Investigation of genotype by country interactions for growth traits for Charolais populations in Australia,Canada, New Zealand and USA

Evidence of heterogeneity of parameters and genotype by country interactions was investigated for birth weight (BWT), weaning weight (WWT) and postweaning gain (PWG) between Australian (AUS), Canadian (CAN), New Zealand (NZ) and USA populations of Charolais cattle. An animal model was fit to data sets for each individual country to compare the within-country parameter estimates for homogeneity. The direct heritability estimates of BWT in AUS (0.34) and NZ (0.31) were less than CAN (0.55) and USA (0.47). Maternal BWT heritabilities (0.13–0.18), direct WWT heritabilities (0.22–0.27), and maternal WWT heritabilities (0.12–0.18) were similar across all four countries. Direct PWG heritability for AUS (0.14) was smaller than the same estimate in the other three countries (0.24–0.31). The phenotypic variances for all three traits were similar across AUS, CAN and USA; however, NZ was higher for BWT and WWT and lower for PWG. A multiple trait animal model that considered each trait as a different trait in each country was also fit to the data for pairs of countries. Direct (maternal) estimated genetic correlations for BWT for AUS–CAN, AUS–USA, USA–CAN, NZ–CAN and NZ–USA were 0.88 (0.86), 0.85 (0.82), 0.88 (0.82), 0.85 (0.83), and 0.84 (0.80), respectively. Direct (maternal) estimated genetic correlations for WWT for AUS–CAN, AUS–USA, USA–CAN, NZ–CAN and NZ–USA were 0.96 (0.91), 0.95 (0.90), 0.95 (0.91), 0.95 (0.92), and 0.95 (0.92), respectively. Direct estimated genetic correlations for PWG for AUS–CAN, AUS–USA, USA–CAN, NZ–CAN and NZ–USA were 0.89, 0.91, 0.94, 0.90, and 0.91, respectively. The magnitude of the across-country genetic correlations indicates that genotype by country interactions were biologically unimportant. However, strong evidence exists for heterogeneity of parameters across the countries for some traits and effects. Therefore, combining these countries into one single analysis to produce a common set of genetic values will depend on the development of methods to adjust for heterogeneous parameters for models containing both direct and maternal effects, and for circumstances where constant variance ratios or heritabilities are not present across populations.