Heterosis for litter size and growth in crosses of four strains of Iberian pig

Data collected from a complete diallel cross scheme with four ancient lines of Iberian pigs have been analyzed in order to estimate genetic line and heterotic effects on litter size and growth traits. Records of the number of piglets born alive (NBA) and total number of piglets born (TB) from 2768 litters of 817 dams were analyzed using different animal models fitting crossbreeding parameters. Estimated values of genetic correlations between NBA or TB records at early and later parities were close to 0.60. Significant differences in litter size at first and second parities were observed between some dam lines, but not at the third and later parities. Negligible progeny specific heterotic effects on litter size traits were estimated in early parities, but they were significant at the third and later parities (ranging from +0.5 to +1.0 alive piglets and from +0.6 to +0.9 born piglets). All these results reinforce the evidence supporting the hypothesis of a partially different genetic basis of litter size at early and later parities. Growth performance in the final fattening period (100–160kg) of purebred and crossbred pigs was analyzed with a Bayesian procedure modeled by a linear growth function. Data consisted of 2103 weight records from 579 pigs of 352–493 days of age. Relevant heterotic effects on weight at 420 days of age (intercept of the growth function) were inferred for the different crosses with mean values ranging from +12.7 to +17.7kg. Heterotic values for daily growth rate (slope of the linear function) were not significantly different from zero for five out of the six combinations of lines but an important heterosis (+66g/d) was estimated between the remaining combinations. The implications of this remarkable heterosis between Iberian pig lines on breeding schemes are discussed in the context of the current breeding structures of this breed.