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The influence of season of lambing and lactation on reproductive activity and plasma LH concentrations in Merino ewes

Summary. 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.