Romney ewes with consistently high (H, n=5) or low L, n=5) seasonal changes in wool growth and with differences in ovulation rate and the onset of anoestrus, were blood sampled every 30 min for 24 h at the winter and summer solstices and at the spring and autumn equinoxes. Plasma samples were assayed for melatonin by direct radioimmunoassay. Mean melatonin levels were higher during the hours of darkness than during daylight at all sampling times. However, the ratio between day- and night-time melatonin levels tended to be smaller in spring (1.8) and summer (1.5) than autumn (2.9) and winter (1988 - 3.8; 1989 - 4.1). There were no significant differences between H and L ewes in mean melatonin concentrations during daylight or darkness at any of the seasons. There was however, considerable variation between individual ewes in the diurnal pattern of melatonin secretion in spring and summer. The high levels of melatonin usually associated with darkness were sometimes found during the day. These disrupted patterns of melatonin secretion occurred in all ewes at one or other of these two seasons. The results suggest that during anoestrus, melatonin profiles may differ from the normal pattern of secretion that is aligned to photoperiod.
Proceedings of the New Zealand Society of Animal Production, Volume 52, , 111-114, 1992
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