Tropical and Subtropical Agroecosystems, 11 (2009): 131-1 34 131 NOTA CORTA [SHORT NOTE] EFFECT OF MELATONIN DURING THE SEASONAL ANOESTROUS ON THE REACTIVATION OF THE SEXUAL ACTIVITY AND SEMEN PRODUCTION AT THE NORMAL BREEDING SEASON ON MEDITERRANEAN BUCKS [EFECTO DE LA MELATONINA DURANTE EL ANESTRO ESTACIONAL EN LA REACTIVACIÓN DE LA ACTIVIDAD SEXUAL Y PRODUCCIÓN SEMINAL DURANTE LA ESTACIÓN REPRODUCTIVA DE MACHOS CABRIOS DEL MEDITERRANEO] L. A. Zarazaga, M. C. Gatica 1 , J. L. Guzmán, J. P. Casas 2 Departamento de Ciencias Agroforestales, Universidad de Huelva, Carretera de Palos de la Frontera s/n, 21819 Palos de la Frontera, Huelva, Spain. 1 Departamento de Agricultura del Desierto, Universidad Arturo Prat, Av. Arturo Prat 2120, Iquique, Chile 2 Ceva Salud Animal. C/ Carabela La niña, 12, 5ª Planta, 08017 Barcelona Spain. Corresponding author: zarazaga@uhu.es . SUMMARY One experiment was conducted to determine if the onset of the reproductive activity and semen production could be modified by a previous treatment with exogenous melatonin, used to enhance reproductive activity during the seasonal anoestrous in Mediterranean bucks. Two balanced groups of bucks were used. The 18 th march 2005, one group (M group) received 3 s.c. implants of melatonin (N=7) and other group (N=4) was used like control (C group). From June to September, body weight and testosterone was measured weekly and testicular weight every 15 days. The reproductive activity of each buck was assessed using characteristics of the testosterone profile. During the first 8 days of each month, volume of ejaculate and sperm concentration was assessed. Each of these 8-d periods was divided into 3-d periods of daily sperm collection separated by 2 d of rest. The semen was collected using an artificial vagina. The effect of treatment and month was studied on each variable. An interaction, month-treatment was observed on ejaculate volume, July was the month with higher volume (0.62 ± 0.06 mL vs 1.07 ± 0.15 mL for M and C group respectively, P < 0.05). Sperm concentration was influenced by treatment (6.13 x10 9 ± 2.49 x10 8 vs 4.26 x10 9 ± 3.12 x10 8 sperm/mL, for M and C, respectively, P < 0.05). The onset of the reproductive activity, after the study of the testosterone concentrations was similar for both groups (31st August ± 7.89 days and 4th September ± 25.66 days for M and C group, respectively). Results demonstrate that melatonin treatment during the seasonal anoestrous does not influence the onset of the normal breeding season or ejaculate volume but it seems that could increases the sperm concentration at the normal breeding season in Mediterranean goat males. Keywords: Buck, Melatonin, Seasonality, Semen. INTRODUCTION The influence of photoperiod on sexual activity is greatest at high latitudes, decreasing gradually towards the equator (Hafez, 1952; Chemineau et al., 1995). Daylight length influences the secretion of the pineal gland, being responsible for transducing the photoperiodic cue into a hormonal signal (melatonin) (Hansen, 1985). Photoperiod information is conveyed to the neuro-endocrine system by the circadian secretion of melatonin from the pineal gland (Bittman et al., 1983). The profile of melatonin secretion is therefore an endocrine signal that relays photoperiodic information to the reproductive axis. As short days are characterized by a longer duration of melatonin secretion than long days, different attempts have been made to mimic the effect of short days by artificially prolonging the duration of high levels of circulating melatonin. Two mechanisms can be put forward to explain that action of melatonin implants: either they provide a short-day signal (very long duration) or they remove photoperiodic information (which is inhibitory at that time) because of the absence of a detectable rhythm. Short-day treatment can therefore be replaced by melatonin treatments so that the maintenance of constantly high levels of melatonin mimics a short day effect. Studies do not exist about the effect of exogenous melatonin used during the seasonal anoestrous on the onset of the normal breeding season in Mediterranean bucks. MATERIAL AND METHODS The study was conducted at the experimental farm of the University of Huelva (latitude 37º 15’ N). The males were distributed at random into two groups balanced according to their live weight and testicular Tropical and Subtropical Agroecosystems