Abstract :
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
