Estimation of genetic parameters for mature cow size in North American and Australian Angus cattle.

Mature cow size can be effectively controlled through genetic selection. Understanding the genetic relationships between mature cow weight (MWT), mature cow height (MHT), and body condition score (BCS) is essential for implementing effective selection programs to enhance cow efficiency. The objectives of this study were to 1) estimate variance components and genetic parameters for MWT, MHT, and BCS measured in the United States and Australia; 2) estimate the genetic correlation between these traits and early growth and carcass traits; and 3) estimate the genetic correlations among these traits across the two countries. The dataset provided by the American Angus Association comprised of 434,746; 213,875; and 382,156 records for MWT, MHT, and BCS, respectively. The dataset provided by Angus Australia comprised of 206,003; 15,379; and 36,184 records for MWT, MHT, and BCS, respectively. Single-trait repeatability models were used to estimate heritabilities and multiple-trait models were used to estimate genetic correlations between traits and across countries. Heritabilities (SE) for MWT were 0.45 (0.005) and 0.40 (0.006); for MHT 0.57 (0.006) and 0.63 (0.02); and for BCS 0.18 (0.004) and 0.18 (0.01), in the United States and Australia, respectively, highlighting their moderate-to-high genetic control. Genetic correlations within the United States and Australian datasets between MWT and MHT, and MWT and BCS were >0.50, and <0.20 between MHT and BCS. Genetic correlations between MWT, MHT and early growth traits were generally positive and moderate-to-high, ranging from 0.51(0.01) to 0.92(0.003) in the United States and 0.41(0.03) to 0.79(0.05) in Australia. Genetic correlations between BCS and early growth traits were in general positive (<0.30), while genetic correlations between mature cow traits and ultrasound traits ranged from strong positive to strong negative. Genetic correlations between the traits in the two countries were high for MWT = 0.91 (0.02) and MHT = 0.97 (0.02); and moderate for BCS = 0.65 (0.08). Our results suggest that optimizing selection for mature cow traits is feasible, and that a joint evaluation between the United States and Australia could be beneficial.