Kaplan–Meier survival plots were constructed for each of the categorical variables. A univariate Cox proportional hazards model was used to check for assumed proportion hazards using univariate analysis looking at log plots and testing survival time vs. scaled Schoenfeld residuals using both chi squared tests and visual examination of plots. DIM at ESD was included in all models to correct for DIM at entry into the study. Significance of each variable in the univariate model was analyzed using the likelihood ratio test. Variables with a P < 0.2 were retained for inclusion in a multivariable Cox proportional hazards model. Farm was included in the analysis as strata effect. Explanatory variables were removed in a backwards elimination based on Likelihood ratio test comparison of reduced model with full model using P > 0.05 as reason for removal. Interaction between variables was examined and each of the removed variables individually entered at the end to ensure correctness of the model.