Effect of Disease on Length of Productive Life of French Holstein Dairy Cows Assessed by Survival Analysis

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Abstract
Data from a survey performed from 1986 to 1990 were analyzed to assess the effects of diseases on length of productive life of 3589 Holstein cows from 47 herds, using a proportional hazard model. The probability of a cow being culled, or hazard function, was supposed to be the product of an unspecified baseline hazard function and log-linear, time-dependent explanatory variables that possibly influence culling rate (Cox's regression). The effect of 16 health events was studied according to lactation number of occurrence. The model included adjustments for effects of herd-year-season (considered to be random), month of calving, stage of lactation, lactation number, reproductive performance, and milk production. The probability of a cow being culled increased in early and late stages of lactation in older cows, in low producing cows, and in cows with poor reproductive performance. Mastitis before the peak of lactation or during the dry period increased the risk (relative culling rate in first lactation, 1.3 and 4.0, respectively). Teat injuries and nontraumatic udder disorders had a large impact on longevity. Cows with late metritis or early abortion had poor survival. The decrease in median length of productive life could be over a standard lactation in particular cases. Expected survivor curves, computed after assumption of a priori values of covariates and their evolution over time, appear to be powerful tools for examining the effect of health disorders on length of productive life of cows.