Effect of Melatonin Implants on Semen Quality of Assaf Rams during Long-Day Photoperiod

Melatonin is the neuroendocrine basis of photoperiodic regulation of sperm production. In short-day breeders, such as sheep, longer nocturnal melatonin synthesis improves fertility, whereas sperm quantity and quality progressively decreases through winter and spring months. The aim of this study was to assess whether melatonin implants would prevent the seminal quality loss in rams observed during the seasonal low fertility periods. Fifteen Assaf rams (Ovis aries) housed at University of León, Spain, received a melatonin implant (18 mg; Melovine®, CEVA, Spain) in February with the objective to maintain increased blood melatonin levels during 60-90 d. A total of 45 semen doses were collected via electroejaculation immediately before treatment, and at d 30 (March) and 150 (July) of the study. Ejaculates were assessed for sperm concentration, individual and progressive motility, morphological abnormalities, viability and acrosome integrity. Data were analyzed using logistic regression and linear mixed models, and expressed as % and least square means ± SE. Compared to February, sperm concentration did not differ in March following melatonin treatment (1028 ± 191 vs. 1283 ± 257 x 10⁶ mL^-1; P > 0.05) and was significantly lower in July upon exhaustion of the implants (446 ± 184 x 10⁶ mL^-1, P < 0.001). Conversely, morphological abnormalities were higher in March compared to both February and July (19.7 % vs. 5.2 % and 11.6%, respectively; P < 0.05). Sperm motility, viability and acrosome integrity did not differ between sampling periods. In conclusion, melatonin implants may be used to maintain sperm concentration in Assaf rams during the seasonal low fertility periods but may decrease sperm quality.