Effects of photoperiod and slow-release preparations of bromocryptine and melatonin on reproductive activity and prolactin secretion in female goats.

The present study was conducted to investigate seasonal reproductive patterns in a Boer Goat x German Fawn cross and ways to modulate these patterns by injecting slow-release bromocryptine and melatonin preparations. Twenty does were exposed to a light program of alternating periods of 90 long days (LD) followed by 100 short days (SD) twice in succession (LD1, SD1, LD2, SD2). At the beginning of LD2, does were treated as follows: vehicle injection (CTR, n = 7), slow-release bromocryptine (BCR, n = 6), or slow-release melatonin (MLT, n = 7). Blood samples were collected twice weekly, and from October 5 to December 14, 1989, weekly, to be analyzed for progesterone, prolactin, and melatonin. Long days inhibited cyclicity, and short days stimulated it. Prolactin was increased during long days and decreased during short days. High ambient temperature resulted in increased prolactin concentrations. Under long photoperiod, long-acting BCR suppressed prolactin secretion for 68 d, and the estrous season was extended compared with CTR. Under long photoperiod, long-acting MLT increased serum melatonin for 96 d, suppressed prolactin concentration, and substantially lengthened the period of cyclicity compared with the CTR- and BCR-treated groups. The resumption of cyclicity during the subsequent short day period occurred later in MLT- than in CTR- and BCR-treated groups. In conclusion, we modulated the day length-controlled seasonal cyclicity of goats by treating them with injectable slow-release preparations of BCR and MLT, the latter being more effective.