Taking into account that semen quality depends on animal age and climate conditions and that oxidative stress has been
reported to be a common cause of infertility, the objective of this study was to monitor indicators of oxidative stress and
antioxidant protection during four seasonal periods in service bulls of various age to get better insight into the significance of these
factors upon evaluating service bull semen. The research was conducted over a year on 19 Simmental service bulls. Animals were
divided into two groups according to age; Group I consisted of younger bulls aged two to four yrs (n 9), and Group II was
comprised of older bulls aged five to ten yrs (n 10). Semen samples were obtained once in the middle of every seasonal period
and blood samples for biochemical analysis were collected by jugular venipuncture immediately after ejaculate collection.