| During the last half of the 20th century, the world pork industry has achieved astonishing developments in pig breeding. Now swine farms are larger, ownership more concentrated, and farms have become more industrialized. Artificial insemination (AI) plays an important role in animal breeding increasing utilization of genetically superior sires. Currently boars selected for commercial use as AI sires are evaluated on grow-finish performance and carcass characteristics. The objectives of this study were to (A) estimate genetic correlations between production and semen traits in the boar; average daily gain (ADG), back fat thickness (BF) and muscle depth (MD) as production traits, and total sperm cells (TSC), total concentration (TC), volume collected (SV), number of extended doses (ND), and acceptance rate of ejaculates (AR) as semen traits; (B) to model the variances and covariances of total sperm cells (×109) over the active lifetime of AI boars; and (C) to compare multiple traits and random regression analyses applied to total sperm cells (TSC). Average heritability estimates were 0.39 for ADG, 0.32 for BF, 0.15 for MD, and repeatability estimates were 0.38 for SV, 0.37 for TSC, 0.09 for TC, 0.39 for ND, and 0.16 for AR. Semen traits showed negative genetic correlations with MD. Genetic correlations would indicate that current selection objectives are having a negative effect on semen traits. Therefore, current AI boar selection practices may be having a detrimental effect on semen production. In random regression analysis for total sperm cells, maximum log likelihood value was observed for sixth, fifth, and seventh order polynomials for fixed, additive genetic and permanent environmental effects, respectively. Best fit as determined by Akaike's Information Criterion was based on a model with sixth, fourth, and seventh order polynomials for fixed, additive genetic and permanent environmental effects, respectively. Best fit as determined by Schwarz Criterion was by fitting fourth, second, and seventh order polynomials for fixed, additive genetic and permanent environmental effects, respectively. Heritability estimates for total sperm cells ranged from 0.27 to 0.61 across age of boar classifications. Heritability for total sperm cells tended to increase with age of boar classification. The cyclic nature of heritability for total sperm cells that was observed over the active lifetime of boars may be due in part to number of observations across seasons limiting our ability to correct for seasonal effects on sperm production. In MTDFREML analysis, heritability estimates of 9, 12, 15, 18, 21, 24, and 27 months of age were, respectively, 0.28, 0.29, 0.26, 0.27, 0.30, 0.79, and 0.41. The results from MTDFREML seemed to be overestimated when compared to random regression. Therefore, random regression methods are the most appropriate to analyze semen traits as they are longitudinal data measured over the boars lifetime. |
