Pituitary responsiveness to LH-RH, the occurrence of oestradiol-17β-induced LH-positive feedback and the resumption of oestrous cycles in ewes post partum

The relationship between pituitary responsiveness to exogenous LH-RH, return of oestradiol-17β-induced positive-feedback effect on LH release and the resumption of oestrous cycles were studied in Merino ewes lambing in the breeding season. The mean (± s.e.m.) increase in plasma LH 2 h after 50 μg LH-RH i.v. was reduced (P<0.001) in ewes tested in the last 4 weeks of pregnancy (8.6 ± 2.1 ng/ml, N = 29) but was similar (P<0.05) for non-parturient dioestrous ewes (N = 31) and for anoestrous ewes tested around 14 (N = 50) and 28 (N = 51) days post partum (30 ± 3.9, 37.8 ± 4.0, 39.0 ± 3.2 ng/ml, respectively). Oestradiol-positive feedback was absent in 68.6% (35/51) and 54% (27/50) of ewes tested around 14 and 28 days post partum, respectively. Neither LH-RH responsiveness nor the presence of oestradiol-positive feedback was related to the incidence of (61.9%, 166/268) or interval to (62 ± 2.2 days, N = 99) first oestrus post partum. We suggest that these endocrine responses do not represent factors limiting the resumption of ovarian cyclicity post partum.