The patterns of seasonal variation in reproductive activity were observed over a period of 15 mo for approximately 15 females of each of three breeds: Finnish Landrace (Finn), Tasmanian Merino (Merino) and Scottish Blackface (Blackface). The incidence of oestrus was measured by teasing with vasectomized rams, and the incidence and rate of ovulation were determined frequently by laparoscopy. Luteal function was assessed from peripheral venous blood progesterone concentration on days 7 and 11 of the oestrous cycle. Nutritional status was monitored by recording body weight and plasma-free fatty acid levels throughout the study.The breeding seasons differed significantly: Finn, October to May; Merino, September to February; and Blackface, October to February. Variation in the incidence of ovulation was similar to that in the incidence of oestrus for each breed. The incidence of silent ovulation varied with the breed, being greatest in the Merino and least in the Finn. The ovulation rate varied among breeds (Finn, 2-99; Merino, 1-08 and Blackface, 1-30), and during the breeding season (e.g. Finn: November, 3-5; March, 2-6). Follicles were observed in each breed throughout the period of study.The pattern of variation in progesterone concentration was similar for each breed despite their different breeding seasons. In addition to breed differences in ovulation rate and in onset and end of the breeding season, the sensitivity to oestrogen was apparently such that, with the Finn if oestrogen secretion was high enough to stimulate ovulation it would usually also stimulate oestrus, whereas with the Merino ovulation often occurred without oestrus; this suggests that in the Merino the centres controlling ovulation are more sensitive to oestrogen than those controlling behaviour.

The concentrations of oestradiol, androstenedione, progesterone and prostaglandin F2alpha (PGF2alpha) were measured in utero-ovarian autotransplants. The secretion of oestradiol was closely correlated with that of androstenedione (r = 0-67, P less than 0-001) indicating a common origin from the Graafian follicle. The concentration of these two steroids fluctuated at random throughout the luteal phase with the maximum secretion occurring about 2 days before the onset of oestrus. Functional regression of the corpus luteum, as indicated by a fall in the secretion of progesterone, began on day 12 or day 13, i.e. about 4 days before the onset of oestrus. In five of the six cycles the first significant rise in the secretion of PGF2alpha occurred on days 12-14 at the time of decline of progesterone secretion, although the release of PGF2alpha was maximal on the day before the onset of oestrus. There was very little release of PGF2alpha from the uterus before day 12. The temporal relationship of these events suggests that the uterus will only release PGF2alpha after it has been primed for 7-10 days with progesterone. The initiation of luteal regression is independent of secretion of oestradiol by the pre-ovulatory follicle which may, however, stimulate the further release of PGF2alpha responsible for irreversible structural luteolysis on the day of pro-oestrus.

The discharge of LH following the injection of 50 j i g oestradiol benzoate was studied in i2 ovariectomized Blackface and 12 ovariectomized Finn ewes on 6 occasions (November, January, March, May, July and November). Both breeds were more sensitive to the oestradiol during the breeding season than during anoestrus. The Finn ewes were less sensitive than the Blackfaces as judged by the proportion of ewes discharging LH, the maximum plasma concentration of LH attained, the interval to the discharge of LH, and the absolute reduction in LH concentration immediately following injection. The failure of oestrogen to trigger the preovulatory discharge of LH may be one of the components of seasonal anoestrus and it is suggested that some of the variation in the ovulation rate of sheep may arise in part from differences in sensitivity to feedback from developing follicles.

No abstract

The new CL is formed from the granulosa cells of the preovulatory follicle. This process involves well-characterized changes in the morphology of the granulosa cells that are usually accompanied by an increase in progesterone secretion (functional luteinization). In many animals, morphological luteinization of the granulosa cells is apparent some hours before the rupture of the follicle al., 1975;