The effect of melatonin treatment in rams on seasonal variation of testicular size and semen production parameters

The administration of exogenous melatonin can mimic the stimulatory effect of short days in advancing the onset of maximum reproductive parameters in seasonal rams. However, the animals need to be previously exposed to natural or artificial long days before responding to the melatonin treatment. The objective of this study was to test the effect of melatonin treatment on rams, initiated in mid May (end of spring) - when rams have already been exposed to some months of increasing day length, but not to artificial long day treatment on testicular size and semen characteristics. Sixteen rams (8 Texel and 8 Charollais) were treated with melatonin subcutaneous implants (REGULIN®) on 22 May and 20 June (spring/summer) (n = 8) or remained untreated (n = 8). Blood samples were taken from all rams over a 24-h period to determine the plasma melatonin concentration profile. The ram's testes were all measured four times at 2-week intervals, and testicular volume and scrotal circumference determined. Semen was collected using the artificial vagina at two occasions (end of June and end of August), when 5 ejaculates were obtained from each ram and analysed for semen volume and sperm concentration. The plasma melatonin levels in the melatonin-treated rams were significantly higher than for the untreated rams (P < 0.01). During the dark phase of the day, the plasma melatonin concentrations were elevated (P < 0.01) in the control, but not in the melatonin-treated rams. Scrotal circumference and testicular volume increased (P < 0.001) from May to July (late spring to mid-summer) in both groups of rams, with no significant difference being detected between groups. Semen volume and sperm concentration increased (P < 0.01) from June (summer) to July/September (summer/autumn) in both treatment groups. Melatonin treatment of the rams did not affect the semen volume or sperm concentration during any of the collection periods. Data suggest that the treatment of rams with melatonin implants initiated in late spring, without a priming period of long days preceding the treatment, is not sufficient stimulus to increase testis growth and sperm production.