Abstract
The onset of reproductive activity in Poll Dorset and North Country Cheviot ewes, kept in local (51 ° 43′N) and in equatorial (13L:11D) light was assessed following lambing in the autumn (n = 10) and in the spring (n = 12). The relationship between the levels of plasma and milk prolactin and the duration of post-partum anoestrus was also investigated. A clear seasonal variation in the duration of post-partum anoestrus and in the incidence of oestrus was observed in the local light environment. Ewes lambing in the autumn showed earlier and more ovarian and behavioural activity than those lambing in the spring. The mean prolactin levels in plasma and milk of ewes lambing in the local light, were significantly higher (P < 0·001) in the spring than in the autumn. In contrast, under equatorial light, prolactin levels did not show a significant seasonal change.
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