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The objective of this study was to investigate individual fixed effects in an animal model for breeding value estimation for semen traits of pig sire breeds. Data (151,755 ejaculates collected from 2000 through 2007 from 2,077 Duroc, sire line of Large White, Piétrain, and single cross boars between these breeds) were from 20 AI centers in the Czech Republic. Traits considered per ejaculate were semen volume, sperm concentration, motility, percentage of abnormal sperm, total number of sperm, and number of functional sperm. Fixed effects in the animal model were month of collection, age of the boar at collection, interval between subsequent collections, combined effect of AI center and year, and breed or crossbred combination. Semen volume was greatest from October through December and was least in March and April. Sperm concentration was greatest in winter and early spring and least in late summer and early autumn. Both total sperm number and number of functional sperm were greatest in winter and least in summer. Semen volume increased until about 2 yr of age and remained relatively constant thereafter. Sperm concentration increased sharply until 11 mo of age, followed by a long-term moderate decrease until 3 yr of age and stabilization thereafter. Motility decreased steadily with age, whereas the percentage of abnormal sperm increased over the entire productive lifetime of the boar. There were initial steep increases with advancing age in total sperm number and number of functional sperm, both reaching their maxima at about 2 yr of age and then dropping slightly to the end of the time scale investigated. The interval between subsequent collections had a large effect on sperm concentration. Motility tended to decrease and the percentage of abnormal sperm tended to increase with lengthening time interval between collections. Both total sperm number and number of functional sperm rose as the interval between collections increased to 10 d. Although boars will continue to be selected mainly for their breeding values for production and female reproduction traits, AI centers should also place economically optimal emphasis on boars with favorable estimated breeding values for semen traits.

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Artificial insemination (AI) has become an important procedure in the global pig industry. Compared with natural service, artificial insemination allows the greater use of genetically superior sires (Oh et al., 2006). Selection practices for AI boars are universally based on the genetic evaluations for economically important traits (Robinson and Buhr, 2005). For example, boars of dam breeds kept in the Czech Republic are mainly selected for average daily gain from birth till the end of the performance test, lean meat content at the end of the test and number of piglets born alive . However, no AI centre can restrict itself to the selection for production and female reproduction traits only, it must also consider factors that enhance the efficiency of the centre such as boar conformation and temperament and sperm quantity and quality. Semen volume, sperm concentration and gross sperm morphology are semen traits that affect the profitability of an AI centre (Robinson and Buhr, 2005).It has been shown that semen traits are heritable traits with heritabilities in the same order of magnitude or higher than those for litter size traits (Rothschild, 1996;Grandjot et al., 1997;Smital et al., 2005;Oh et al., 2006;Wolf, 2009). Therefore, genetic evaluation of semen traits and selection for these traits are possible. On this basis, an animal model was developed and put into practical use for the genetic evaluation of semen traits of pig dam breeds kept in the Czech Republic. The genetic evaluation is based on a large data set and includes all AI boars of both dam breeds. The objective of the present investigation is to present and discuss the effects of the individual fixed factors in the animal model for semen traits. Fixed effects were estimated from a four-trait animal model for semen volume, sperm concentration, motility and percentage of abnormal spermatozoa and from single-trait animal models for the total number of spermatozoa and the number of functional spermatozoa. Both the total number of spermatozoa and the number of functional spermatozoa were highest in winter and lowest in summer. Boar's age had a strong influence on semen volume, the total number and the functional number of spermatozoa; these traits increased especially in the first phase. The percentage of abnormal spermatozoa also increased with age. An interval between successive collections of 7 to 10 days yielded the best values for all semen traits. As semen traits are of direct economic importance for AI centres, it can be expected that the estimation of breeding value for semen traits will become important and that AI centres will choose among top boars for production and female reproduction traits the boars with better semen production.