Growth and reproduction genomic-polygenic and polygenic parameters and prediction trends as Brahman fraction increases in an Angus-Brahman multibreed population

• We estimated genomic and polygenic parameters for 4 reproductive and growth traits.
• Breed composition of cattle spanned the range from 100% Angus to 100% Brahman.
• Genomic-polygenic and polygenic heritabilities and correlations were similar.
• High % Brahman were older at first calving and had shorter first-calving intervals.
• High, medium, and low EBV for all 4 traits existed across all Brahman percentages.

The objectives of this research were to estimate genomic-polygenic and polygenic parameters and to evaluate prediction trends as Brahman fraction increased from 0% to 100% in a subtropical multibreed Angus-Brahman (MAB) population for four growth and reproduction traits using single-step genomic-polygenic (GPM) and polygenic models (PM). Traits were 365-d yearling weight (YW), yearling reproductive tract score (RTS), age at first calving (AFC), and first calving interval (FCI). Numbers of phenotypic records were 1758 for YW, 381 for RTS, 1385 for AFC, and 985 for FCI. The pedigree file had 6869 calves, sires, and dams, and genotype file contained 115,711 actual and imputed GGPHD150k SNP markers from 1547 animals. The 4-trait GPM and PM included contemporary group, age of dam (YW only), sex of calf (YW only), direct heterosis, maternal heterosis (YW only) as fixed effects, and animal and residual as random effects. Genetic parameters were estimated using REML procedures and computed using AIREMLF90. Heritabilities were slightly higher for GPM than PM (0.47 vs. 0.45 for YW, 0.31 vs. 0.30 for RTS, 0.14 vs. 0.12 for AFC, and 0.31 vs. 0.29 for FCI). Genetic correlations were positive between YW and RTS (GPM: 0.55; PM: 0.60), negative between RTS and AFC (GPM: −0.22; PM: −0.55) and between AFC and FCI (GPM: −0.68; PM: −0.67), and near zero for all other trait pairs. The similarity between GPM and PM heritabilities and genetic correlations indicated that the 115,711 GGPHD150k SNP markers added little additional information to that contained in the pedigree. Regression coefficients of breed group EBV means on Brahman fraction were negative (P=0.0005) for YW, RTS, and FCI, and positive (P<0.0001) for AFC as Brahman fraction increased. This indicated that heifers with higher Brahman percentages tended to be lighter and less mature as yearlings, older at first calving, and have shorter FCI than heifers with higher Angus percentages under the subtropical environmental conditions of the MAB population. Regression coefficients of individual animal EBV on Brahman fraction showed similar trends, although absolute values were smaller. However, there was a high degree of variation in EBV values within breed groups. Consequently, animals with high, medium, and low EBV existed across all Brahman percentages, thus allowing the selection of replacement animals of all Brahman percentages based on a common set of objectives.