Effects of maintaining solstice light and temperature on reproductive activity, coat growth, plasma prolactin and melatonin in goats

Interactive effects of light and temperature on aspects of seasonality were studied in female British Saanen dairy goats. Four groups of adult non-pre gnant non-lactating goats (n = 5) were housed under the following condition s: controls (July-June): natural photoperiod and temperature; group 1 (July -December): long days (16 h light: 8 h dark) and natural temperature; group 2 (July-December): long days and average summer temperature (17.6 degrees C); group 3 (December-June): short days (8 h light: 16 h dark) and winter t emperature (8.4 degrees C). Plasma prolactin and progesterone were measured once a week, circadian changes in prolactin and melatonin were determined in December and May, and coat development was assessed. Seasonal variation in prolactin was influenced by manipulation of both daylength and temperatu re. In group 1, prolactin concentrations decreased as the environmental tem perature decreased, despite maintenance of long days. When light and temper ature were maintained under summer (group 2) and winter (group 3) condition s, prolactin remained relatively constant, although at different high and l ow set points, respectively, but with indications of a seasonal rhythm. An asymptotic relationship between prolactin and temperature was maintained un der all daylengths. The circadian pattern of melatonin was related to dayle ngth and was not influenced significantly by temperature. Onset of oestrus was unaltered. In group 3 (maintained winter solstice light and temperature ), anoestrus was delayed (P < 0.05) from a median control date of 17 March to a median date of 28 April. Winter coat development was delayed in group 1; group 2 showed premature moulting of the winter coat; and in group 3, de velopment of the summer coat was delayed. The results imply that temperatur e modifies the influence of daylength on prolactin secretion and hair folli cle growth by mechanisms that do not involve melatonin.