A review of the purebred-crossbred correlation in pigs: theory, estimates, and reporting

Pig and poultry production relies on crossbreeding of purebred populations to produce production animals. Thus, the breeding goal is to improve crossbred performance, while selection typically takes place within the purebred populations. The genetic correlation between purebred and crossbred performance (rpc) is known to be lower than unity for many traits. A low value of rpc indicates that use of crossbred performance in selection is required to achieve sizable genetic progress. We aimed to 1) review estimates of rpc in pigs, 2) review the different components of rpc and their contribution to it, 3) give guidelines for future studies estimating rpc. In total, 195 rpc estimates from 27 studies were used, published between 1964 and 2016. The rpc estimates had an average value of 0.63, with 50% of the estimates between 0.43 and 0.88. Standard errors were on average 0.16, with 50% of the values between 0.07 and 0.2. Standard errors of rpc reduced with increasing numbers of common sires between purebred and crossbred animals. For all different trait categories, e.g. growth, meat amount, meat quality, feed, and fertility, the average rpc was below 0.8. The rpc has three components 1) genotype by environment interaction between nucleus and commercial herds, 2) differences in genetic background of purebred and crossbred animals, and 3) differences in trait definition or measurement used in purebred and crossbred animals. Genotype by environment interaction appeared to have a smaller contribution to rpc than differences in genetic background. Across traits and studies, the rpc did not show a relation with the heritability of the trait. Future studies are advised to consider to keep both purebred and crossbred animals under nucleus and commercial conditions, report characteristics of the herd environments in detail, estimate separate rpc for different pure lines, and genotype the animals under study.