RESUMEN
An attempt has been made to define semen seasonality in a horse in the Southern Hemisphere. Repeated measurements of three variables in the semen were made for 36 months (Jan/90-Dec/92) in a 21-year old "Mangalarga" stallion living under natural photoperiod and temperature conditions in a farm situated in São José do Rio Pardo, São Paulo, Brazil (latitude 21 degrees) 36'S; longitude 46 degrees 53' W). The horse fed on natural pasture and a nutritionally balanced feed twice a day (11:00 and 17:00 h). Water and mineral supplement were available ad libitum. Semen was collected almost daily by an artificial vagina and showed annual changes in volume of ejaculate, sperm motility, and sperm concentration. Spectral and rhythmometric analyses of the semen data were performed according to the fast Fourier transform (FFT) and cosinor techniques (tau = 365 days). Statistically significant annual rhythms for semen volume, motility, and concentration of sperm were present. Acrophases occurred in the spring (October) for ejaculate volume and in autumn (May) for sperm concentration and motility.
