THE EFFECT OF SUPPLEMENTARY LONG DAYS, A SUBCUTANEOUS MELATONIN IMPLANT AND EXPOSURE TO A RAM ON THE ONSET OF THE BREEDING-SEASON IN EWES

Seasonality of breeding in sheep occurs as a consequence of exposure to photoperiodic, social and other environmental stimuli. The objective of this experiment was to determine if the onset of the breeding season in ewes could be advanced by giving supplementary long days during pregnancy, followed by a subcutaneous melatonin implant at lambing. The additional effect of exposing ewes treated in this way to a ram was also investigated. Twin bearing ewes (mean +/- SEM lambing date 7 March +/- 2 days) maintained on natural photoperiod were randomly allocated within breed to one of the following treatments: (1) control, ewes maintained on natural photoperiod throughout (n=22); (2) melatonin, ewes maintained on natural photoperiod throughout and given a subcutaneous melatonin implant at lambing (n=23); (3) long days plus melatonin, ewes were exposed to 18 h light:6 h dark for an average of 44 days before lambing, given a melatonin implant at lambing and then maintained on natural photoperiod (n=21); (4) long days plus melatonin plus ram, as Treatment 3, followed by continuous exposure to a ram from the summer solstice (n=22). Reproductive activity was assessed from serum progesterone concentrations in blood collected once weekly from I June. Control ewes began their breeding season in September (median date 21 September, range 50 days). Ewes given melatonin alone became reproductively active around the same time (median date 13 September, range 77 days; P> 0.05). Ewes exposed to long days during pregnancy followed by a melatonin implant at lambing began their breeding season earlier than controls (median date 6 September, range 93 days; P< 0.05). Exposure to a ram from the summer solstice in addition to treatment with long days and melatonin resulted in an additional advancement in the onset of the breeding season (median date 29 July, range 77 days) compared with ewes exposed to long days and given melatonin (P< 0.01). Thus, the combined treatment of long days during pregnancy followed by a melatonin implant at lambing can advance the breeding season of ewes. Additional exposure to a ram in such ewes can further advance the onset of the breeding season.