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Nine ewes of each of two breeds, Dorset Horn (long breeding season) and Welsh Mountain (short breeding season), were ovariectomized after insertion of subcutaneous implants containing oestradiol-17 beta. A further 9 ewes of each breed were left entire. All of the ewes were placed in an artificial photoperiod of 8L : 16D on 12 December 1980. After 5 weeks half of the ewes of each breed and physiological state were abruptly changed into a long-day photoperiod (16L :8D) while the rest remained in short days. The time of onset of seasonal anoestrus in entire animals was significantly advanced in ewes changed to long days, with the end of the breeding season coming at the normal time of year in ewes maintained in short days. These differences in oestrous cycle activity were reflected by differences in the time at which LH concentrations in ovariectomized, oestrogen-treated ewes on the two light treatments fell to basal values. Prolactin concentrations showed an immediate and sustained rise in ewes changed to long days, but remained low in ewes maintained in short days. Since the onset of seasonal anoestrus occurred in the absence of high levels of prolactin (in short-day ewes), it is concluded that prolactin is not the major vehicle by which seasonal changes in hypothalamic responsiveness to the negative feedback effects of oestradiol are produced. The results suggest that anoestrus may be due to photorefractoriness.

Evidence that the onset of seasonal anoestrus in the ewe may be independent of increasing prolactin concentrations and daylength