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Cutting edge methodology
P1-51 Comparison of five, with and without time-dependent covariates, survival models as descriptors for culling distributions in dairy cows
  1. H Sharifi1,
  2. A R Bahonar2,
  3. A Sharifi3,
  4. P Kostoulas4
  1. 1Department of Food Hygiene and Public Health, Faculty of Veterinary Medicine, Shahid Bahonar University of Kerman, Kerman, Iran
  2. 2Department of Food Hygine and Quality Control, Faculty of Veterinary Medicine, University of Tehran, Tehran, Iran
  3. 3Department of Ophthalmology, Kerman University of Medical Science, Kerman, Iran
  4. 4Laboratory of Epidemiology, Biostatistics and Animal Health Economics, University of Thessaly, Karditsa, Greece


Introduction This paper presents the results of a study to evaluate survival analysis the effect of treated diseases on the culling rate (remove from herd) in dairy cows.

Methods Five different models, with and without time-dependent covariates, using Gompertz distribution were studied. Model 1 treated diseases as a binary and time-independent covariates. Models 2 through 5 treated diseases as time-dependent covariates. For each observation, we split follow-up time in intervals each corresponding to a different lactation month. In other words, each observation from study entry until culling or censoring was split into several one-month observations by Lexis expansion of the original dataset. Model 2 assumed an animal experience a certain disease from the beginning of the occurrence of that disease by the end of follow-up period. Model 3 assumed cows are at risk from the begging of the study until the disease occurred (inverse of model 2). In model 4 and 5 an animal was assumed to experience a certain disease for 1 month if the disease occurred during this period. The only difference is in model 4 assumed diseases occurred only one time and in model 5 multiple disease occurrences at different months were considered as different episodes.

Results According to Akaike's Information Criterion (AIC) value and Cox-Snell residuals model 5 was the best model.

Conclusion A comparison of culling models with and without time-dependent covariates found that models without time dependency tended to seriously underestimate the risk of a disease on culling.

  • Survival analysis
  • time-dependent covariate
  • culling

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