Genetic analyses of udder conformation traits and daily milk yield measured by robotic milking systems using repeatability and random regression models in American Holstein cattle

estimates ranges of 0.33 to 0.71 (UD), 0.36 to 0.63 (FTD), 0.10 to 0.47 (RTD), 0.40 to 0.64 (DFR), 0.15 to 0.44 (UB), and 0.13 to 0.33 (DMY). The genetic correlation estimates for the same udder conformation trait across lactations were consistently high (>0.80), indicating substantial genetic stability across parities. Within lactation, genetic correlations across DIM were also high (>0.70), particularly in later parities. Across all lactations, FTD and RTD were moderately and positively genetically correlated, whereas UD exhibited negative genetic correlations with teat-spacing and geometric traits (e.g., FTD, RTD, and DFR). Genetic correlations between udder conformation traits and DMY were weak to moderate (up to 0.37 under the repeatability model). Collectively, these results indicate that AMS-derived udder conformation traits are under moderate-to-high genetic control, which is consistent across DIM and lactations, supporting their incorporation into genomic selection programs in dairy cattle. The RRM provided additional resolution on within-lactation dynamics, whereas repeatability models captured most of the additive genetic signal required for routine genetic evaluations.