Genetic and phenotypic variation in reproductive traits of AI boars

The primary objective of this paper is to review our current understanding of phenotypic variation in reproductive traits of AI boars. The proportion of boars that cannot be trained for collection in commercial studs is low and differences among genetic lines are small. In contrast, there is a considerable variation in sperm produciton and significant differences are present among gtenotypes. The general pattern is for sperm numbers to increase rapidlly betwenn 9 and 13 months of age and then gradually reach a plateau. This initial period of enhanced produciton occurs over a longer period in some genetic line,s resulting in differences of 30 x 10(9) sperm cells or more per ejaculate. There also are genetic lines of boars that seem tohave a high "heat tolerance". Decreases in sperm produciton during periods of high enviruomental temperatures averate 5-7% in these lines, compared with 15-20% in others. Finally, there are boars currently being used in the industry that are capable of producing exceptional fertility results with low numbers of sperm. Unfortunately, several breeding practices common to swine AI make their routine identification difficult. Based on the phenotypic variation observed in modern terminal sire lines of AI boars, current prospects for influencing sperm production, boar fertility, and mounting behaviours through genetic selection are viewed as being good, moderate to low, and poor, respectively. (C) 2008 Elsevier Inc. All rights reserved.