Summary. Thirty-two out-of-season Saanen dairy goats were involved in a 2 × 2 factorial experiment using (1) extra-light treatment (equivalent to long days) from 18 January to 19 March, at the end of gestation and the onset of lactation and (2) daily melatonin injections (equivalent to short days) from 20 March to 30 June. Hence, there were 4 groups of 8 goats each: CC (control–control), EC (extra-light–control), CM (control–melatonin) and EM (extra-light–melatonin). In each group teasers (males and androgenized females) were introduced on 30 May. Serial blood samples were taken for measurement of LH pulsatility on 30 April and 30 May. Ovarian activity was followed by plasma progesterone assay and laparoscopy. Oestrous behaviour was checked daily from 30 May to 15 July.
The number of LH pulses was not modified by the treatments (extra light or melatonin). Melatonin treatment increased the basal concentration of LH (Groups CC + EC = 0·30 ng/ml; Groups CM + EM = 0·38 ng/ml; P < 0·02) and the amplitude of pulses (Groups CC + EC = 1·24; Groups CM + EM = 2·44 ng/ml; P < 0·02); the highest amplitude was observed for females in Group EM. Melatonin-treated females ovulated earlier after introduction of teasers than did untreated goats (Groups CM + EM = 2·3 days; Groups CC + EC = 6·1 days; P < 0·001), but treatments did not modify ovulation rate (mean 2·1) or percentage of goats experiencing a short luteal phase. Plasma progesterone concentration during this short luteal phase was higher in melatonin-treated goats (Groups CM + EM = 1·61 ng/ml; Groups CC + EC = 1·05 ng/ml; P < 0·05). Ovarian cyclicity persisted longer in Group EM goats than in the other groups: at 2·5 months after introduction of teasers 75% of the females were still ovulating compared to 12% in Group CM and none in Groups CC or EC (P = 0·003). The number of goats which exhibited oestrus on two or more occasions was higher after melatonin-treatment than in controls (P = 0·013).
We conclude that exposure to long days is essential before melatonin treatment to obtain and maintain maximum sexual activity in the out-of-season dairy goat.
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