Extended day length in late winter/early spring, with a return to natural day length of shorter duration, increased plasma testosterone and sexual performance in rams with or without melatonin implants.

Abstract

Sixteen rams were used to quantify the effects of long days, imposed during late winter/early spring, with or without exogenous melatonin, on plasma testosterone concentrations and ram serving capacity. Rams were assigned to two groups: photoperiod-treated rams (Artificial Photoperiod, AP; n=8), exposed to 2months of long days (16hr of light/day) between 22 December and 22 February, and control rams (Natural Photoperiod, NP; n=8). At the end of the long-day period, AP rams were returned to the natural photoperiod, and each ram in the two groups either did (+M) or did not (-M) receive three subcutaneous melatonin implants. Four groups were created as follows: AP+M (n=4), AP-M (n=4), NP+M (n=4) and NP-M (n=4). Thirty days after of the onset of photoperiodic treatment, AP rams (13.5±2.8ng/ml) had significantly (p<.05) lower testosterone levels than NP rams (36.7±1.0), and similar differences were not apparent at the end of the photoperiod treatment. A month later, AP rams (24.3±7.9) had higher (p<.10) testosterone levels than NP rams (13.1±5.0), with no effect of melatonin treatment. Fifty days after melatonin implantations, rams were exposed for 20min to three oestrous ewes. AP rams (2.50±0.42) exhibited significantly (p<.05) more serves than did NP rams (1.11±0.39), and melatonin treatment had no significant effect; however, the interaction between treatments was significant. Time to first serve was significantly (p<.05) shorter in AP (2.30±1.20min) than it was in NP rams (5.58±0.68min). In conclusion, exposure to 2months of long days in late winter/early spring, with a return to natural day length of shorter duration, increased plasma testosterone concentrations and sexual performance in rams with or without exogenous melatonin. This particular management is an option if a non-hormonal reproductive strategy is scheduled; yet, if the use of exogenous hormones is feasible, melatonin implants increase the mating efficiency of rams.