Individual fixed effects in an animal model for breeding value estimation for semen traits of pig sire breeds were investigated. Data (151,755 ejaculates from 2,077 boars) were from 20 AI centers in the Czech Republic. Semen volume was highest from October through December and was lowest in March and April. Sperm concentration was highest in winter and lowest in late summer. Both total sperm number and number of functional sperm were highest in winter and lowest in summer. Semen volume increased until about two years of age and sperm concentration increased sharply until 11 months of age. There were initial steep increases with advancing age in both total sperm number and number of functional sperm, both reaching their maxima at about two years of age.The interval between subsequent collections had a large effect on sperm concentration. Both total sperm number and number of functional sperm rose as the interval between collections increased to 10 days.