EFFECT OF DISEASE ON LENGTH OF PRODUCTIVE LIFE OF FRENCH HOLSTEIN DAIRY-COWS ASSESSED BY SURVIVAL ANALYSIS

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 probabilit y of a cow being culled, or hazard function, was supposed to be the pr oduct 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 accordin g to lactation number of occurrence. The model included adjustments fo r effects of herd-year-season (considered to be random), month of calv ing, stage of lactation, lactation number, reproductive performance, a nd milk production. The probability of a cow being culled increased in early and late stages of lactation in older cows, in low producing co ws, and in cows with poor reproductive performance. Mastitis before th e peak of lactation or during the dry period increased the risk (relat ive culling rate in first lactation, 1.3 and 4.0, respectively). Teat injuries and nontraumatic udder disorders had a large impact on longev ity. 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 afte r 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.