Abstract
Functional longevity (FL) or length of productive life (LPL) is an important trait to measure the global functional aptitude of a cow. A proportional hazard mixed model by using the software package Survival Kit was used. The model included time-dependent variables taking the current value of the covariate at each calving date. The model included fixed effects of parity and stage of lactation, within herd and parity production level, and age at first calving (time-independent) and the random effect of herd– year– season interaction. The tested factors revealed a significant effect (p < 0.001) on the functional longevity. The study showed that milk yield was the most important factor affecting the length of productive life. Indeed, the hazard of culling increased as the milk production decreased. Though, the relative risk of culling increased with lactation stage throughout the first three lactations, and the highest one was found at the beginning and the end of the first lactation and at the end of next lactations
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