The spermiogram of 10 mature bulls, ranging in age between three and 10 years and comprising five indigenous (Bos indicus) and five exotic (Bos taurus) bulls, was studied for 12 months. There were no significant seasonal variations in sperm cell concentration, percentage live sperm cells and sperm cell abnormalities in the indigenous bulls but the exotic breed showed significant seasonal fluctuations with higher sperm cell abnormalities, lower percentage live-sperm cells and lower sperm cell concentration during the hot periods. In both groups the sperm cell concentration was significantly higher in the relatively younger, mature bulls (three to seven years) than in the older bulls (seven-and-a-half to 10 years) Histological study of the testes of 240 indigenous bulls over the same period showed that percentage spermatogenesis was lower in the older than in the younger bulls. This lower sperm output was associated with degenerative changes in the seminiferous tubules. It is suggested that thermal stress in the tropics has significant adverse effects on spermatogenesis only in bulls of non-indigenous breeds. Senile testicular degeneration could be a significant factor in the low reproductive performance of the local bulls.
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