Effect of health disorders on culling in dairy cows: a review and a critical discussion

Risk of culling consequent to the main health disorders occurring in the cu rrent production systems is reviewed. Survival analysis including health di sorders as lime-dependent variables is considered to be the most appropriat e method to assess their effects because they allow a better description of the exact follow-up of disease history. Farmers preferentially consider he alth events in the current lactation and/or those occurring in early stages of lactation for making culling decisions. The unfavourable direct effects on culling of dystocia and udder disorders (mastitis and teat injuries) ar e clearly demonstrated, whereas there ale variations between studies on the association between metabolic and reproductive disorders and culling. Thes e variations may be due to differences in study designs, populations involv ed and methods. Consequences, in terms of estimated effect of health disord ers, of methodological choices (e.g. whether or not including in the models descriptors for milk yield and/or reproductive performance) are discussed. Metabolic and reproductive disorders may act indirectly through a subseque nt decrease in milk yield and reproductive performance. The impact of healt h disorders on longevity is on average weak;, compared to the impact of low milk yield potential and pool reproductive performance. Herd characteristi cs (availability of heifers, quota, farmer's attitude towards risk and unce rtainty...) modify the risk for a cow to be culled for a given health disor der. Aims of further studies could be (1) to interpret the meaning and to a nalyse the reliability of culling reasons information, (2) to evaluate the relative effect on culling of health disorders and performance (milk yield and reproduction) in different parities, (3) to investigate the role of com ponents of the herd effect on the risk of culling.