Genetic relationships between milk production, meat production, and cow size traits in Nordic Red dairy cattle

The beef-on-dairy approach, which uses surplus dairy calves for meat production, is considered environmentally sustainable and an efficient use of dairy resources. Inseminating dairy cows with beef semen improves the meat traits of dairy-origin beef; however, the focus on crossbreeding with beef sires often overlooks the role of the cow's genetics on meat traits and does not directly target genetic improvement of the cow. This issue is further compounded in the Nordic countries, where a Saved Feed index is included in the Nordic Total Merit index. This index lowers maintenance energy requirement by applying a negative index weight on metabolic body weight. Although this improves feed efficiency, its influence on other economically important traits has not been addressed so far. In this study, we estimated genetic parameters for cow carcass traits (carcass weight, carcass conformation, carcass fatness), 305-d milk production traits (milk yield, protein yield, fat yield), cow size traits (stature, metabolic body weight), and bull calf growth traits (daily carcass gain, carcass conformation, carcass fatness) in Nordic Red dairy cattle using data from 18,757 cows and 41,838 bull calves. Variance components were estimated by fitting a multivariate animal model and applying a Monte Carlo expectation maximization restricted maximum likelihood algorithm implemented in MiX99 software. Heritability estimates for cow carcass traits ranged from 0.37 (carcass conformation) to 0.68 (carcass weight); for milk production traits from 0.31 (fat yield) to 0.38 (milk yield); and for cow size traits from 0.64 (stature) to 0.66 (metabolic body weight). Bull calf growth traits showed heritability estimates ranging from 0.19 (bull carcass conformation) to 0.34 (daily carcass gain). Unfavorable genetic correlations were observed between cow carcass and 305-d milk production traits, ranging from -0.27 (carcass weight and protein yield) to -0.67 (carcass fatness and fat yield). In contrast, positive genetic correlations were noted between cow carcass and progeny growth traits, ranging from 0.20 (bull carcass conformation and cow carcass weight) to 0.67 (cow carcass weight and daily carcass gain), except for bull carcass fatness, which was positively correlated only with cow carcass fatness. Cow carcass traits were also positively correlated with metabolic body weight, ranging from 0.39 (cow carcass fatness) to 0.90 (cow carcass weight). These results provide insight into the genetic relationship among traits relevant for milk and dairy-beef production and may support the optimization of total merit index selection for balanced improvement of milk and meat traits.