Genetic parameter estimates and genetic trends for cow longevity indicators in Holstein cattle based on different culling reasons and random regression models.

Selection for increased longevity in dairy cows can enhance farm profitability and animal welfare. The main objective of this study was to estimate variance components and genetic parameters for 8 functional longevity indicators defined based on different culling reasons recorded in 7,188,910 Holstein cows. Culling reason groups included age, workability issues, economic reasons, conformation problems, production level, health issues, reproduction performance, and unknown factors. Longevity was analyzed longitudinally, assigning a score of 1 or 2 based on the cow's lifetime in the herd across age classes ranging from 1 (12-23 mo) to 15 (180 mo) years. A single-trait random regression model (RRM) using fifth-order Legendre orthogonal polynomial provided the optimal fit, and it was used in subsequent analyses. Heritability estimates varied according to age and culling reasons: 0.17 to 0.38 (age-related), 0.04 to 0.52 (workability issues), 0.002 to 0.20 (economic reasons), 0.02 to 0.28 (conformation), 0.02 to 0.26 (production), 0.02 to 0.25 (health issues), 0.02 to 0.24 (reproduction), and 0.02 to 0.22 (unknown reasons). The genetic correlations between adjacent ages were high and positive (≈1.0) but became negative (≈-1.0) between more distant ages. The average genetic correlations among the culling reasons were generally low or moderate (≈-0.50 to ≈0.80). However, the age-related culling, workability, and economic reason groups showed the lowest correlations among them compared with the others. In contrast, health, reproduction, and unknown reason groups exhibited the highest correlations, reaching values close to 0.80. Genetic trends showed similar patterns across culling reasons, with genetic gains being observed at younger ages and losses at older ages. Overall, this study demonstrates that cow longevity in Canadian Holstein cattle is heritable with substantial additive genetic variance. The heritability estimates varied depending on the culling reason, but at least one age always had a moderate heritability estimate. Moreover, cow longevity defined based on different culling reasons may represent genetically distinct traits. Therefore, it is recommended to consider culling reasons when genetically evaluating cow longevity in Holstein cattle populations.