title: Comparison of culling rates among dairy cows grouped on the basis of serologic status for bovine leukemia virus.
abstract: OBJECTIVE
  To determine the association between serologic status for bovine leukemia virus (BLV) and culling rates by use of survival times in a commercial Holstein dairy herd.
  
  
  METHODS
  Longitudinal study.
  
  
  METHODS
  593 milking cows.
  
  
  METHODS
  Cattle were tested for antibodies against BLV by use of agar gel immunodiffusion or ELISA 4 times each year from 1989 to 1993 and then annually through 1999. Dates of birth, first calving, and culling or death were obtained from Dairy Herd Improvement Association records. Most cows were enrolled in the study on the date of first calving. Survival times were compared among seropositive, seronegative, and seroconverted cows with the Kaplan-Meier method and a Cox regression model stratified on the basis of year of birth.
  
  
  RESULTS
  Complete records were available for 593 of 685 (87%) cattle in the dairy herd during the study period. Median survival time for all cows was 31.7 months. Survival times, which correspond to cull rates, did not differ significantly between seropositive and seronegative cattle, whereas cattle that seroconverted during the study had a significantly longer survival time. Year of birth was positively and significantly associated with survival time.
  
  
  CONCLUSIONS
  BLV serologic status was not associated with cull rate as measured by survival time in this dairy herd. This finding is in contrast to results of studies that used survival analysis techniques; our results may influence management decisions concerning BLV.
authors.affiliation: Avrum Gudelsky Veterinary Center, University of Maryland, College Park, MD 20742, USA.
authors.forename: J Kelly
authors.initials: JK
authors.lastname: Rhodes
authors.forename: Kevin D
authors.initials: KD
authors.lastname: Pelzer
authors.forename: Yvette J
authors.initials: YJ
authors.lastname: Johnson
authors.forename: Estelle
authors.initials: E
authors.lastname: Russek-Cohen
mesh_terms: D000818:Animals; D000914:Antibodies, Viral / Q000097:blood; D002417:Cattle; D016583:Enzootic Bovine Leukosis / Q000453:epidemiology / Q000401:mortality*; D005260:Female; D001909:Leukemia Virus, Bovine / Q000276:immunology / Q000302:isolation & purification; D008137:Longitudinal Studies; D008396:Maryland / Q000453:epidemiology; D015995:Prevalence; D016016:Proportional Hazards Models; D016036:Seroepidemiologic Studies; D016019:Survival Analysis
chemical_list: D000914:Antibodies, Viral