Emergence and deviation of follicles during the development of follicular waves in cattle.

The nature of emergence and deviation of follicles during follicular waves in cattle was studied in 3 experiments by re-examining data from previous projects. Wave emergence was defined as the day or examination (when more than 1 examination per day) the future dominant follicle was 4 mm (Day 0 or Examination 0). Deviation was defined as the beginning of the greatest difference in growth rates between the 2 largest follicles and between 2 consecutive examinations. The search for deviation in an individual wave was done retrospectively from the examination with the maximum diameter of the second largest follicle. In Experiment 1, follicles were assessed ultrasonically for 28 waves every 8 h. The number of examinations that encompassed the emergence of all growing 3-mm follicles was 10.0 +/-0.5 (mean +/-SEM; equivalent to 3.3 d) and extended from mean Examination -3.1 +/-0.3 to mean Examination 6.0 +/-0.6. A mean of 24 growing 3-mm follicles was found, and the maximal attained diameters were 4 mm (46%), 5 mm (25%), and >/=6 mm (29%). More (P<0.05) 3-mm follicles at Examinations -2 and -1 grew to >/=6 mm than to 4 or 5 mm, whereas more 3-mm follicles at Examinations 2 to 6 grew to only 4 mm. On average, the future dominant follicle appeared as a 3-mm follicle (Examination -2.1 +/-0.2) 6 and 10 h earlier (P<0.03) than for the largest (Examination -1.4 +/-0.3) and second-largest (Examination -0.8 +/-0.4) future subordinates, respectively. This result supported the hypothesis that the future dominant follicle has, on the average, an early developmental advantage. In Experiment 2 (n=33 waves), data were normalized to the day at the beginning of deviation (Day 2.8 +/-0.2) when the mean diameters of the dominant and largest subordinate follicle were 8.5 +/-0.2 mm and 7.2 +/-0.2 mm, respectively. This result suggests that the follicle selected to become dominant, as manifested by deviation, is the first follicle to develop to a decisive stage. In Experiment 3 (n=19 waves), FSH concentrations were lower (P<0.05) on the day at the beginning of deviation (8.5 +/-0.5 ng/ml) than on the day before (10.1 +/-0.8 ng/ml), with no continuing decrease after deviation. This temporal result suggests that the attainment of approximate basal levels of FSH is a component of the deviation mechanism.