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J. P. Ravindra and N. C. Rawlings

Daily transrectal ovarian ultrasonography was performed in ten ewes for 5 consecutive days, once in early July, once in late July (anoestrus) and then continuously until from mid-August until ewes had completed one ovulatory cycle. During anoestrus the size range and numbers of ovarian antral follicles were similar to those seen during the breeding season. However, numbers of small antral follicles (2–3 mm in diameter) decreased during late anoestrus, and maximum follicle diameter increased just before the short period of progesterone secretion preceding the first observed ovulation. The ovarian antral follicles that ovulated first and second in the breeding season grew from 2 mm in diameter to 5.7 ± 0.3 mm and 6.2 ± 0.3 mm diameter over 4.7 ± 0.3 days and 4.6 ± 0.3 days, respectively, and the interovulatory interval was 16.6 ± 0.2 days. During the first ovulatory cycle, follicles emerged to grow from the 2 mm size class on 11 of the 17 days, but peaks of emergence were seen on days 2 and 11. The first observed ovulation was preceded by a transient increase in serum concentrations of progesterone (6 days duration), with a peak concentration of 1.30 ± 0.22 nmol l−1. With ultrasonography, no evidence of ovulation was seen before the increase in progesterone secretion and no luteal structure was detected during the small increase in progesterone secretion; however, luteal structures are normally detected by ultrasonography only from 3 to 5 days after ovulation. An LH surge similar to a preovulatory LH surge preceded the first increase in progesterone secretion in five ewes. Oestrus occurred consistently with ovulation only at the second observed ovulation of the breeding season, after a normal luteal phase. LH pulse frequency and mean and basal serum concentrations of LH all increased in late anoestrus, but no major trends in serum concentrations of FSH and oestradiol were seen during this period. It was concluded that at the end of anoestrus there is no major change in ovarian antral follicle dynamics. At this time, increased LH secretion was seen as was a reduction in numbers of small antral follicles and a greater maximum diameter of follicles. A surge release of LH resulted in a short-lived secretion of progesterone, the source of which was unclear; this was followed by the first observed ovulation and the first ovulatory cycle of the breeding season. Oestrus occurred consistently only at the second observed ovulation of the season and the peak concentration of progesterone at each period of progesterone secretion increased to at least the second ovulatory cycle.

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J. P. Ravindra, N. C. Rawlings, A. C. O. Evans, and G. P. Adams

Transrectal ovarian ultrasonography was performed daily in eight ewes during one interovulatory interval, using a 7.5 MHz, rigid, human prostate transducer, and a realtime B-mode scanner to record the numbers, diameters and position of all follicles ≥ 2 mm in diameter and the corpora lutea in both ovaries. Blood samples were taken once a day and were analysed for concentrations of FSH, progesterone and oestradiol. During the interovulatory interval of 17.2 ± 0.4 days, antral follicles (follicles > 2 mm in diameter) emerged on all days except for days 1, 5, 15, 16 and 17. A significant increase in the numbers of follicles emerging was seen on days 2 and 11. The ovulatory follicle (6.9 ± 0.1 mm diameter) was retrospectively traced to emergence on day 11.1 ± 0.3 and grew over a period of 4.1 ± 0.1 days at a growth rate of 1.2 ± 0.04 mm day−1. The largest nonovulatory follicles of the same period grew at the same rate as ovulatory follicles and regressed over a period of 2.6 ± 0.2 days at a rate of 1.2 ± 0.07 mm day−1. The mean diameter of the largest follicles seen on each day of the oestrous cycle was lowest on the day of ovulation (2.9 ± 0.2 mm), increased from day 3 to day 5 (4.1 ± 0.4 mm) and again from day 11 to the day before ovulation (6.9 ± 0.1 mm; P < 0.05). The mean number of antral follicles ≥2 mm in diameter increased over the oestrous cycle from 4.5 ± 0.4 on day 3 to 7.2 ± 0.7 on day 11 and showed a sharp decline starting on day 15, to a low of 3.5 ± 0.3 on the day of ovulation (day 17.2 ± 0.4). The corpus luteum could be identified by day 3 of the oestrous cycle in all the ewes, at a mean diameter of 11.5 ± 0.3 mm. The diameter increased to 13.3 ± 0.6 mm on day 5, and declined from day 11 to a diameter of 7.5 ± 0.3 mm on the day of ovulation. Apart from increases before ovulation, there were no clear associations between serum concentrations of FSH or oestradiol and the pattern of follicular growth and regression. We concluded that follicle emergence appeared on many days of the oestrous cycle of ewes, with two phases of increased emergence. There was no discernible connection between follicle emergence and FSH secretion; the overall pattern of growth and regression of follicles was not as distinctly wave-like as in cattle. Follicular dominance was noted only just before ovulation, again, in contrast to cattle.