Ovarian weight has been shown to increase markedly in the newborn ewe lamb, and factors which might contribute to this growth were examined. Follicle development was studied in the ovaries of 28 lambs aged 0, 2, 4, 6, 8 and 10 weeks. Plasma samples, pituitaries and one ovary were assayed for gonadotrophin or steroid hormone content.No significant differences between age groups were found in pituitary LH concentration or mean plasma LH. Pituitary FSH concentration increased with age, except for a small decrease at 8 weeks, but no significant changes were seen in plasma FSH. Ovarian progesterone and oestradiol concentrations did not appear to be associated with follicular development.Growing follicles were most numerous at 2 weeks whilst total vesicular follicles reached a peak at 4 weeks. Most vesicular follicles were less than 1 ·4 mm in diameter. Advanced atresia in vesicular follicles became apparent by 4 weeks although early signs were present in younger lambs. Fluctuations in gonadotrophin levels do not appear to be responsible for variations in number and size of vesicular and growing follicles in the lamb ovary. Other possible explanations are discussed.

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The numbers of primordial follicles in ovaries of ewe lambs, from two groups of Peppin Merinos that had been selected for (T group) and against (0 group) mUltiple births, were estimated at birth, 7 days and 5 months of age. Peripheral plasma and anterior pituitary levels of LH and FSH were determined by radioimmunoassay in the lambs at 5 months of age. These lambs were either entire or unilaterally ovariectomized (hemispayed) at 7 days of age.There was no difference in primordial follicle numbers between lambs at birth and 7 days of age, but when paired ovaries of hemispayed lambs at 7 days and 5 months of age were compared there was a significant increase in follicle numbers with age (34903 v. 48047). O-group lambs had more primordial follicles than T-group lambs at birth and 7 days of age (65501 v. 37797) and there was a similar but not significant difference at 5 months of age (53934 v. 44057).Peripheral plasma LH was higher in T-group than in O-group lambs and plasma LH levels were correlated to pituitary LH content (r = o· 75). Vesicular follicles larger than 2 mm in diameter were present only in ovaries of lambs with plasma LH levels above 3· 0 ng/m!. Pituitary FSH levels were considerably higher in hemispayed lambs than in entire lambs and there was a trend to higher levels in T-group lambs when compared with O-group lambs.It is concluded that genetic selection for the incidence of multiple births has resulted in changes in the number of ovarian primordial follicles of the post-natal lamb and it is suggested that these changes may be related to the levels of pituitary gonadotrophins.

Plasma luteinizing hormone (LH) concentrations were measured in normal Merino ewes and in Merino ewes with lowered fertility which had resulted from prolonged grazing of Trifolium subterraneum L. cv. Dinninup. During the anoestrous season, LH was measured at frequent intervals before and following administration of oestradiol-17 P or gonadotrophin releasing hormone (GnRH), and around the time of spontaneous oestrus.All ewes responded to GnRH treatment and there were no differences between the two groups in terms of the amount of LH released or the time to reach maximal plasma hormone concentration.Comparable proportions of ewes from both groups responded to oestradiol-17 p treatment with LH levels showing initially a negative feedback response (i.e. suppression to approximately 80 % of pretreatment levels) followed by positive feedback response. During the positive feedback phase, the quantity of LH which was released was comparable in both groups; however, the time interval over which this hormone was secreted was significantly shorter for clover-infertile ewes (P < O' 05).During the anoestrous season the mean LH level was lower in normal ewes than in clover-infertile ewes [0·45±0·41 (n = 31) versus 0·82±0·73 (n = 48) ng/ml, P < 0'01]. In the oestrous season the mean LH level for normal ewes rose to 1·23 ± O· 65 ng/ml (n = 76) but there was no change for the clover-infertile ewes (0·77 ± O· 59 ng/ml, n = 73). These experiments indicate that in the clover-infertile ewe LH concentration remains static throughout the breeding and non-breeding seasons but the results suggest that this altered endocrine status is not due to any differing level of sensitivity to negative feedback effects of oestradiol-17 p as compared with normal ewes.

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