Ovarian function in ewes during the transition from breeding season to anoestrus

Transrectal ovarian ultrasonography was conducted in six Western white-face d ewes for 35 days from the last oestrus of the breeding season, to record the number and size of all ovarian follicles greater than or equal to 3 mm in diameter and luteal structures. Blood samples were collected once a day for estimation of serum concentrations of follicle-stimulating hormone (FSH ), oestradiol and progesterone. Each ewe had five follicular waves (follicl es growing from 3 to greater than or equal to 5 mm in diameter) over the sc anning period. The duration of the growth phase of the largest ovarian foll icles did not differ (P > 0.05) between waves, but follicular static and re gressing phases decreased significantly (P < 0.05) after the decline in ser um progesterone concentrations at the end of the last luteal phase of the b reeding season. The intervals between the five follicular waves were: 9.2 /- 0.4, 5.2 +/- 0.7, 8.3 +/- 0.8 and 5.8 +/- 0.7 days; the two shorter inte rvals differed (P < 0.05) from the two longer intervals. Using the cycle-de tection program, rhythmic increases in serum FSH concentrations were detect ed in all ewes; the amplitude, duration and periodicity of FSH fluctuations did not vary (P > 0.05) throughout the period of study. The number of iden tified FSH peaks (7.8 +/- 0.5 peaks per ewe, per scanning period) was great er (P < 0.05) than the number of emerging follicular waves. Serum concentra tions of oestradiol remained low (less than or equal to 1 pg/ml) on most da ys, in five out of the six ewes studied, and sporadic elevations in oestrad iol secretion above the non-detectable level were not associated with the e mergence of follicular waves. The ovulation rate was lower than that seen d uring the middle portion of the breeding season (November-December) in whit e-faced ewes but the transitional ewes had larger corpora lutea (CL). Maxim al serum concentrations of progesterone appeared to be lower and the platea u phase of progesterone secretion appeared to be shorter during the last lu teal phase of the ovulatory season in comparison to the mid-breeding season of Western white-faced ewes. During the transition into anoestrus in ewes, the endogenous rhythm of FSH release is remarkably robust but the pattern of emergence of sequential follicular waves is dissociated from FSH and oes tradiol secretion. Luteal progesterone secretion is suppressed because of f ewer ovulations and diminished total luteal volume, but it may also result from diminished gonadotropic support. These season-related alterations in t he normal pattern of ovine ovarian cycles appear to be due to reduction in ovarian responsiveness to gonadotropins and/or attenuation in secretion of luteinizing hormone (LH) occurring at the onset of the anovulatory season i n ewes. (C) 1999 Elsevier Science B.V. All rights reserved.