Summary. Early oestrous cycles were induced in adult, maiden, 18-month-old Suffolkcross ewes, maintained from birth in natural photoperiod by the following treatments applied from mid-June: (a) subcutaneous implantation of melatonin (1 g) in Silastic packets, (b) daily, oral, melatonin administration (3 mg/ewe) at 15:30 h, (c) an artificial photoperiod of 8L:16D (lights on 07:30 h). Ovarian cycles began 5–10 weeks before those of control ewes maintained in a natural photoperiod. In contrast, the onset of ovarian cycles in ewes given s.c. implants of melatonin (1 g) in April, and a further group in May, was highly variable, and not significantly different from that of the control ewes.
Plasma melatonin profiles in sheep with implants showed a night-time rise superimposed on a constant level, which was itself within the physiological night-time range. Implant-derived melatonin declined with time but remained at or above physiological night-time levels for at least 3½ months. These results indicate that melatonin implants in June, but not in April or May, advance onset of oestrus in the non-lactating, adult ewe. The effects of melatonin implants in June on onset of ovarian cycles were indistinguishable from those of melatonin feeding or artificial short photoperiod initiated at this time of year.
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