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

• Follicle-stimulating hormone surges can be delineated by examining FSH concentration every 12 h.
• Determination of temporal relationships between FSH surges and follicle emergence.
• Emerging 2- and 3-mm follicles were detected during the minor FSH surge.
• Follicles that were 5- or 6-mm were detected only during the periovulatory surge.

The number and day of emergence (first detection) of 2-mm follicles and the number and day when the 2-mm follicles reached 3-, 4-, 5-, and 6-mm during wave 1 were determined every 0.5 d (n = 9 heifers). Emergence of the follicles at each of the indicated diameters was normalized to the beginning and ending nadir and the peak of each of a minor FSH surge, the preovulatory surge, and the periovulatory surge. Relative to the day of ovulation (day 0), the minor FSH surge, preovulatory surge, and periovulatory surge encompassed (nadir to nadir) days −7.0 to −2.5 (peak, day −4.0), days −2.5 to −0.5 (peak, day −1.0), and days −0.5 to 4 (peak, day 0), respectively. Distinct mean nadirs occurred between the minor and preovulatory surges and between the preovulatory and periovulatory surges. A small percentage of 2-mm follicles (12%) and 3-mm follicles (2%) emerged during the minor FSH surge. The 4-mm follicles emerged during the preovulatory surge (24% of follicles) and periovulatory surge (76%). The 5-mm and 6-mm follicles emerged only during the periovulatory surge. The first increase (P < 0.05) in number of 2-, 3-, and 4-mm follicles began at 1.5, 1.0, and 0 d, respectively, before the nadir at the beginning of the preovulatory surge. The first increase (P < 0.05) in number of 5- and 6-mm follicles began at 0.5 and 0 d, respectively, before the intervening nadir between the preovulatory and periovulatory surges. Results demonstrated that each of the 3 surges including the minor surge contributed to the emergence of follicles at various diameters during wave 1. The emergence of 2-mm follicles during the descending portion of the minor surge indicated that smaller follicles (eg, 1 mm) apparently emerged during the major portion of the minor surge. The increasing diameter of the 2 largest follicles was not interrupted during the distinct intervening nadir between the preovulatory and periovulatory FSH surges.