of the next IOI. The primary function of the FSH booster surge as indicated by close temporality is to stimulate resurgence of a preovulatory follicle that has been lagging in growth rate. It is not known whether the booster surge and resurgence of the preovulatory follicle are essential for complete and normal maturation of the follicle and oocyte and for ovulation.