Groups of Merino ewes which were lactating for 40 days (Group I) or had had their lambs removed at birth (Group II) after lambing in the winter (June) or spring (November) were fed on a high plane of nutrition. Ovarian inspections were carried out at 15 and 30 days after lambing and plasma LH levels were measured at 6-h intervals for 20-30 days. First ovulation was earlier in ewes lambing in the winter (16-6 days, range 11-26) than in the spring (24-7 days, range 15-30) but there was no difference in the number of ewes ovulating. LH levels were higher in winter-lambing ewes (2-79 +/- 3-4 ng/ml) than in those lambing in the spring (1-78 +/- 0-23 ng/ml). LH peaks were usually associated with an ovulation in spring lambing ewes but were not consistently so in the others. More ewes ovulated in Group II (72%) than in Group I (40%) but the mean time of first ovulation was similar. In the winter-lambing ewes the mean daily LH concentration was 2-40 +/- 0-32 ng/ml in Group I and 3-18 +/- 0-31 ng/ml in Group II but there were no differences between the spring-lambing ewes, (I, 1-75 +/- 0-20 ng/ml; II, 1-80 +/- 0-26 ng/ml). There were more elevations in LH levels in Group II ewes (64%) than in Group I ewes (43-8%). After lambing the LH levels increased slowly, indicating a gradual recovery of pituitary function.

No abstract

The release of LH from the pituitary of lactating ewes was studied. In Exp. 1, ewes were injected with 50 microng oestradiol benzoate (OB), 2-0 mg testosterone propionate (TP) or oil only (control) on days 5, 10, or 20 after lambing. LH was measured in peripheral plasma samples obtained 20-38 h after treatment, and the ovulations were recorded. The number of ewes in which an LH release was detected, and the amount released, declined between Day 5 and 20 after OB treatment but increased after TP treatment. The releases of LH were not always accompanied by ovulation and the incidence of ovulation was higher in ewes treated with TP. In Exp. 2, lactating ewes were injected with 1 or 5 (at 2-h intervals) doses of 50 microng Gn-RH, on Days 12 or 25 after lambing. LH was measured in peripheral plasma samples collected every 2 h for 10 h and every 3 h for a further 70 h. Release of LH occurred in all ewes, the amount being greater in ewes receiving multiple injections and in ewes treated on Day 25. The incidence of ovulation was higher after treatment on Day 25. Multiple injections of Gn-RH appeared to reduce the incidence of abnormal corpora lutea.