Quantification of factors affecting semen traits in artificial insemination boars from animal model analyses.

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.