Animal board invited review: The purebred-crossbred genetic correlation in poultry.

The purebred-crossbred genetic correlation (r) is a key parameter to determine whether the optimal selection of purebred animals to improve crossbred performance should rely on crossbred phenotypes, purebred phenotypes, or both. We reviewed published estimates of the r in poultry. In total, 19 studies were included, of which four were on broilers and 15 on laying hens, with 150 r estimates for nine different trait categories. Average reported r estimates were highest for egg weight, egg quality and egg colour (0.74-0.82), intermediate for BW, maturity and mortality (0.61-0.70) and egg number (0.58), and low for resilience (0.40) and body conformation (0.14). Most studies were based on measuring purebred and crossbred phenotypes in the same environment and thus did not capture the contribution of genotype by environment interactions to the r, suggesting that the presented average estimates may be higher than values that apply in practice. Nearly all studies were based on two-way crossbred animals. We hypothesised that r values for a two-way cross are good proxies for r of a four-way cross. Only eight out of 19 studies were published in the last 25 years, and only two of those used genomic data. We expect that more studies using genomic data may be published in the coming years, as the required data may be generated when implementing genomic selection for crossbred performance, which will lead to more accurate r estimates. Future studies that aim to estimate r are encouraged to capture the genotype by environment interaction component by housing purebred and crossbred animals differently as is done in practice. Moreover, there is a need for further studies that enable to explicitly estimate the magnitude of genotype by environment versus genotype by genotype interactions for multiple trait categories. Further, studies are advised to report: the specific housing conditions of the animals, any differences between measurements of purebred versus crossbred performance, and the heritabilities of purebred and crossbred performance.