Estimation & Prediction

Evaluating the Impact of Breed Proportion on Genetic Parameter Estimates and Sire Re-Ranking in an Open Herd Book Simmental Population

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Abstract

Open herd book systems allow animals of different breed compositions to be registered within a single breed association, promoting genetic diversity but potentially increasing heterogeneity in genetic evaluations. This study aimed to quantify the impact of including breed proportion and expected heterosis on the estimation of genetic parameters and the ranking of sires in a Simmental population. Phenotypic data for birth weight (BW), weaning weight (WW), and yearling weight (YW) from 5,502 animals were analyzed through three alternative trait animal models using BLUPF90+ software. Model 1 (M1) assumed all animals were pure Simmental, ignoring breed composition; Model 2 (M2) included the proportion of the animals as a linear covariate in the fixed structure; and Model 3 (M3) incorporated the expected heterosis based on the individual breed proportion as an additional covariate. For BW and WW, maternal additive effects and maternal heterosis were also included in the model. Heritability estimates varied across models. For BW, h² values were 0.52 (M1), 0.44 (M2), and 0.49 (M3). For WW, estimates were 0.36 (M1), 0.34 (M2), and 0.35 (M3). For YW, heritability remained consistently moderate across models (0.45-0.48). Maternal heritability estimates for BW were 0.13, and for WW were 0.29 for all models. These results demonstrate that incorporating breed composition and/or heterosis leads to shifts in the partitioning of phenotypic variance, particularly between additive and residual components, reflecting improvements in model fit and accounting for biological variation associated with crossbreeding. To evaluate the consequences for selection, EBVs from the top sires (top 10% with the most progeny) were compared across models. Spearman rank correlations for BW were 0.822 (M1 vs M2), 0.981 (M1 vs M3), and 0.904 (M2 vs M3). For WW, correlations were 0.860, 0.927, and 0.954, respectively. For YW, correlations were higher (0.943, 0.926, and 0.989), indicating less re-ranking in post-weaning performance. These results show that model choice meaningfully affects sire ranking for early-life growth traits, where maternal and heterosis effects are most pronounced. Overall, the inclusion of breed composition and heterosis altered genetic parameter estimates and led to moderate re-ranking of sires, particularly between M1 and the models that account for breed proportion. These findings highlight that ignoring breed proportion and heterosis in open herd book populations can bias genetic evaluations and impact selection decisions. Future research should explore advanced multi-breed evaluation strategies, such as metafounders and genomic relationships, to more accurately model the genetic architecture of admixed beef cattle populations.

Keywords: 2026

How to Cite:

Mulim, H. & Rojas de Oliveira, H., (2026) “Evaluating the Impact of Breed Proportion on Genetic Parameter Estimates and Sire Re-Ranking in an Open Herd Book Simmental Population”, World Congress on Genetics Applied to Livestock Production Digital Archive 2026(1): 2285993. doi: https://doi.org/10.31274/wcgalp.23831

Rights: 1