Ovarian antral follicular dynamics and their relationships with endocrine variables throughout the oestrous cycle in breeds of sheep differing in prolificacy

Transrectal ultrasonography of ovaries was performed each day in non-prolif ic Western white-faced (n = 12) and prolific Finn ewes (n = 7), during one oestrous cycle in the middle portion of the breeding season (October-Decemb er), to record the number and size of all follicles greater than or equal t o 3 mm in diameter. Blood samples collected once a day were analysed by rad ioimmunoassay for concentrations of LH, FSH and oestradiol. A cycle-detecti on computer program was used to identify transient increases in concentrati ons of FSH and oestradiol in individual ewes. Follicular and hormonal data were then analysed for associations between different stages of the lifespa n of the largest follicles of follicular waves, and detected fluctuations i n serum concentrations of FSH and oestradiol. A follicular wave was defined as a follicle or a group of follicles that began to grow from 3 to greater than or equal to 5 mm in diameter within a 48 h period. An average of four follicular waves per ewe emerged during the interovulatory interval in bot h breeds of sheep studied. The last follicular wave of the oestrous cycle c ontained ovulatory follicles in all ewes, and the penultimate wave containe d ovulatory follicles in 10% of white-faced ewes but in 57% of Finn ewes. T ransient increases in serum concentrations of FSH were detected in all anim als and concentrations reached peak values on days that approximated to fol licle wave emergence. Follicular wave emergence was associated with the ons et of transient increases in serum concentrations of oestradiol, and the en d of the growth phase of the largest follicles (greater than or equal to 5 mm in diameter) was associated with peak serum concentrations of oestradiol . Serum FSH concentrations were higher in Finn than in Western white-faced ewes during the follicular phase of the cycle (P < 0.05). There were no sig nificant differences in serum concentrations of LH between Western white-fa ced and Finn ewes (P > 0.05). Mean serum concentrations of oestradiol were higher in Finn compared with Western white-faced ewes (P < 0.01). It was co ncluded that follicular waves (follicles growing from 3 to greater than or equal to 5 mm in diameter) occurred in both prolific and non-prolific genot ypes of ewes and were closely associated with increased secretion of FSH an d oestradiol. The increased ovulation rate in prolific Finn ewes appeared t o be due primarily to an extended period of ovulatory follicle recruitment.