Use of termination events and mortality data recorded during the lactation as a proxy to predict the genetics of resilience and health of dairy cattle

ABSTRACT: Increasing production and environmental challenges in dairy cattle means that selecting for resilience is becoming more important. This study explored whether data on cows that exit before completing their lactation and those that die during lactation can be used to predict resilience. To identify predictors of resilience, exiting the herd by 60, 120, 180, and 240 d were defined as traits. Additional traits were defined by including all the cows that died during the entire lactation with the cows that exited at different times up to 240 d of lactation. For all traits, cows that exited the herd or died were coded as 1, otherwise as 0, at the end of the lactation. We used performance and exit data of Holstein (HOL) and Jersey (JER) cows that calved between 1998 and 2023. The data were analyzed using a multitrait sire model to estimate heritability and correlations with milk yield (MY), SCC, calving interval (CIN), and selected type traits. The results showed that the proportion of cows that exited by 60 d was 2%, increasing by about 2% every 2 mo until exit by 240 d. The trend over the years in the proportion of exits, taking exit by 180 d and exit by 180 d + death as an example, showed an undesirable increase from 5.6% in 2000 to 9.4% in 2022. Heritability of all exit traits was low, increasing from below 1% for exit by 60 d to 2.8% for exit by 240 d + all deaths over the lactation. The genetic correlation of early exit (i.e., 60 or 120 d) with first test-day MY was positive (unfavorable) and higher at the beginning (0.4), decreasing over time to be favorable in JER (−0.2) and near zero in HOL (0.1) by the end of the lactation. In contrast, the genetic correlation of exit with first test-day SCC became stronger (favorable) at the end of the lactation (0.3 to 0.4). Exit at any time during the lactation had the strongest genetic correlation with CIN (i.e., fertility). The genetic correlation of exit traits with BCS and angularity showed that the likelihood of cow exit, especially up to 180 d, was higher for thin and more angular cows. The genetic correlation estimates imply that cows with high potential for MY, poor fertility, poor BCS, and high scores for angularity are more likely to exit early due to metabolic stress. The change in genetic correlation between exit and MY early from unfavorable to favorable in JER due to more culling for milk and less for fertility and udder health is leading to an undesirable genetic trend for exit by 180 d as well as exit by 180 d + all death. However, the increasing phenotypic trend of exit rates in both breeds suggests a need for close monitoring. The selective use of exit data can help to develop genetic evaluations for resilience and health traits and validate and complement data collected to improve health and welfare during the transition period.