The effect of melatonin implants on semen quality in fresh and frozen/thawed semen, Scrotal circumference and plasma testosterone levels has been studied During two consecutive years in the Patagonia region of Argentina (43ºS). Sixteen Dohne Merino rams (experiment 1) and 12 Dohne Merino and Merino rams (experiment 2) were used from September to December 2011 and 2012, respectively. Half of the rams were treated with melatonin implants (M groups), the remaining rams being the non-treated, control group (C groups). Differences between groups (P<0.01) for scrotal circumference were observed from 30 days after melatonin implantations up to the end of experiment 1, M rams presenting a higher scrotal circumference than animals in the C group. In the second experiment, similar differences between groups (P<0.01) were observed from day 45 to day 75. No differences between groups were detected for any of seminal parameters under study (volume, sperm concentration, mass motility and percentage of live sperm) and plasma testosterone levels. Semen quality of frozen/thawed semen was similar between treated and non-treated rams (rectilinear progressive individual motility, percentage of live spermatozoa, and membrane permeability test). No significant differences for the parameter derived (mitochondrial functionality) and the chlortetracycline (acrosomal state) tests were observed. In conclusion, the use of melatonin implants during the seasonal anoestrous at 43ºS provokes a significant increment of the scrotal circumference of rams without changing seminal quality parameters of fresh and frozen/ thawed semen.

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