Fertility, mastitis and longevity in dairy cattle analyzed using survival models

The aim of the thesis was to investigate whether survival analysis (SA) results in a better genetic evaluation of female fertility and mastitis traits and to study the effects of mastitis and pregnancy status as risk factors for culling. Sire breeding values for interval between calving and last insemination (CLI) and interval between first and last insemination (FLI) were predicted using SA and mixed linear models (LM). Correlations between simulated true breeding values for conception rate and breeding values for CLI and FLI predicted by SA were higher than corresponding correlations with LM. When pregnancy status was known, SA was better than LM for genetic evaluation of conception rate when using observations on CLI and FLI. If selection were carried out on these predicted breeding values, this would translate into 8 to 12% higher genetic progress for FLI and CLI, respectively. Clinical mastitis was analyzed with LM (binary trait), and time to first mastitis with SA. The higher accuracies for SA could be translated into a higher genetic response, approximately, 3% for first parity and 25 % for later parities. The effect of pregnancy status and mastitis on culling in Swedish dairy cattle was analyzed with SA. Mastitis affected culling decisions throughout the lactation, but its effect depended on pregnancy status and the stage during which the cow was treated. The risk of culling was low for pregnant cows, whether or not they had been treated for mastitis. Different patterns were observed for the risk of culling between open and pregnant cows. For both groups, the risk of being culled was higher for cows treated for mastitis than for untreated cows. For open cows, the risk was higher for cows treated in earlier stages, whereas for pregnant cows, the risk was similar across different stages of lactation. This thesis showed the potential of using SA for genetic evaluation of fertility traits and clinical mastitis in dairy cattle. SA was also found to be a useful method to analyze the effect of pregnancy status and clinical mastitis as risk factors for culling by treating them as time-dependent covariates.