The purebred-crossbred correlation in pigs: a review of theory, estimates and implications

Pig and poultry production relies on crossbreeding of purebred populations to produce production animals. In those breeding schemes, selection takes place within the purebred population to improve crossbred performance. The genetic correlation between purebred and crossbred performance (r_pc) is, however, lower than unity for many traits. When r_pc is low, the use of crossbred performance in selection is required to achieve sizable genetic progress. The objectives of this paper were to describe the different components and importance of r_pc, and to review existing literature that report estimates of the r_pc in pigs. The r_pc has three components 1) genotype by genotype interactions, 2) genotype by environment interactions, and 3) differences in trait measurements. We theoretically showed that selecting based on crossbred performance is beneficial when the accuracy of selection for crossbred performance in purebred animals is at least r_pc times the accuracy for purebred performance. In total, 201 r_pc estimates from 27 studies were used, published between 1964 and 2017. The average r_pc estimate was 0.63, with 50% of the estimates between 0.45 and 0.87. Standard errors of the estimates were on average 0.16, with 50% of the standard errors between 0.06 and 0.19. For all different trait categories, e.g. growth, meat amount, meat quality, feed, and fertility, the average r_pc was around 0.6. Genotype by environment interaction appeared to have a smaller contribution to r_pc than genotype by genotype interactions. More research regarding the impact of the different components on the r_pc can help to improve breeding programs. Future studies are advised to report characteristics of the herd environments in detail, to report estimated h², additive genetic variances for purebreds and crossbreds, and the estimated r_pc with standard errors or confidence intervals, to estimate separate r_pc for different pure lines, and to genotype the animals under study.