Performance of selected and control lines of Duroc and Yorkshire pigs and their reciprocal crossbred progeny.

Performance records for 223 boars and gilts in Duroc and Yorkshire selected and control lines and for 231 boar and gilt progeny from reciprocal breed crosses were analyzed to evaluate (1) maternal effects, (2) effects of mass selection in purebreds on performance of crossbred progeny and (3) heterosis effects. Selection of purebred herd replacements was based mainly on an index (Index) of average daily gain (ADG), average scan backfat thickness (BF) and scan longissimus muscle (loin eye) area (LEA) among tested pigs. Mixed models were fitted separately to the purebred and crossbred populations, including effects of breeding group, sires within breeding group, sex, pertinent interactions and residual on Index and its component traits, ADG, BF and LEA. Crossbred pigs from Yorkshire dams had slightly larger (P less than .10) LEA's and higher (P less than .05) Indexes, suggesting a small maternal effect favoring Yorkshire dams over Duroc dams in crosses of these breeds. Crossbred pigs from Yorkshire dams also had a higher ADG and slightly less BF than crossbred pigs from Duroc dams, but the differences were not significant. Selection differentials were over three times as large for sires as for dams in purebreds and were substantially larger in selected lines than in controls where selection pressure was essentially zero for ADG and Index. In parental purebreds, selected lines exceeded controls for ADG, LEA and Index and had less BF. Selected lines crossbred progeny exceeded control lines crossbred progeny for all traits including BF. Responses in crossbreds to purebred selection varied with the trait, suggesting effects of genetic correlations, gene frequency differences, nonadditive genetic variance and sampling variation. Heterosis appeared not to be influenced by performance level or intensity or direction of selection in parental purebreds.