Economic modeling and selection for longevity traits in dairy cattle.

Different definitions have been proposed for longevity, making it challenging to compare methods for estimating the economic significance of survival traits. Simple profit functions often fail to capture biological complexity, whereas bioeconomic models, though comprehensive, can be difficult to apply. This study aimed to develop a versatile framework for estimating the economic value (EV) of longevity traits and designing optimal selection indices. The approach can accommodate various definitions of longevity (e.g., herd life, productive life, number of lactations, stayability or survival across lactations) and be applied across production systems. The economic model is founded on a stable herd age structure at equilibrium, where changes in survival rates influence profitability and age distribution. Average survival rates between parities determine the distribution of cows across parities and the number of replacements needed to maintain a steady-state herd structure. Iranian production and economic data were used for parameterization, and EVs were estimated accordingly. Seven selection indices were developed, incorporating production, reproduction, somatic cell score (SCS), longevity and type traits. The estimated economic weights were $0.12/kg for milk yield, $5.56/kg for fat yield, $5.50/kg for protein yield, $2.73/month for productive life, $22.6/year for herd life, $0.18/percentage point for survival, $37.59/lactation for number of lactations, $-3.84/day for days open and $-122.87/unit for SCC. Excluding longevity from the breeding goal led to the lowest observed genetic gain, demonstrating that including longevity traits positively influences overall genetic improvement. The greatest genetic progress was achieved by indices enhancing productive life through indirect selection on fore udder conformation. This study demonstrates that (1) the flexible approach enables estimation of EVs under multiple longevity definitions and (2) selection for productive life, particularly via indirect selection on fore udder traits, improves longevity, enhances production and reduces SCC.