Abstract
Goats indigenous to non-equatorial climates are seasonal short-day (SD) breeders, initiating reproductive activity in response to decreasing day-length in the autumn. The annual caprine breeding season is regulated by changing photoperiod throughout the year. Photoperiodic information is transduced in the form of melatonin (aMT) from the pineal gland, probably via changing duration of secretion. Summer SD and aMT treatment, to create artificial winter photoperiod conditions, induced a small advance in breeding activity: goats are not as responsive to these treatments as sheep. The sensitivity of goats to light, as assessed by suppression of plasma aMT, was high, the threshold for suppression being between 0. 26-0. 68 W/cm2 (0. 87-2. 3 lux). Treatment of goatlings with a 20 h light: 4 h dark (20L:4D) photoperiod for 2 months during the winter removed the photorefractory state imposed by prevailing SD conditions. When followed by 3 months s. c. aMT implantation a 3 month oestrous advance was induced. The goats’ breeding season was completely reversed by advancing 2 consecutive oestrous onsets in this way. 20L:4D was more effective at inducing early breeding activity than a skeleton photoperiod (lights on 0400-0900 h and 2200-2400 h) when followed by aMT treatment in goatlings. Winter 20L:4D photoperiod treatment for 50 days induced out-of-season breeding, and was more successful in multiparous than in nulliparous goats; subsequent aMT treatment increased successful conception. Multiparous goats breeding out-of-season, following light and aMT treatment, had reduced milk yields. Milk composition and yield from goatlings induced to breed out-of-season were normal when compared to naturally breeding goatlings. Melatonin was secreted into the milk of goats during the first week after treatment with a aMT intraruminal bolus. Melatonin residues did not accumulate in the tissues of goats in large amounts. Exposure to 20L:4D during the winter, followed by aMT treatment, induced the growth of an early winter coat in goats, probably via effects on PRL production. Autumn-born kids reached puberty at a much earlier age than spring-born kids (12. 8 wks vs. 25-30 wks). In utero 20L:4D treatment seasonally phase delayed puberty in autumn-born kids; puberty occurred at 16. 5 wks. Putative aMT receptors were demonstrated in the pars tuberalis, suprachiasmatic nucleus, preoptic area, medial and lateral septal areas, fornix, hippocampus and cerebral cortex of the caprine brain by in vitro autoradiography.