Ovarian follicular dynamics in Suffolk ewes at different periods of the year

Five Suffolk ewes underwent daily laparoscopy for 18 days at the onset, middle and end of the breeding season (August, November and February, respectively) and during anoestrus (May) to examine their ovaries. Ovarian follicles > or = 2 mm were measured and their relative locations reported on an ovarian map to follow the sequential development of each individual follicle. Some aspects of follicular growth (mean day of emergence and mean number of small, medium-sized and large follicles, maximum size before regression and ovulation rate) were also studied and compared. Three waves of follicular growth and atresia were observed during the oestrous cycle; two waves occurred during the luteal phase and one during the follicular phase. This cyclic pattern of growth and atresia was not influenced by the stage of the breeding season. A similar turnover of follicles occurred during anoestrus. Features of follicular growth were similar at all stages studied except for the size of the large follicles which were smaller during the two waves of the luteal phase (P < 0.05). The number of growing follicles did not vary and the mean ovulation rate was 1.2 +/- 0.4 at all stages of the breeding season.