Genetic parameters and predictions for direct and maternal growth traits in a multibreed Angus–Brahman cattle population using genomic–polygenic and polygenic models

• We computed genomic–polygenic parameters under 4 subtropical data scenarios.
• We predicted direct and maternal animal EBV for 3 growth traits.
• The breed composition of animals ranged from 100% Angus to 100% Brahman.
• Genetic parameters from the polygenic and genomic–polygenic model 1 were similar.
• Brahman animals had higher EBV for direct and lower EVB for maternal growth traits.

The objectives of this research were to compare variance components, genetic parameters, and EBV rankings for birth weight (BW) direct and maternal, weaning weight (WW) direct and maternal, and postweaning gain from 205 d to 365 d (PWG) direct using three genomic–polygenic and one polygenic model representing four plausible beef cattle genetic evaluation scenarios for growth traits under subtropical conditions in the US southern region. In addition, EBV trends as percentage Brahman increased from 0% to 100% were evaluated for each trait and model. The dataset included 5264 animals from a multibreed Angus–Brahman population born from 1987 to 2013. Genomic–polygenic models 1 (GP1; pedigree relationships for all animals; genomic relationships for genotyped animals), 2 (GP2; pedigree relationships for non-genotyped animals; genomic relationships for genotyped animals), and 3 (GP3; no pedigree relationships; genomic relationships for genotyped animals) used actual and imputed genotypes from 46,768 SNP markers. Variance components and genetic parameters were estimated using REML procedures. Variance component and genetic parameter estimates from GP1 were the most similar to those from the polygenic model, followed by those from GP2, and the least similar (especially for maternal traits) were those from GP3. Similarly, the highest rank correlations were those between animal EBV from the polygenic model and GP1, followed by those between animal EBV from GP1 and GP2 and between the polygenic model and GP2. Model GP3 performed poorly for maternal traits due to ignoring calf–dam relationships. These results indicated that the polygenic model and genomic–polygenic model 1 should be preferred. However, high genotyping costs still make the polygenic model preferable for commercial beef cattle operations. Brahman animals tended to have higher EBV for BW direct and WW direct, and lower EBV for PWG direct, BW maternal, and WW maternal. However, low regression coefficients for EBV on Brahman fraction ensured that high, medium, and low EBV animals from all breed compositions existed in this multibreed population.