Inbreeding and Swine Productivity Traits

Various multiple regression models were used primarily to examine the regression of swine litter traits from farrowing through 154 days of age on differences in inbreeding and in age of dam. The records included were for 7,075 litters from eight cooperating agricultural experiment stations in the Regional Swine Breeding Laboratory. Only first parity litters, from gilts aged 10 to 19 mo., were studied. An initial study indicated that the inbreeding of the sire of the litter had little or no effect on litter size at farrowing. The inbreeding of the dam significantly depressed litter size, total litter weight, and average weight per live pig at farrowing. Nonlinearity was apparent with the traits involving weight. Each of the litter traits at farrowing increased curvilinearly as age of dam at first parity increased. The predicted peak performance was by gilts of age 15 to 17 mo. at farrowing. The inbreeding of the litter showed practically no influence on litter size but evidenced a significant effect on pig weights at farrowing. The inbreeding and age of dam had practically no effect on number weaned over their indirect effects on litter traits at farrowing. However, the inbreeding of the dam continued to exert a significant direct influence on both pig and litter weights at weaning, while age of dam retained a significant effect on average weaning weight per pig. The quadratic equation provided a significantly closer fit than the linear equation in relating the inbreeding of the dam to the total weight of the litter at weaning. The inbreeding of the litter showed an increased influence on weaning traits over that shown at farrowing. Significant curvilinear relationships were noted for each trait. The inbreeding of the dam retained a significant direct effect on total litter weight at 154 days, in addition to its indirect influence through the litter traits at farrowing. No such direct effect was noted for the inbreeding of the dam on litter size or average pig weight at 154 days. However, the inbreeding of the litter exerted a significant influence on the 154-day litter traits even when litter farrowing or weaning traits were considered as covariables. The significant curvilinear effects of the inbreeding of the dam and litter suggest that complex inter-locus genetic interactions may be involved. Differential opportunities for selection also may have modified the apparent effects of the inbreeding of the dam. Average pig weight at weaning was heaviest in litters in which about nine live pigs had been farrowed and six to seven pigs weaned. The birth weight of the pig showed a highly significant influence on the litter traits at weaning and at 154 days.