The aim of this study was to analyze the main influences on the quality and quantity of boar semen. A total of 230,705 records of semen collections were utilised to estimate statistics of semen traits of 2712 boars belonging to the following breeds: Czech Meat Pig, Duroc, Hampshire, Landrace, Large White, Czech Large White and Pietrain, and various crosses of these breeds. The evaluation was based on semen volume (VO), concentration of spermatozoa (CO), progressive motion of spermatozoa (MO), abnormal spermatozoa (AB), total number of spermatozoa (NO(T)) and corrected number of spermatozoa (NO(C)). The breeds differed significantly for all examined traits. The maximal differences between the breeds were 95 ml for VO, 109 x 10(3)mm(-3) for CO, 9% for MO, 1.6% for AB, 24 x 10(9) for NO(T) and 19 x 10(9) for NO(C). The maximal heterosis effect reached 12% for VO, 17% for CO, 4% for MO, -14% for AB, and 8% for NO(T) and NO(C.) The results demonstrate that the year-season effect has a clear effect on semen quality. The lowest values of semen traits were observed in summer while the highest values were found in autumn and winter. Age of boar was found to have a strong impact on sperm output. Sperm output tended to increase up to a boars' age of 3.5 years. An acceptable level of semen volume occurred after a sexual pause of 3 days and the pool of spermatozoa was restoring after 5-7 days and fully after 10-11 days.